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% Format this file with latex. % Format this file with latex.
%\documentstyle[11pt,myformat]{article} \documentstyle[myformat]{report}
\documentstyle[palatino,11pt,myformat]{article}
\title{\bf \title{\bf
Python Tutorial \\ Python Tutorial
(DRAFT)
} }
\author{ \author{
...@@ -25,29 +23,30 @@ ...@@ -25,29 +23,30 @@
\noindent \noindent
Python is a simple, yet powerful programming language that bridges the Python is a simple, yet powerful programming language that bridges the
gap between C and shell programming, and is thus ideally suited for rapid gap between C and shell programming, and is thus ideally suited for
prototyping. ``throw-away programming''
Its syntax is put together from constructs borrowed from a variety of other and rapid prototyping. Its syntax is put
languages; most prominent are influences from ABC, C, Modula-3 and Icon. together from constructs borrowed from a variety of other languages;
most prominent are influences from ABC, C, Modula-3 and Icon.
The Python interpreter is easily extended with new functions and data The Python interpreter is easily extended with new functions and data
types implemented in C. types implemented in C. Python is also suitable as an extension
Python is also suitable as an extension language for highly language for highly customizable C applications such as editors or
customizable C applications such as editors or window managers. window managers.
Python is available for various operating systems, amongst which Python is available for various operating systems, amongst which
several flavors of \UNIX, Amoeba, and the Apple Macintosh O.S. several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S.,
and MS-DOS.
This tutorial introduces the reader informally to the basic concepts and This tutorial introduces the reader informally to the basic concepts
features of the Python language and system. and features of the Python language and system. It helps to have a
It helps to have a Python interpreter handy for hands-on experience, Python interpreter handy for hands-on experience, but as the examples
but as the examples are self-contained, the tutorial can be read are self-contained, the tutorial can be read off-line as well.
off-line as well.
For a description of standard objects and modules, see the Library For a description of standard objects and modules, see the {\em
Reference document. Library Reference} document. The {\em Language Reference} document
The Language Reference document (XXX not yet existing) (when it is ever written)
gives a more formal definition of the language. will give a more formal definition of the language.
\end{abstract} \end{abstract}
...@@ -59,156 +58,148 @@ gives a more formal definition of the language. ...@@ -59,156 +58,148 @@ gives a more formal definition of the language.
\pagenumbering{arabic} \pagenumbering{arabic}
\section{Whetting Your Appetite} \chapter{Whetting Your Appetite}
If you ever wrote a large shell script, you probably know this feeling: If you ever wrote a large shell script, you probably know this
you'd love to add yet another feature, but it's already so slow, and so feeling: you'd love to add yet another feature, but it's already so
big, and so complicated; or the feature involves a system call or other slow, and so big, and so complicated; or the feature involves a system
funcion that is only accessible from C \ldots call or other funcion that is only accessible from C \ldots Usually
Usually the problem at hand isn't serious enough to warrant rewriting the problem at hand isn't serious enough to warrant rewriting the
the script in C; perhaps because the problem requires variable-length script in C; perhaps because the problem requires variable-length
strings or other data types (like sorted lists of file names) that strings or other data types (like sorted lists of file names) that are
are easy in the shell but lots of work to implement in C; or perhaps easy in the shell but lots of work to implement in C; or perhaps just
just because you're not sufficiently familiar with C. because you're not sufficiently familiar with C.
In such cases, Python may be just the language for you. In such cases, Python may be just the language for you. Python is
Python is simple to use, but it is a real programming language, offering simple to use, but it is a real programming language, offering much
much more structure and support for large programs than the shell has. more structure and support for large programs than the shell has. On
On the other hand, it also offers much more error checking than C, and, the other hand, it also offers much more error checking than C, and,
being a being a {\em very-high-level language}, it has high-level data types
{\em very-high-level language}, built in, such as flexible arrays and dictionaries that would cost you
it has high-level data types built in, such as flexible arrays and days to implement efficiently in C. Because of its more general data
dictionaries that would cost you days to implement efficiently in C. types Python is applicable to a much larger problem domain than {\em
Because of its more general data types Python is applicable to a Awk} or even {\em Perl}, yet most simple things are at least as easy
much larger problem domain than in Python as in those languages.
{\em Awk}
or even Python allows you to split up your program in modules that can be
{\em Perl}, reused in other Python programs. It comes with a large collection of
yet most simple things are at least as easy in Python as in those standard modules that you can use as the basis of your programs ---
languages. or as examples to start learning to program in Python. There are also
built-in modules that provide things like file I/O, system calls, and
Python allows you to split up your program in modules that can be reused even a generic interface to window systems (STDWIN).
in other Python programs.
It comes with a large collection of standard modules that you can use as
the basis for your programs --- or as examples to start learning to
program in Python.
There are also built-in modules that provide things like file I/O,
system calls, and even a generic interface to window systems (STDWIN).
Python is an interpreted language, which saves you considerable time Python is an interpreted language, which saves you considerable time
during program development because no compilation and linking is during program development because no compilation and linking is
necessary. necessary. The interpreter can be used interactively, which makes it
The interpreter can be used interactively, which makes it easy to easy to experiment with features of the language, to write throw-away
experiment with features of the language, to write throw-away programs, programs, or to test functions during bottom-up program development.
or to test functions during bottom-up program development.
It is also a handy desk calculator. It is also a handy desk calculator.
Python allows writing very compact and readable programs. Python allows writing very compact and readable programs. Programs
Programs written in Python are typically much shorter than equivalent C written in Python are typically much shorter than equivalent C
programs: programs, for several reasons:
No declarations are necessary (all type checking is \begin{itemize}
dynamic); statement grouping is done by indentation instead of begin/end \item
brackets; and the high-level data types allow you to express complex the high-level data types allow you to express complex operations in a
operations in a single statement. single statement;
\item
Python is statement grouping is done by indentation instead of begin/end
{\em extensible}: brackets;
if you know how to program in C it is easy to add a new built-in module \item
to the interpreter, either to perform critical operations at maximum no variable or argument declarations are necessary.
speed, or to link Python programs to libraries that may be only available \end{itemize}
in binary form (such as a vendor-specific graphics library).
Once you are really hooked, you can link the Python interpreter into an Python is {\em extensible}: if you know how to program in C it is easy
application written in C and use it as an extension or command language. to add a new built-in
function or
\subsection{Where From Here} module to the interpreter, either to
perform critical operations at maximum speed, or to link Python
Now that you are all excited about Python, you'll want to examine it in programs to libraries that may only be available in binary form (such
some more detail. as a vendor-specific graphics library). Once you are really hooked,
Since the best introduction to a language is using it, you are invited you can link the Python interpreter into an application written in C
here to do so. and use it as an extension or command language.
In the next section, the mechanics of using the interpreter are \section{Where From Here}
explained.
This is rather mundane information, but essential for trying out the Now that you are all excited about Python, you'll want to examine it
examples shown later. in some more detail. Since the best introduction to a language is
using it, you are invited here to do so.
In the next chapter, the mechanics of using the interpreter are
explained. This is rather mundane information, but essential for
trying out the examples shown later.
The rest of the tutorial introduces various features of the Python The rest of the tutorial introduces various features of the Python
language and system though examples, beginning with simple expressions, language and system though examples, beginning with simple
statements and data types, through functions and modules, and finally expressions, statements and data types, through functions and modules,
touching upon advanced concepts like exceptions and classes. and finally touching upon advanced concepts like exceptions.
When you're through with the turtorial (or just getting bored), you
should read the Library Reference, which gives complete (though terse)
reference material about built-in and standard types, functions and
modules that can save you a lot of time when writing Python programs.
\section{Using the Python Interpreter} \chapter{Using the Python Interpreter}
The Python interpreter is usually installed as The Python interpreter is usually installed as {\tt /usr/local/python}
{\tt /usr/local/python} on those machines where it is available; putting {\tt /usr/local} in
on those machines where it is available; putting your {\UNIX} shell's search path makes it possible to start it by
{\tt /usr/local}
in your {\UNIX} shell's search path makes it possible to start it by
typing the command typing the command
\bcode\begin{verbatim} \bcode\begin{verbatim}
python python
\end{verbatim}\ecode \end{verbatim}\ecode
to the shell. to the shell. Since the choice of the directory where the interpreter
Since the choice of the directory where the interpreter lives is an lives is an installation option, other places are possible; check with
installation option, other places instead of your local Python guru or system administrator.
{\tt /usr/local}
are possible; check with your local Python guru or system The interpreter operates somewhat like the {\UNIX} shell: when called
administrator. with standard input connected to a tty device, it reads and executes
commands interactively; when called with a file name argument or with
The interpreter operates somewhat like the {\UNIX} shell: when called with a file as standard input, it reads and executes a {\em script} from
standard input connected to a tty device, it reads and executes commands that file.
interactively; when called with a file name argument or with a file as
standard input, it reads and executes a Note that there is a difference between ``{\tt python file}'' and
{\em script} ``{\tt python $<$file}''. In the latter case, input requests from the
from that file.% program, such as calls to {\tt input()} and {\tt raw\_input()}, are
\footnote{ satisfied from {\em file}. Since this file has already been read
There is a difference between ``{\tt python file}'' and until the end by the parser before the program starts executing, the
``{\tt python $<$file}''. In the latter case {\tt input()} and program will encounter EOF immediately. In the former case (which is
{\tt raw\_input()} are satisfied from {\em file}, which has usually what you want) they are satisfied from whatever file or device
already been read until the end by the parser, so they will read is connected to standard input of the Python interpreter.
EOF immediately. In the former case (which is usually what
you want) they are satisfied from whatever file or device is
connected to standard input of the Python interpreter.
}
A third possibility is ``{\tt python -c command [arg] ...}'', which A third possibility is ``{\tt python -c command [arg] ...}'', which
executes the statement(s) in {\tt command}, in analogy of the shell's executes the statement(s) in {\tt command}, analogous to the shell's
{\tt -c} option. {\tt -c} option. Usually {\tt command} will contain spaces or other
When available, the script name and additional arguments thereafter are characters that are special to the shell, so it is best to quote it.
passed to the script in the variable
{\tt sys.argv}, When available, the script name and additional arguments thereafter
which is a list of strings. are passed to the script in the variable {\tt sys.argv}, which is a
list of strings.
When {\tt -c command} is used, {\tt sys.argv} is set to {\tt '-c'}. When {\tt -c command} is used, {\tt sys.argv} is set to {\tt '-c'}.
When commands are read from a tty, the interpreter is said to be in When commands are read from a tty, the interpreter is said to be in
{\em interactive\ mode}. {\em interactive\ mode}. In this mode it prompts for the next command
In this mode it prompts for the next command with the with the {\em primary\ prompt}, usually three greater-than signs ({\tt
{\em primary\ prompt}, >>>}); for continuation lines it prompts with the {\em secondary\
usually three greater-than signs ({\tt >>>}); for continuation lines prompt}, by default three dots ({\tt ...}). Typing an EOF (Control-D)
it prompts with the at the primary prompt causes the interpreter to exit with a zero exit
{\em secondary\ prompt}, status.
by default three dots ({\tt ...}).
Typing an EOF (Control-D) at the primary prompt causes the interpreter
to exit with a zero exit status.
When an error occurs in interactive mode, the interpreter prints a When an error occurs in interactive mode, the interpreter prints a
message and a stack trace and returns to the primary prompt; with input message and a stack trace and returns to the primary prompt; with
from a file, it exits with a nonzero exit status. input from a file, it exits with a nonzero exit status after printing
(Exceptions handled by an the stack trace. (Exceptions handled by an {\tt except} clause in a
{\tt except} {\tt try} statement are not errors in this context.) Some errors are
clause in a unconditionally fatal and cause an exit with a nonzero exit; this
{\tt try} applies to internal inconsistencies and some cases of running out of
statement are not errors in this context.) memory. All error messages are written to the standard error stream;
Some errors are unconditionally fatal and cause an exit with a nonzero normal output from the executed commands is written to standard
exit; this applies to internal inconsistencies and some cases of running output.
out of memory.
All error messages are written to the standard error stream; normal
output from the executed commands is written to standard output.
Typing an interrupt (normally Control-C or DEL) to the primary or Typing an interrupt (normally Control-C or DEL) to the primary or
secondary prompt cancels the input and returns to the primary prompt. secondary prompt cancels the input and returns to the primary prompt.
Typing an interrupt while a command is being executed raises the Typing an interrupt while a command is being executed raises the {\tt
{\tt KeyboardInterrupt} KeyboardInterrupt} exception, which may be handled by a {\tt try}
exception, which may be handled by a
{\tt try}
statement. statement.
When a module named When a module named
...@@ -223,79 +214,67 @@ i.e., a list of colon-separated directory names. ...@@ -223,79 +214,67 @@ i.e., a list of colon-separated directory names.
When When
{\tt PYTHONPATH} {\tt PYTHONPATH}
is not set, an installation-dependent default path is used, usually is not set, an installation-dependent default path is used, usually
{\tt .:/usr/local/lib/python}.% {\tt .:/usr/local/lib/python}.
\footnote{ (Modules are really searched in the list of directories given by the
Modules are really searched in the list of directories given by variable {\tt sys.path} which is initialized from {\tt PYTHONPATH} or
the variable {\tt sys.path} which is initialized from from the installation-dependent default. See the section on Standard
{\tt PYTHONPATH} or from the installation-dependent default. Modules later.)
See the section on Standard Modules later.
}
As an important speed-up of the start-up time of short programs, if a As an important speed-up of the start-up time for short programs, if a
file called {\tt foo.pyc} exists in the directory where {\tt foo.py} file called {\tt foo.pyc} exists in the directory where {\tt foo.py}
is found, this is assumed to contain an already-``compiled'' version is found, this is assumed to contain an already-``compiled'' version
of the module {\tt foo}. The last modification time of {\tt foo.py} of the module {\tt foo}. The last modification time of {\tt foo.py}
is recorded in {\tt foo.pyc}, and if these don't match, {\tt foo.pyc} is recorded in {\tt foo.pyc}, and the file is ignored if these don't
is ignored. Whenever {\tt foo.py} is successfully compiled, an match. Whenever {\tt foo.py} is successfully compiled, an attempt is
attempt is made to write the compiled version to {\tt foo.pyc}. made to write the compiled version to {\tt foo.pyc}.
On BSD'ish {\UNIX} systems, Python scripts can be made directly executable, On BSD'ish {\UNIX} systems, Python scripts can be made directly
like shell scripts, by putting the line executable, like shell scripts, by putting the line
\bcode\begin{verbatim} \bcode\begin{verbatim}
#! /usr/local/python #! /usr/local/python
\end{verbatim}\ecode \end{verbatim}\ecode
(assuming that's the name of the interpreter) at the beginning of the (assuming that's the name of the interpreter) at the beginning of the
script and giving the file an executable mode. script and giving the file an executable mode. (The {\tt \#!} must be
(The the first two characters of the file.)
{\tt \#!}
must be the first two characters of the file.)
\subsection{Interactive Input Editing and History Substitution} \section{Interactive Input Editing and History Substitution}
Some versions of the Python interpreter support editing of the current Some versions of the Python interpreter support editing of the current
input line and history substitution, similar to facilities found in the input line and history substitution, similar to facilities found in
Korn shell and the GNU Bash shell. the Korn shell and the GNU Bash shell. This is implemented using the
This is implemented using the {\em GNU\ Readline} library, which supports Emacs-style and vi-style
{\em GNU\ Readline} editing. This library has its own documentation which I won't
library, which supports Emacs-style and vi-style editing. duplicate here; however, the basics are easily explained.
This library has its own documentation which I won't duplicate here;
however, the basics are easily explained. Perhaps the quickest check to see whether command line editing is
supported is typing Control-P to the first Python prompt you get. If
If supported,% it beeps, you have command line editing. If nothing appears to
\footnote{ happen, or if \verb/^P/ is echoed, you can skip the rest of this
Perhaps the quickest check to see whether command line editing section.
is supported is typing Control-P to the first Python prompt
you get. If it beeps, you have command line editing. If supported, input line editing is active whenever the interpreter
If not, you can skip the rest of this section. prints a primary or secondary prompt. The current line can be edited
} using the conventional Emacs control characters. The most important
input line editing is active whenever the interpreter prints a primary of these are: C-A (Control-A) moves the cursor to the beginning of the
or secondary prompt. line, C-E to the end, C-B moves it one position to the left, C-F to
The current line can be edited using the conventional Emacs control the right. Backspace erases the character to the left of the cursor,
characters. C-D the character to its right. C-K kills (erases) the rest of the
The most important of these are: line to the right of the cursor, C-Y yanks back the last killed
C-A (Control-A) moves the cursor to the beginning of the line, C-E to string. C-underscore undoes the last change you made; it can be
the end, C-B moves it one position to the left, C-F to the right. repeated for cumulative effect.
Backspace erases the character to the left of the cursor, C-D the
character to its right. History substitution works as follows. All non-empty input lines
C-K kills (erases) the rest of the line to the right of the cursor, C-Y issued are saved in a history buffer, and when a new prompt is given
yanks back the last killed string. you are positioned on a new line at the bottom of this buffer. C-P
C-underscore undoes the last change you made; it can be repeated for moves one line up (back) in the history buffer, C-N moves one down.
cumulative effect.
History substitution works as follows.
All non-empty input lines issued are saved in a history buffer,
and when a new prompt is given you are positioned on a new line at the
bottom of this buffer.
C-P moves one line up (back) in the history buffer, C-N moves one down.
Any line in the history buffer can be edited; an asterisk appears in Any line in the history buffer can be edited; an asterisk appears in
front of the prompt to mark a line as modified. front of the prompt to mark a line as modified. Pressing the Return
Pressing the Return key passes the current line to the interpreter. key passes the current line to the interpreter. C-R starts an
C-R starts an incremental reverse search; C-S starts a forward search. incremental reverse search; C-S starts a forward search.
The key bindings and some other parameters of the Readline library can The key bindings and some other parameters of the Readline library can
be customized by placing commands in an initialization file called be customized by placing commands in an initialization file called
{\tt \$HOME/.initrc}. {\tt \$HOME/.inputrc}. Key bindings have the form
Key bindings have the form
\bcode\begin{verbatim} \bcode\begin{verbatim}
key-name: function-name key-name: function-name
\end{verbatim}\ecode \end{verbatim}\ecode
...@@ -314,50 +293,42 @@ Meta-h: backward-kill-word ...@@ -314,50 +293,42 @@ Meta-h: backward-kill-word
Control-u: universal-argument Control-u: universal-argument
\end{verbatim}\ecode \end{verbatim}\ecode
Note that the default binding for TAB in Python is to insert a TAB Note that the default binding for TAB in Python is to insert a TAB
instead of Readline's default filename completion function. instead of Readline's default filename completion function. If you
If you insist, you can override this by putting insist, you can override this by putting
\bcode\begin{verbatim} \bcode\begin{verbatim}
TAB: complete TAB: complete
\end{verbatim}\ecode \end{verbatim}\ecode
in your in your {\tt \$HOME/.inputrc}. (Of course, this makes it hard to type
{\tt \$HOME/.inputrc}. indented continuation lines.)
(Of course, this makes it hard to type indented continuation lines.)
This facility is an enormous step forward compared to previous versions of This facility is an enormous step forward compared to previous
the interpreter; however, some wishes are left: versions of the interpreter; however, some wishes are left: It would
It would be nice if the proper indentation were suggested on be nice if the proper indentation were suggested on continuation lines
continuation lines (the parser knows if an indent token is required (the parser knows if an indent token is required next). The
next). completion mechanism might use the interpreter's symbol table. A
The completion mechanism might use the interpreter's symbol table. function to check (or even suggest) matching parentheses, quotes etc.
A function to check (or even suggest) matching parentheses, quotes would also be useful.
etc. would also be useful.
\section{An Informal Introduction to Python} \chapter{An Informal Introduction to Python}
In the following examples, input and output are distinguished by the In the following examples, input and output are distinguished by the
presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat the presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat the
example, you must type everything after the prompt, when the prompt example, you must type everything after the prompt, when the prompt
appears; everything on lines that do not begin with a prompt is output appears;
from the interpreter. lines that do not begin with a prompt are output from the interpreter.
Note that a secondary prompt on a line by itself in an example means you Note that a secondary prompt on a line by itself in an example means
must type a blank line; this is used to end a multi-line command. you must type a blank line; this is used to end a multi-line command.
\subsection{Using Python as a Calculator} \section{Using Python as a Calculator}
Let's try some simple Python commands. Let's try some simple Python commands. Start the interpreter and wait
Start the interpreter and wait for the primary prompt, for the primary prompt, {\tt >>>}.
{\tt >>>}.
The interpreter acts as a simple calculator: you can type an expression The interpreter acts as a simple calculator: you can type an
at it and it will write the value. expression at it and it will write the value. Expression syntax is
Expression syntax is straightforward: the operators straightforward: the operators {\tt +}, {\tt -}, {\tt *} and {\tt /}
{\tt +},
{\tt -},
{\tt *}
and
{\tt /}
work just as in most other languages (e.g., Pascal or C); parentheses work just as in most other languages (e.g., Pascal or C); parentheses
can be used for grouping. can be used for grouping. For example:
For example:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> # This is a comment >>> # This is a comment
>>> 2+2 >>> 2+2
...@@ -370,8 +341,8 @@ For example: ...@@ -370,8 +341,8 @@ For example:
2 2
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
As in C, the equal sign ({\tt =}) is used to assign a value to a variable. As in C, the equal sign ({\tt =}) is used to assign a value to a
The value of an assignment is not written: variable. The value of an assignment is not written:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> width = 20 >>> width = 20
>>> height = 5*9 >>> height = 5*9
...@@ -379,15 +350,21 @@ The value of an assignment is not written: ...@@ -379,15 +350,21 @@ The value of an assignment is not written:
900 900
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
There is some support for floating point, but you can't mix floating A value can be assigned to several variables simultaneously:
point and integral numbers in expression (yet): \bcode\begin{verbatim}
>>> # Zero x, y and z
>>> x = y = z = 0
>>>
\end{verbatim}\ecode
There is full support for floating point; operators with mixed type
operands convert the integer operand to floating point:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> 10.0 / 3.3 >>> 4 * 2.5 / 3.3
3.0303030303 3.0303030303
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Besides numbers, Python can also manipulate strings, enclosed in single Besides numbers, Python can also manipulate strings, enclosed in
quotes: single quotes:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> 'foo bar' >>> 'foo bar'
'foo bar' 'foo bar'
...@@ -395,12 +372,14 @@ quotes: ...@@ -395,12 +372,14 @@ quotes:
'doesn\'t' 'doesn\'t'
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Strings are written inside quotes and with quotes and other funny Strings are written
characters escaped by backslashes, to show the precise value. the same way as they are typed for input:
(There is also a way to write strings without quotes and escapes.) inside quotes and with quotes and other funny characters escaped by
Strings can be concatenated (glued together) with the backslashes, to show the precise value. (There is also a way to write
{\tt +} strings without quotes and escapes.)
operator, and repeated with~{\tt *}:
Strings can be concatenated (glued together) with the {\tt +}
operator, and repeated with {\tt *}:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word = 'Help' + 'A' >>> word = 'Help' + 'A'
>>> word >>> word
...@@ -409,13 +388,12 @@ operator, and repeated with~{\tt *}: ...@@ -409,13 +388,12 @@ operator, and repeated with~{\tt *}:
'<HelpAHelpAHelpAHelpAHelpA>' '<HelpAHelpAHelpAHelpAHelpA>'
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Strings can be subscripted; as in C, the first character of a string has Strings can be subscripted; as in C, the first character of a string
subscript 0. has subscript 0.
There is no separate character type; a character is simply a string of There is no separate character type; a character is simply a string of
size one. size one. As in Icon, substrings can be specified with the {\em
As in Icon, substrings can be specified with the slice} notation: two subscripts (indices) separated by a colon.
{\em slice}
notation: two subscripts (indices) separated by a colon.
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word[4] >>> word[4]
'A' 'A'
...@@ -434,8 +412,8 @@ notation: two subscripts (indices) separated by a colon. ...@@ -434,8 +412,8 @@ notation: two subscripts (indices) separated by a colon.
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Degenerate cases are handled gracefully: an index that is too large is Degenerate cases are handled gracefully: an index that is too large is
replaced by the string size, an upper bound smaller than the lower bound replaced by the string size, an upper bound smaller than the lower
returns an empty string. bound returns an empty string.
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word[1:100] >>> word[1:100]
'elpA' 'elpA'
...@@ -446,8 +424,7 @@ returns an empty string. ...@@ -446,8 +424,7 @@ returns an empty string.
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Slice indices (but not simple subscripts) may be negative numbers, to Slice indices (but not simple subscripts) may be negative numbers, to
start counting from the right. start counting from the right. For example:
For example:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word[-2:] # Take last two characters >>> word[-2:] # Take last two characters
'pA' 'pA'
...@@ -459,14 +436,9 @@ For example: ...@@ -459,14 +436,9 @@ For example:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
The best way to remember how slices work is to think of the indices as The best way to remember how slices work is to think of the indices as
pointing pointing {\em between} characters, with the left edge of the first
{\em between} character numbered 0. Then the right edge of the last character of a
characters, with the left edge of the first character numbered 0. string of {\tt n} characters has index {\tt n}, for example:
Then the right edge of the last character of a string of
{\tt n}
characters has index
{\tt n},
for example:
\bcode\begin{verbatim} \bcode\begin{verbatim}
+---+---+---+---+---+ +---+---+---+---+---+
| H | e | l | p | A | | H | e | l | p | A |
...@@ -474,29 +446,22 @@ for example: ...@@ -474,29 +446,22 @@ for example:
0 1 2 3 4 5 0 1 2 3 4 5
-5 -4 -3 -2 -1 -5 -4 -3 -2 -1
\end{verbatim}\ecode \end{verbatim}\ecode
The first row of numbers gives the position of the indices 0...5 in the The first row of numbers gives the position of the indices 0...5 in
string; the second row gives the corresponding negative indices. the string; the second row gives the corresponding negative indices.
For nonnegative indices, the length of a slice is the difference of the For nonnegative indices, the length of a slice is the difference of
indices, if both are within bounds, the indices, if both are within bounds, e.g., the length of {\tt
e.g., word[1:3]} is 3--1 = 2.
the length of
{\tt word[1:3]}
is 3--1 = 2.
Finally, the built-in function {\tt len()} computes the length of a The built-in function {\tt len()} computes the length of a string:
string:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> s = 'supercalifragilisticexpialidocious' >>> s = 'supercalifragilisticexpialidocious'
>>> len(s) >>> len(s)
34 34
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Python knows a number of Python knows a number of {\em compound} data types, used to group
{\em compound} together other values. The most versatile is the {\em list}, which
data types, used to group together other values. can be written as a list of comma-separated values between square
The most versatile is the
{\em list},
which can be written as a list of comma-separated values between square
brackets: brackets:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a = ['foo', 'bar', 100, 1234] >>> a = ['foo', 'bar', 100, 1234]
...@@ -522,9 +487,8 @@ Lists can be sliced, concatenated and so on, like strings: ...@@ -522,9 +487,8 @@ Lists can be sliced, concatenated and so on, like strings:
['foo', 'bar', 100, 'foo', 'bar', 100, 'foo', 'bar', 100, 'Boe!'] ['foo', 'bar', 100, 'foo', 'bar', 100, 'foo', 'bar', 100, 'Boe!']
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Unlike strings, which are Unlike strings, which are {\em immutable}, it is possible to change
{\em immutable}, individual elements of a list:
it is possible to change individual elements of a list:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a >>> a
['foo', 'bar', 100, 1234] ['foo', 'bar', 100, 1234]
...@@ -556,23 +520,32 @@ The built-in function {\tt len()} also applies to lists: ...@@ -556,23 +520,32 @@ The built-in function {\tt len()} also applies to lists:
4 4
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
It is possible to nest lists (create lists containing other lists),
for example:
\bcode\begin{verbatim}
>>> p = [1, [2, 3], 4]
>>> len(p)
3
>>> p[1]
[2, 3]
>>> p[1][0]
2
>>> p[1].append('xtra')
>>> p
[1, [2, 3, 'xtra'], 4]
>>>
\end{verbatim}\ecode
\subsection{Tuples and Sequences} \section{First Steps Towards Programming}
XXX To Be Done.
\subsection{First Steps Towards Programming}
Of course, we can use Python for more complicated tasks than adding two Of course, we can use Python for more complicated tasks than adding
and two together. two and two together. For instance, we can write an initial
For instance, we can write an initial subsequence of the subsequence of the {\em Fibonacci} series as follows:
{\em Fibonacci}
series as follows:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> # Fibonacci series: >>> # Fibonacci series:
>>> # the sum of two elements defines the next >>> # the sum of two elements defines the next
>>> a, b = 0, 1 >>> a, b = 0, 1
>>> while b < 100: >>> while b < 10:
... print b ... print b
... a, b = b, a+b ... a, b = b, a+b
... ...
...@@ -582,68 +555,53 @@ series as follows: ...@@ -582,68 +555,53 @@ series as follows:
3 3
5 5
8 8
13
21
34
55
89
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
This example introduces several new features. This example introduces several new features.
\begin{itemize} \begin{itemize}
\item \item
The first line contains a The first line contains a {\em multiple assignment}: the variables
{\em multiple\ assignment}: {\tt a} and {\tt b} simultaneously get the new values 0 and 1. On the
the variables last line this is used again, demonstrating that the expressions on
{\tt a} the right-hand side are all evaluated first before any of the
and
{\tt b}
simultaneously get the new values 0 and 1.
On the last line this is used again, demonstrating that the expressions
on the right-hand side are all evaluated first before any of the
assignments take place. assignments take place.
\item \item
The The {\tt while} loop executes as long as the condition (here: {\tt b <
{\tt while} 100}) remains true. In Python, as in C, any non-zero integer value is
loop executes as long as the condition (here: $b < 100$) remains true. true; zero is false. The condition may also be a string or list value,
In Python, as in C, any non-zero integer value is true; zero is false. in fact any sequence; anything with a non-zero length is true, empty
The condition may also be a string or list value, in fact any sequence; sequences are false. The test used in the example is a simple
anything with a non-zero length is true, empty sequences are false. comparison. The standard comparison operators are written as {\tt <},
The test used in the example is a simple comparison. {\tt >}, {\tt =}, {\tt <=}, {\tt >=} and {\tt <>}.%
The standard comparison operators are written as
{\tt <},
{\tt >},
{\tt =},
{\tt <=},
{\tt >=}
and
{\tt <>}.%
\footnote{ \footnote{
The ambiguity of using {\tt =} The ambiguity of using {\tt =}
for both assignment and equality is resolved by disallowing for both assignment and equality is resolved by disallowing
unparenthesized conditions at the right hand side of assignments. unparenthesized conditions on the right hand side of assignments.
Parenthesized assignment is also disallowed; instead it is
interpreted as an equality test.
} }
\item \item
The The {\em body} of the loop is {\em indented}: indentation is Python's
{\em body} way of grouping statements. Python does not (yet!) provide an
of the loop is intelligent input line editing facility, so you have to type a tab or
{\em indented}: indentation is Python's way of grouping statements. space(s) for each indented line. In practice you will prepare more
Python does not (yet!) provide an intelligent input line editing complicated input for Python with a text editor; most text editors have
facility, so you have to type a tab or space(s) for each indented line. an auto-indent facility. When a compound statement is entered
In practice you will prepare more complicated input for Python with a interactively, it must be followed by a blank line to indicate
text editor; most text editors have an auto-indent facility. completion (since the parser cannot guess when you have typed the last
When a compound statement is entered interactively, it must be line).
followed by a blank line to indicate completion (since the parser
cannot guess when you have typed the last line).
\item \item
The The {\tt print} statement writes the value of the expression(s) it is
{\tt print} given. It differs from just writing the expression you want to write
statement writes the value of the expression(s) it is given. (as we did earlier in the calculator examples) in the way it handles
It differs from just writing the expression you want to write (as we did multiple expressions and strings. Strings are written without quotes,
earlier in the calculator examples) in the way it handles multiple and a space is inserted between items, so you can format things nicely,
expressions and strings. like this:
Strings are written without quotes and a space is inserted between
items, so you can format things nicely, like this:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> i = 256*256 >>> i = 256*256
>>> print 'The value of i is', i >>> print 'The value of i is', i
...@@ -662,15 +620,16 @@ A trailing comma avoids the newline after the output: ...@@ -662,15 +620,16 @@ A trailing comma avoids the newline after the output:
\end{verbatim}\ecode \end{verbatim}\ecode
Note that the interpreter inserts a newline before it prints the next Note that the interpreter inserts a newline before it prints the next
prompt if the last line was not completed. prompt if the last line was not completed.
\end{itemize} \end{itemize}
\subsection{More Control Flow Tools} \chapter{More Control Flow Tools}
Besides the {\tt while} statement just introduced, Python knows the Besides the {\tt while} statement just introduced, Python knows the
usual control flow statements known from other languages, with some usual control flow statements known from other languages, with some
twists. twists.
\subsubsection{If Statements} \section{If Statements}
Perhaps the most well-known statement type is the {\tt if} statement. Perhaps the most well-known statement type is the {\tt if} statement.
For example: For example:
...@@ -687,21 +646,20 @@ For example: ...@@ -687,21 +646,20 @@ For example:
... ...
\end{verbatim}\ecode \end{verbatim}\ecode
There can be zero or more {\tt elif} parts, and the {\tt else} part is There can be zero or more {\tt elif} parts, and the {\tt else} part is
optional. optional. The keyword `{\tt elif}' is short for `{\tt else if}', and is
The keyword `{\tt elif}' is short for `{\tt else if}', and is useful to useful to avoid excessive indentation. An {\tt if...elif...elif...}
avoid excessive indentation. sequence is a substitute for the {\em switch} or {\em case} statements
An {\tt if...elif...elif...} sequence is a substitute for the found in other languages.
{\em switch} or {\em case} statements found in other languages.
\subsubsection{For Statements} \section{For Statements}
The {\tt for} statement in Python differs a bit from what you may be The {\tt for} statement in Python differs a bit from what you may be
used to in C or Pascal. used to in C or Pascal. Rather than always iterating over an
Rather than always iterating over an arithmetic progression of numbers arithmetic progression of numbers (as in Pascal), or leaving the user
(as Pascal), or leaving the user completely free in the iteration test completely free in the iteration test and step (as C), Python's {\tt
and step (as C), Python's {\tt for} statement iterates over the items for} statement iterates over the items of any sequence (e.g., a list
of any sequence (e.g., a list or a string). or a string), in the order that they appear in the sequence. For
For example (no pun intended): example (no pun intended):
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> # Measure some strings: >>> # Measure some strings:
>>> a = ['cat', 'window', 'defenestrate'] >>> a = ['cat', 'window', 'defenestrate']
...@@ -713,23 +671,34 @@ window 6 ...@@ -713,23 +671,34 @@ window 6
defenestrate 12 defenestrate 12
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
It is not safe to modify the sequence being iterated over in the loop
(this can only happen for mutable sequence types, i.e., lists). If
you need to modify the list you are iterating over, e.g., duplicate
selected items, you must iterate over a copy. The slice notation
makes this particularly convenient:
\bcode\begin{verbatim}
>>> for x in a[:]: # make a slice copy of the entire list
... if len(x) > 6: a.insert(0, x)
...
>>> a
['defenestrate', 'cat', 'window', 'defenestrate']
>>>
\end{verbatim}\ecode
\subsubsection{The {\tt range()} Function} \section{The {\tt range()} Function}
If you do need to iterate over a sequence of numbers, the built-in If you do need to iterate over a sequence of numbers, the built-in
function {\tt range()} comes in handy. function {\tt range()} comes in handy. It generates lists containing
It generates lists containing arithmetic progressions, arithmetic progressions, e.g.:
e.g.:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> range(10) >>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
The given end point is never part of the generated list; The given end point is never part of the generated list; {\tt range(10)}
{\tt range(10)} generates a list of 10 values, generates a list of 10 values, exactly the legal indices for items of a
exactly the legal indices for items of a sequence of length 10. sequence of length 10. It is possible to let the range start at another
It is possible to let the range start at another number, or to specify a number, or to specify a different increment (even negative):
different increment (even negative):
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> range(5, 10) >>> range(5, 10)
[5, 6, 7, 8, 9] [5, 6, 7, 8, 9]
...@@ -739,10 +708,10 @@ different increment (even negative): ...@@ -739,10 +708,10 @@ different increment (even negative):
[-10, -40, -70] [-10, -40, -70]
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
To iterate over the indices of a sequence, combine {\tt range()} To iterate over the indices of a sequence, combine {\tt range()} and
and {\tt len()} as follows: {\tt len()} as follows:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a = ['Mary', 'had', 'a', 'little', 'boy'] >>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)): >>> for i in range(len(a)):
... print i, a[i] ... print i, a[i]
... ...
...@@ -750,20 +719,23 @@ and {\tt len()} as follows: ...@@ -750,20 +719,23 @@ and {\tt len()} as follows:
1 had 1 had
2 a 2 a
3 little 3 little
4 boy 4 lamb
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Break Statements and Else Clauses on Loops} \section{Break and Continue Statements, and Else Clauses on Loops}
The {\tt break} statement, like in C, breaks out of the smallest
enclosing {\tt for} or {\tt while} loop.
The {\tt continue} statement, also borrowed from C, continues with the
next iteration of the loop.
The {\tt break} statement breaks out of the smallest enclosing {\tt for}
or {\tt while} loop.
Loop statements may have an {\tt else} clause; it is executed when the Loop statements may have an {\tt else} clause; it is executed when the
loop terminates through exhaustion of the list (with {\tt for}) or when loop terminates through exhaustion of the list (with {\tt for}) or when
the condition becomes false (with {\tt while}) but not when the loop is the condition becomes false (with {\tt while}), but not when the loop is
terminated by a {\tt break} statement. terminated by a {\tt break} statement. This is exemplified by the
This is exemplified by the following loop, which searches for a list following loop, which searches for a list item of value 0:
item of value 0:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> for n in range(2, 10): >>> for n in range(2, 10):
... for x in range(2, n): ... for x in range(2, n):
...@@ -784,7 +756,7 @@ item of value 0: ...@@ -784,7 +756,7 @@ item of value 0:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Pass Statements} \section{Pass Statements}
The {\tt pass} statement does nothing. The {\tt pass} statement does nothing.
It can be used when a statement is required syntactically but the It can be used when a statement is required syntactically but the
...@@ -796,11 +768,7 @@ For example: ...@@ -796,11 +768,7 @@ For example:
... ...
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Conditions Revisited} \section{Defining Functions}
XXX To Be Done.
\subsection{Defining Functions}
We can create a function that writes the Fibonacci series to an We can create a function that writes the Fibonacci series to an
arbitrary boundary: arbitrary boundary:
...@@ -816,29 +784,24 @@ arbitrary boundary: ...@@ -816,29 +784,24 @@ arbitrary boundary:
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
The keyword The keyword {\tt def} introduces a function {\em definition}. It must
{\tt def} be followed by the function name and the parenthesized list of formal
introduces a function parameters. The statements that form the body of the function starts at
{\em definition}. the next line, indented by a tab stop.
It must be followed by the function name and the parenthesized list of
formal parameters. The {\em execution} of a function introduces a new symbol table used
The statements that form the body of the function starts at the next for the local variables of the function. More precisely, all variable
line, indented by a tab stop. assignments in a function store the value in the local symbol table;
The whereas
{\em execution} variable references first look in the local symbol table, then
of a function introduces a new symbol table used for the local variables in the global symbol table, and then in the table of built-in names.
of the function. Thus,
More precisely, all variable assignments in a function store the value global variables cannot be directly assigned to from within a
in the local symbol table; variable references first look in the local function, although they may be referenced.
symbol table, then in the global symbol table, and then in the table of
built-in names.
Thus, the global symbol table is
{\em read-only}
within a function.
The actual parameters (arguments) to a function call are introduced in The actual parameters (arguments) to a function call are introduced in
the local symbol table of the called function when it is called; the local symbol table of the called function when it is called; thus,
thus, arguments are passed using arguments are passed using {\em call\ by\ value}.%
{\em call\ by\ value}.%
\footnote{ \footnote{
Actually, {\em call by object reference} would be a better Actually, {\em call by object reference} would be a better
description, since if a mutable object is passed, the caller description, since if a mutable object is passed, the caller
...@@ -848,13 +811,14 @@ thus, arguments are passed using ...@@ -848,13 +811,14 @@ thus, arguments are passed using
When a function calls another function, a new local symbol table is When a function calls another function, a new local symbol table is
created for that call. created for that call.
A function definition introduces the function name in the global symbol A function definition introduces the function name in the
table. current
The value has a type that is recognized by the interpreter as a symbol table. The value
user-defined function. of the function name
This value can be assigned to another name which can then also be used has a type that is recognized by the interpreter as a user-defined
as a function. function. This value can be assigned to another name which can then
This serves as a general renaming mechanism: also be used as a function. This serves as a general renaming
mechanism:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> fib >>> fib
<function object at 10042ed0> <function object at 10042ed0>
...@@ -863,17 +827,13 @@ This serves as a general renaming mechanism: ...@@ -863,17 +827,13 @@ This serves as a general renaming mechanism:
1 1 2 3 5 8 13 21 34 55 89 1 1 2 3 5 8 13 21 34 55 89
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
You might object that You might object that {\tt fib} is not a function but a procedure. In
{\tt fib} Python, as in C, procedures are just functions that don't return a
is not a function but a procedure. value. In fact, technically speaking, procedures do return a value,
In Python, as in C, procedures are just functions that don't return a albeit a rather boring one. This value is called {\tt None} (it's a
value. built-in name). Writing the value {\tt None} is normally suppressed by
In fact, technically speaking, procedures do return a value, albeit a the interpreter if it would be the only value written. You can see it
rather boring one. if you really want to:
This value is called {\tt None} (it's a built-in name).
Writing the value {\tt None} is normally suppressed by the interpreter
if it would be the only value written.
You can see it if you really want to:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> print fib(0) >>> print fib(0)
None None
...@@ -896,103 +856,339 @@ the Fibonacci series, instead of printing it: ...@@ -896,103 +856,339 @@ the Fibonacci series, instead of printing it:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
This example, as usual, demonstrates some new Python features: This example, as usual, demonstrates some new Python features:
\begin{itemize} \begin{itemize}
\item \item
The The {\tt return} statement returns with a value from a function. {\tt
{\tt return} return} without an expression argument is used to return from the middle
statement returns with a value from a function. of a procedure (falling off the end also returns from a proceduce), in
{\tt return} which case the {\tt None} value is returned.
without an expression argument is used to return from the middle of a
procedure (falling off the end also returns from a proceduce).
\item \item
The statement The statement {\tt result.append(b)} calls a {\em method} of the list
{\tt ret.append(b)} object {\tt result}. A method is a function that `belongs' to an
calls a object and is named {\tt obj.methodname}, where {\tt obj} is some
{\em method} object (this may be an expression), and {\tt methodname} is the name
of the list object of a method that is defined by the object's type. Different types
{\tt ret}. define different methods. Methods of different types may have the
A method is a function that `belongs' to an object and is named same name without causing ambiguity. (It is possible to define your
{\tt obj.methodname}, own object types and methods, using {\em classes}. This is an
where advanced feature that is not discussed in this tutorial.)
{\tt obj} The method {\tt append} shown in the example, is defined for
is some object (this may be an expression), and list objects; it adds a new element at the end of the list. In this
{\tt methodname} example
is the name of a method that is defined by the object's type. it is equivalent to {\tt result = result + [b]}, but more efficient.
Different types define different methods.
Methods of different types may have the same name without causing
ambiguity.
See the section on classes, later, to find out how you can define your
own object types and methods.
The method
{\tt append}
shown in the example, is defined for list objects; it adds a new element
at the end of the list.
In this case it is equivalent to
{\tt ret = ret + [b]},
but more efficient.%
\footnote{
There is a subtle semantic difference if the object
is referenced from more than one place.
}
\end{itemize} \end{itemize}
The list object type has two more methods:
\chapter{Odds and Ends}
This chapter describes some things you've learned about already in
more detail, and adds some new things as well.
\section{More on Lists}
The list data type has some more methods. Here are all of the methods
of lists objects:
\begin{description} \begin{description}
\item[{\tt insert(i, x)}] \item[{\tt insert(i, x)}]
Inserts an item at a given position. Insert an item at a given position. The first argument is the index of
The first argument is the index of the element before which to insert, the element before which to insert, so {\tt a.insert(0, x)} inserts at
so {\tt a.insert(0, x)} inserts at the front of the list, and the front of the list, and {\tt a.insert(len(a), x)} is equivalent to
{\tt a.insert(len(a), x)} is equivalent to {\tt a.append(x)}. {\tt a.append(x)}.
\item[{\tt append(x)}]
Equivalent to {\tt a.insert(len(a), x)}.
\item[{\tt index(x)}]
Return the index in the list of the first item whose value is {\tt x}.
It is an error if there is no such item.
\item[{\tt remove(x)}]
Remove the first item from the list whose value is {\tt x}.
It is an error if there is no such item.
\item[{\tt sort()}] \item[{\tt sort()}]
Sorts the elements of the list. Sort the items of the list, in place.
\item[{\tt reverse()}]
Reverse the elements of the list, in place.
\end{description} \end{description}
For example:
An example that uses all list methods:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a = [10, 100, 1, 1000] >>> a = [66.6, 333, 333, 1, 1234.5]
>>> a.insert(2, -1) >>> a.insert(2, -1)
>>> a.append(333)
>>> a >>> a
[10, 100, -1, 1, 1000] [66.6, 333, -1, 333, 1, 1234.5, 333]
>>> a.index(333)
1
>>> a.remove(333)
>>> a
[66.6, -1, 333, 1, 1234.5, 333]
>>> a.reverse()
>>> a
[333, 1234.5, 1, 333, -1, 66.6]
>>> a.sort() >>> a.sort()
>>> a >>> a
[-1, 1, 10, 100, 1000] [-1, 1, 66.6, 333, 333, 1234.5]
>>> # Strings are sorted according to ASCII:
>>> b = ['Mary', 'had', 'a', 'little', 'boy']
>>> b.sort()
>>> b
['Mary', 'a', 'boy', 'had', 'little']
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsection{Modules} \section{The {\tt del} statement}
There is a way to remove an item from a list given its index instead
of its value: the {\tt del} statement. This can also be used to
remove slices from a list (which we did earlier by assignment of an
empty list to the slice). For example:
\bcode\begin{verbatim}
>>> a
[-1, 1, 66.6, 333, 333, 1234.5]
>>> del a[0]
>>> a
[1, 66.6, 333, 333, 1234.5]
>>> del a[2:4]
>>> a
[1, 66.6, 1234.5]
>>>
\end{verbatim}\ecode
{\tt del} can also be used to delete entire variables:
\bcode\begin{verbatim}
>>> del a
>>>
\end{verbatim}\ecode
Referencing the name {\tt a} hereafter is an error (at least until
another value is assigned to it). We'll find other uses for {\tt del}
later.
\section{Tuples and Sequences}
We saw that lists and strings have many common properties, e.g.,
subscripting and slicing operations. They are two examples of {\em
sequence} data types. As Python is an evolving language, other
sequence data types may be added. There is also another standard
sequence data type: the {\em tuple}.
A tuple consists of a number of values separated by commas, for
instance:
\bcode\begin{verbatim}
>>> t = 12345, 54321, 'hello!'
>>> t[0]
12345
>>> t
(12345, 54321, 'hello!')
>>> # Tuples may be nested:
>>> u = t, (1, 2, 3, 4, 5)
>>> u
((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
>>>
\end{verbatim}\ecode
As you see, on output tuples are alway enclosed in parentheses, so
that nested tuples are interpreted correctly; they may be input with
or without surrounding parentheses, although often parentheses are
necessary anyway (if the tuple is part of a larger expression).
Tuples have many uses, e.g., (x, y) coordinate pairs, employee records
from a database, etc. Tuples, like strings, are immutable: it is not
possible to assign to the individual items of a tuple (you can
simulate much of the same effect with slicing and concatenation,
though).
A special problem is the construction of tuples containing 0 or 1
items: the syntax has some extra quirks to accomodate these. Empty
tuples are constructed by an empty pair of parentheses; a tuple with
one item is constructed by following a value with a comma
(it is not sufficient to enclose a single value in parentheses).
Ugly, but effective. For example:
\bcode\begin{verbatim}
>>> empty = ()
>>> singleton = 'hello', # <-- note trailing comma
>>> len(empty)
0
>>> len(singleton)
1
>>> singleton
('hello',)
>>>
\end{verbatim}\ecode
The statement {\tt t = 12345, 54321, 'hello!'} is an example of {\em
tuple packing}: the values {\tt 12345}, {\tt 54321} and {\tt 'hello!'}
are packed together in a tuple. The reverse operation is also
possible, e.g.:
\bcode\begin{verbatim}
>>> x, y, z = t
>>>
\end{verbatim}\ecode
This is called, appropriately enough, {\em tuple unpacking}. Tuple
unpacking requires that the list of variables on the left has the same
number of elements as the length of the tuple. Note that multiple
assignment is really just a combination of tuple packing and tuple
unpacking!
Occasionally, the corresponding operation on lists is useful: {\em list
unpacking}. This is supported by enclosing the list of variables in
square brackets:
\bcode\begin{verbatim}
>>> a = ['foo', 'bar', 100, 1234]
>>> [a1, a2, a3, a4] = a
>>>
\end{verbatim}\ecode
\section{Dictionaries}
Another useful data type built into Python is the {\em dictionary}.
Dictionaries are sometimes found in other languages as ``associative
memories'' or ``associative arrays''. Unlike sequences, which are
indexed by a range of numbers, dictionaries are indexed by {\em keys},
which are strings. It is best to think of a dictionary as an unordered set of
{\em key:value} pairs, with the requirement that the keys are unique
(within one dictionary).
A pair of braces creates an empty dictionary: \verb/{}/.
Placing a comma-separated list of key:value pairs within the
braces adds initial key:value pairs to the dictionary; this is also the
way dictionaries are written on output.
The main operations on a dictionary are storing a value with some key
and extracting the value given the key. It is also possible to delete
a key:value pair
with {\tt del}.
If you store using a key that is already in use, the old value
associated with that key is forgotten. It is an error to extract a
value using a non-existant key.
The {\tt keys()} method of a dictionary object returns a list of all the
keys used in the dictionary, in random order (if you want it sorted,
just apply the {\tt sort()} method to the list of keys). To check
whether a single key is in the dictionary, use the \verb/has_key()/
method of the dictionary.
Here is a small example using a dictionary:
\bcode\begin{verbatim}
>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139; 'guido': 4127; 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127; 'irv': 4127; 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> tel.has_key('guido')
1
>>>
\end{verbatim}\ecode
\section{More on Conditions}
The conditions used in {\tt while} and {\tt if} statements above can
contain other operators besides comparisons.
The comparison operators {\tt in} and {\tt not in} check whether a value
occurs (does not occur) in a sequence. The operators {\tt is} and {\tt
is not} compare whether two objects are really the same object; this
only matters for mutable objects like lists. All comparison operators
have the same priority, which is lower than that of all numerical
operators.
Comparisons can be chained: e.g., {\tt a < b = c} tests whether {\tt a}
is less than {\tt b} and moreover {\tt b} equals {\tt c}.
Comparisons may be combined by the Boolean operators {\tt and} and {\tt
or}, and the outcome of a comparison (or of any other Boolean
expression) may be negated with {\tt not}. These all have lower
priorities than comparison operators again; between them, {\tt not} has
the highest priority, and {\tt or} the lowest, so that
{\tt A and not B or C} is equivalent to {\tt (A and (not B)) or C}. Of
course, parentheses can be used to express the desired composition.
The Boolean operators {\tt and} and {\tt or} are so-called {\em
shortcut} operators: their arguments are evaluated from left to right,
and evaluation stops as soon as the outcome is determined. E.g., if
{\tt A} and {\tt C} are true but {\tt B} is false, {\tt A and B and C}
does not evaluate the expression C. In general, the return value of a
shortcut operator, when used as a general value and not as a Boolean, is
the last evaluated argument.
It is possible to assign the result of a comparison or other Boolean
expression to a variable, but you must enclose the entire Boolean
expression in parentheses. This is necessary because otherwise an
assignment like \verb/a = b = c/ would be ambiguous: does it assign the
value of {\tt c} to {\tt a} and {\tt b}, or does it compare {\tt b} to
{\tt c} and assign the outcome (0 or 1) to {\tt a}? As it is, the first
meaning is what you get, and to get the latter you have to write
\verb/a = (b = c)/. (In Python, unlike C, assignment cannot occur
inside expressions.)
\section{Comparing Sequences and Other Types}
Sequence objects may be compared to other objects with the same
sequence type. The comparison uses {\em lexicographical} ordering:
first the first two items are compared, and if they differ this
determines the outcome of the comparison; if they are equal, the next
two items are compared, and so on, until either sequence is exhausted.
If two items to be compared are themselves sequences of the same type,
the lexiographical comparison is carried out recursively. If all
items of two sequences compare equal, the sequences are considered
equal. If one sequence is an initial subsequence of the other, the
shorted sequence is the smaller one. Lexicographical ordering for
strings uses the ASCII ordering for individual characters. Some
examples of comparisons between sequences with the same types:
\bcode\begin{verbatim}
(1, 2, 3) < (1, 2, 4)
[1, 2, 3] < [1, 2, 4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1, 2, 3, 4) < (1, 2, 4)
(1, 2) < (1, 2, -1)
(1, 2, 3) = (1.0, 2.0, 3.0)
(1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
\end{verbatim}\ecode
Note that comparing objects of different types is legal. The outcome
is deterministic but arbitrary: the types are ordered by their name.
Thus, a list is always smaller than a string, a string is always
smaller than a tuple, etc. Mixed numeric types are compared according
to their numeric value, so 0 equals 0.0, etc.%
\footnote{
The rules for comparing objects of different types should
not be relied upon; they may change in a future version of
the language.
}
\chapter{Modules}
If you quit from the Python interpreter and enter it again, the If you quit from the Python interpreter and enter it again, the
definitions you have made (functions and variables) are lost. definitions you have made (functions and variables) are lost.
Therefore, if you want to write a somewhat longer program, you are Therefore, if you want to write a somewhat longer program, you are
better off using a text editor to prepare the input for the interpreter better off using a text editor to prepare the input for the interpreter
and run it with that file as input instead. and run it with that file as input instead. This is known as creating a
This is known as creating a {\em script}. As your program gets longer, you may want to split it
{\em script}. into several files for easier maintenance. You may also want to use a
As your program gets longer, you may want to split it into several files handy function that you've written in several programs without copying
for easier maintenance. its definition into each program.
You may also want to use a handy function that you've written in several
programs without copying its definition into each program.
To support this, Python has a way to put definitions in a file and use To support this, Python has a way to put definitions in a file and use
them in a script or in an interactive instance of the interpreter. them in a script or in an interactive instance of the interpreter.
Such a file is called a Such a file is called a {\em module}; definitions from a module can be
{\em module}; {\em imported} into other modules or into the {\em main} module (the
definitions from a module can be collection of variables that you have access to in a script
{\em imported} executed at the top level
into other modules or into the and in calculator mode).
{\em main}
module (the collection of variables that you have access to in A module is a file containing Python definitions and statements. The
a script and in calculator mode). file name is the module name with the suffix {\tt .py} appended. For
instance, use your favorite text editor to create a file called {\tt
A module is a file containing Python definitions and statements. fibo.py} in the current directory with the following contents:
The file name is the module name with the suffix
{\tt .py}
appended.
For instance, use your favorite text editor to create a file called
{\tt fibo.py}
in the current directory with the following contents:
\bcode\begin{verbatim} \bcode\begin{verbatim}
# Fibonacci numbers module # Fibonacci numbers module
...@@ -1003,12 +1199,12 @@ def fib(n): # write Fibonacci series up to n ...@@ -1003,12 +1199,12 @@ def fib(n): # write Fibonacci series up to n
a, b = b, a+b a, b = b, a+b
def fib2(n): # return Fibonacci series up to n def fib2(n): # return Fibonacci series up to n
ret = [] result = []
a, b = 0, 1 a, b = 0, 1
while b <= n: while b <= n:
ret.append(b) result.append(b)
a, b = b, a+b a, b = b, a+b
return ret return result
\end{verbatim}\ecode \end{verbatim}\ecode
Now enter the Python interpreter and import this module with the Now enter the Python interpreter and import this module with the
following command: following command:
...@@ -1018,7 +1214,7 @@ following command: ...@@ -1018,7 +1214,7 @@ following command:
\end{verbatim}\ecode \end{verbatim}\ecode
This does not enter the names of the functions defined in This does not enter the names of the functions defined in
{\tt fibo} {\tt fibo}
directly in the symbol table; it only enters the module name directly in the current symbol table; it only enters the module name
{\tt fibo} {\tt fibo}
there. there.
Using the module name you can access the functions: Using the module name you can access the functions:
...@@ -1037,7 +1233,7 @@ If you intend to use a function often you can assign it to a local name: ...@@ -1037,7 +1233,7 @@ If you intend to use a function often you can assign it to a local name:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{More on Modules} \section{More on Modules}
A module can contain executable statements as well as function A module can contain executable statements as well as function
definitions. definitions.
...@@ -1093,26 +1289,19 @@ There is even a variant to import all names that a module defines: ...@@ -1093,26 +1289,19 @@ There is even a variant to import all names that a module defines:
This imports all names except those beginning with an underscore This imports all names except those beginning with an underscore
({\tt \_}). ({\tt \_}).
\subsubsection{Standard Modules} \section{Standard Modules}
Python comes with a library of standard modules, described in a separate Python comes with a library of standard modules, described in a separate
document (Python Library and Module Reference). document (Python Library Reference). Some modules are built into the
Some modules are built into the interpreter; these provide access to interpreter; these provide access to operations that are not part of the
operations that are not part of the core of the language but are core of the language but are nevertheless built in, either for
nevertheless built in, either for efficiency or to provide access to efficiency or to provide access to operating system primitives such as
operating system primitives such as system calls. system calls. The set of such modules is a configuration option; e.g.,
The set of such modules is a configuration option; e.g., the the {\tt amoeba} module is only provided on systems that somehow support
{\tt amoeba} Amoeba primitives. One particular module deserves some attention: {\tt
module is only provided on systems that somehow support Amoeba sys}, which is built into every Python interpreter. The variables {\tt
primitives. sys.ps1} and {\tt sys.ps2} define the strings used as primary and
One particular module deserves some attention: secondary prompts:
{\tt sys},
which is built into every Python interpreter.
The variables
{\tt sys.ps1}
and
{\tt sys.ps2}
define the strings used as primary and secondary prompts:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> import sys >>> import sys
>>> sys.ps1 >>> sys.ps1
...@@ -1143,16 +1332,140 @@ You can modify it using standard list operations, e.g.: ...@@ -1143,16 +1332,140 @@ You can modify it using standard list operations, e.g.:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsection{Errors and Exceptions} \section{The {\tt dir()} function}
Until now error messages haven't yet been mentioned, but if you have The built-in function {\tt dir} is used to find out which names a module
tried out the examples you have probably seen some. defines. It returns a sorted list of strings:
There are (at least) two distinguishable kinds of errors: \bcode\begin{verbatim}
{\em syntax\ errors} >>> import fibo, sys
and >>> dir(fibo)
{\em exceptions}. ['fib', 'fib2']
>>> dir(sys)
['argv', 'exit', 'modules', 'path', 'ps1', 'ps2', 'stderr', 'stdin', 'stdout']
>>>
\end{verbatim}\ecode
Without arguments, {\tt dir()} lists the names you have defined currently:
\bcode\begin{verbatim}
>>> a = [1, 2, 3, 4, 5]
>>> import fibo, sys
>>> fib = fibo.fib
>>> dir()
['a', 'fib', 'fibo', 'sys']
>>>
\end{verbatim}\ecode
Note that it lists all types of names: variables, modules, functions, etc.
{\tt dir()} does not list the names of built-in functions and variables.
If you want a list of those, they are defined in the standard module
{\tt builtin}:
\bcode\begin{verbatim}
>>> import builtin
>>> dir(builtin)
['EOFError', 'KeyboardInterrupt', 'MemoryError', 'NameError', 'None', 'Runti
meError', 'SystemError', 'TypeError', 'abs', 'chr', 'dir', 'divmod', 'eval',
'exec', 'float', 'input', 'int', 'len', 'long', 'max', 'min', 'open', 'ord'
, 'pow', 'range', 'raw_input', 'reload', 'type']
>>>
\end{verbatim}\ecode
\chapter{Output Formatting}
So far we've encountered two ways of writing values: {\em expression
statements} and the {\tt print} statement. (A third way is using the
{\tt write} method of file objects; the standard output file can be
referenced as {\tt sys.stdout}. See the Library Reference for more
information on this.)
Often you'll want more control over the formatting of your output than
simply printing space-separated values. The key to nice formatting in
Python is to do all the string handling yourself; using string slicing
and concatenation operations you can create any lay-out you can imagine.
The standard module {\tt string} contains some useful operations for
padding strings to a given column width; these will be discussed shortly.
One question remains, of course: how do you convert values to strings?
Luckily, Python has a way to convert any value to a string: just write
the value between reverse quotes (\verb/``/). Some examples:
\bcode\begin{verbatim}
>>> x = 10 * 3.14
>>> y = 200*200
>>> s = 'The value of x is ' + `x` + ', and y is ' + `y` + '...'
>>> print s
The value of x is 31.4, and y is 40000...
>>> # Reverse quotes work on other types besides numbers:
>>> p = [x, y]
>>> ps = `p`
>>> ps
'[31.4, 40000]'
>>> # Converting a string adds string quotes and backslashes:
>>> hello = 'hello, world\n'
>>> hellos = `hello`
>>> print hellos
'hello, world\012'
>>> # The argument of reverse quotes may be a tuple:
>>> `x, y, ('foo', 'bar')`
'(31.4, 40000, (\'foo\', \'bar\'))'
>>>
\end{verbatim}\ecode
Here is how you write a table of squares and cubes:
\bcode\begin{verbatim}
>>> import string
>>> for x in range(1, 11):
... print string.rjust(`x`, 2), string.rjust(`x*x`, 3),
... # Note trailing comma on previous line
... print string.rjust(`x*x*x`, 4)
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
>>>
\end{verbatim}\ecode
(Note that one space between each column was added by the way {\tt print}
works: it always adds spaces between its arguments.)
This example demonstrates the function {\tt string.rjust()}, which
right-justifies a string in a field of a given width by padding it with
spaces on the left. There are similar functions {\tt string.ljust()}
and {\tt string.center()}. These functions do not write anything, they
just return a new string. If the input string is too long, they don't
truncate it, but return it unchanged; this will mess up your column
lay-out but that's usually better than the alternative, which would be
lying about a value. (If you really want truncation you can always add
a slice operation, as in {\tt string.ljust(x,~n)[0:n]}.)
There is another function, {\tt string.zfill}, which pads a numeric
string on the left with zeros. It understands about plus and minus
signs:%
\footnote{
Better facilities for formatting floating point numbers are
lacking at this moment.
}
\bcode\begin{verbatim}
>>> string.zfill('12', 5)
'00012'
>>> string.zfill('-3.14', 7)
'-003.14'
>>> string.zfill('3.14159265359', 5)
'3.14159265359'
>>>
\end{verbatim}\ecode
\chapter{Errors and Exceptions}
Until now error messages haven't been more than mentioned, but if you
have tried out the examples you have probably seen some. There are
(at least) two distinguishable kinds of errors: {\em syntax\ errors}
and {\em exceptions}.
\subsubsection{Syntax Errors} \section{Syntax Errors}
Syntax errors, also known as parsing errors, are perhaps the most common Syntax errors, also known as parsing errors, are perhaps the most common
kind of complaint you get while you are still learning Python: kind of complaint you get while you are still learning Python:
...@@ -1173,10 +1486,14 @@ the arrow: in the example, the error is detected at the keyword ...@@ -1173,10 +1486,14 @@ the arrow: in the example, the error is detected at the keyword
File name and line number are printed so you know where to look in case File name and line number are printed so you know where to look in case
the input came from a script. the input came from a script.
\subsubsection{Exceptions} \section{Exceptions}
Even if a statement or expression is syntactically correct, it may cause Even if a statement or expression is syntactically correct, it may
an error when an attempt is made to execute it: cause an error when an attempt is made to execute it.
Errors detected during execution are called {\em exceptions} and are
not unconditionally fatal: you will soon learn how to handle them in
Python programs. Most exceptions are not handled by programs,
however, and result in error messages as shown here:
\bcode\small\begin{verbatim} \bcode\small\begin{verbatim}
>>> 10 * (1/0) >>> 10 * (1/0)
Unhandled exception: run-time error: integer division by zero Unhandled exception: run-time error: integer division by zero
...@@ -1192,12 +1509,6 @@ Stack backtrace (innermost last): ...@@ -1192,12 +1509,6 @@ Stack backtrace (innermost last):
File "<stdin>", line 1 File "<stdin>", line 1
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Errors detected during execution are called
{\em exceptions}
and are not unconditionally fatal: you will soon learn how to handle
them in Python programs.
Most exceptions are not handled by programs, however, and result
in error messages as shown here.
The first line of the error message indicates what happened. The first line of the error message indicates what happened.
Exceptions come in different types, and the type is printed as part of Exceptions come in different types, and the type is printed as part of
...@@ -1227,25 +1538,24 @@ errors are more serious: these are usually caused by misspelled ...@@ -1227,25 +1538,24 @@ errors are more serious: these are usually caused by misspelled
identifiers.% identifiers.%
\footnote{ \footnote{
The parser does not check whether names used in a program are at The parser does not check whether names used in a program are at
all defined elsewhere in the program, so such checks are all defined elsewhere in the program; such checks are
postponed until run-time. The same holds for type checking. postponed until run-time. The same holds for type checking.
} }
The detail is the offending identifier. The detail is the offending identifier.
\item \item
{\em Type\ errors} {\em Type\ errors} are also pretty serious: this is another case of
are also pretty serious: this is another case of using wrong data (or using wrong data (or better, using data the wrong way), but here the
better, using data the wrong way), but here the error can be glanced error can be gleaned from the object type(s) alone. The detail shows
from the object type(s) alone. in what context the error was detected.
The detail shows in what context the error was detected.
\end{itemize} \end{itemize}
\subsubsection{Handling Exceptions} \section{Handling Exceptions}
It is possible to write programs that handle selected exceptions. It is possible to write programs that handle selected exceptions.
Look at the following example, which prints a table of inverses of Look at the following example, which prints a table of inverses of
some floating point numbers: some floating point numbers:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> numbers = [0.3333, 2.5, 0.0, 10.0] >>> numbers = [0.3333, 2.5, 0, 10]
>>> for x in numbers: >>> for x in numbers:
... print x, ... print x,
... try: ... try:
...@@ -1271,10 +1581,11 @@ If no exception occurs, the ...@@ -1271,10 +1581,11 @@ If no exception occurs, the
{\em except\ clause} {\em except\ clause}
is skipped and execution of the {\tt try} statement is finished. is skipped and execution of the {\tt try} statement is finished.
\item \item
If an exception occurs during execution of the try clause, and its If an exception occurs during execution of the try clause,
type matches the exception named after the {\tt except} keyword, the the rest of the clause is skipped. Then if
rest of the try clause is skipped, the except clause is executed, and its type matches the exception named after the {\tt except} keyword,
then execution continues after the {\tt try} statement. the rest of the try clause is skipped, the except clause is executed,
and then execution continues after the {\tt try} statement.
\item \item
If an exception occurs which does not match the exception named in the If an exception occurs which does not match the exception named in the
except clause, it is passed on to outer try statements; if no handler is except clause, it is passed on to outer try statements; if no handler is
...@@ -1320,12 +1631,7 @@ Standard exception names are built-in identifiers (not reserved ...@@ -1320,12 +1631,7 @@ Standard exception names are built-in identifiers (not reserved
keywords). keywords).
These are in fact string objects whose These are in fact string objects whose
{\em object\ identity} {\em object\ identity}
(not their value!) identifies the exceptions.% (not their value!) identifies the exceptions.
\footnote{
There should really be a separate exception type; it is pure
laziness that exceptions are identified by strings, and this may
be fixed in the future.
}
The string is printed as the second part of the message for unhandled The string is printed as the second part of the message for unhandled
exceptions. exceptions.
Their names and values are: Their names and values are:
...@@ -1358,7 +1664,7 @@ Handling run-time error: integer division by zero ...@@ -1358,7 +1664,7 @@ Handling run-time error: integer division by zero
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Raising Exceptions} \section{Raising Exceptions}
The {\tt raise} statement allows the programmer to force a specified The {\tt raise} statement allows the programmer to force a specified
exception to occur. exception to occur.
...@@ -1373,7 +1679,7 @@ Stack backtrace (innermost last): ...@@ -1373,7 +1679,7 @@ Stack backtrace (innermost last):
The first argument to {\tt raise} names the exception to be raised. The first argument to {\tt raise} names the exception to be raised.
The optional second argument specifies the exception's argument. The optional second argument specifies the exception's argument.
\subsubsection{User-defined Exceptions} \section{User-defined Exceptions}
Programs may name their own exceptions by assigning a string to a Programs may name their own exceptions by assigning a string to a
variable. variable.
...@@ -1395,7 +1701,7 @@ Stack backtrace (innermost last): ...@@ -1395,7 +1701,7 @@ Stack backtrace (innermost last):
Many standard modules use this to report errors that may occur in Many standard modules use this to report errors that may occur in
functions they define. functions they define.
\subsubsection{Defining Clean-up Actions} \section{Defining Clean-up Actions}
The {\tt try} statement has another optional clause which is intended to The {\tt try} statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances. define clean-up actions that must be executed under all circumstances.
...@@ -1415,141 +1721,11 @@ Stack backtrace (innermost last): ...@@ -1415,141 +1721,11 @@ Stack backtrace (innermost last):
The The
{\em finally\ clause} {\em finally\ clause}
must follow the except clauses(s), if any. must follow the except clauses(s), if any.
It is executed whether or not an exception occurred. It is executed whether or not an exception occurred,
or whether or not an exception is handled.
If the exception is handled, the finally clause is executed after the If the exception is handled, the finally clause is executed after the
handler (and even if another exception occurred in the handler). handler (and even if another exception occurred in the handler).
It is also executed when the {\tt try} statement is left via a It is also executed when the {\tt try} statement is left via a
{\tt break} or {\tt return} statement. {\tt break} or {\tt return} statement.
\subsection{Classes}
Classes in Python make it possible to play the game of encapsulation in a
somewhat different way than it is played with modules.
Classes are an advanced topic and are probably best skipped on the first
encounter with Python.
\subsubsection{Prologue}
Python's class mechanism is not particularly elegant, but quite powerful.
It is a mixture of the class mechanisms found in C++ and Modula-3.
As is true for modules, classes in Python do not put an absolute barrier
between definition and user, but rather rely on the politeness of the
user not to ``break into the definition.''
The most important features of classes are retained with full power,
however: the class inheritance mechanism allows multiple base classes,
a derived class can override any method of its base class(es), a method
can call the method of a base class with the same name.
Objects can contain an arbitrary amount of private data.
In C++ terminology, all class members (including data members) are
{\em public},
and all member functions (methods) are
{\em virtual}.
There are no special constructors or destructors.
As in Modula-3, there are no shorthands for referencing the object's
members from its methods: the method function is declared with an
explicit first argument representing the object, which is provided
implicitly by the call.
As in Smalltalk, classes themselves are objects, albeit in the wider
sense of the word: in Python, all data types are objects.
This provides semantics for renaming or aliasing.
But, just like in C++ or Modula-3, the built-in types cannot be used as
base classes for extension by the user.
Also, like Modula-3 but unlike C++, the built-in operators with special
syntax (arithmetic operators, subscripting etc.) cannot be redefined for
class members.%
\footnote{
They can be redefined for new object types implemented in C in
extensions to the interpreter, however. It would require only a
naming convention and a relatively small change to the
interpreter to allow operator overloading for classes, so
perhaps someday...
}
\subsubsection{A Simple Example}
Consider the following example, which defines a class {\tt Set}
representing a (finite) mathematical set with operations to add and
remove elements, a membership test, and a request for the size of the
set.
\bcode\begin{verbatim}
class Set():
def new(self):
self.elements = []
return self
def add(self, e):
if e not in self.elements:
self.elements.append(e)
def remove(self, e):
if e in self.elements:
for i in range(len(self.elements)):
if self.elements[i] = e:
del self.elements[i]
break
def is_element(self, e):
return e in self.elements
def size(self):
return len(self.elements)
\end{verbatim}\ecode
Note that the class definition looks like a big compound statement,
with all the function definitons indented repective to the
{\tt class}
keyword.
Let's assume that this
{\em class\ definition}
is the only contents of the module file
{\tt SetClass.py}.
We can then use it in a Python program as follows:
\bcode\begin{verbatim}
>>> from SetClass import Set
>>> a = Set().new() # create a Set object
>>> a.add(2)
>>> a.add(3)
>>> a.add(1)
>>> a.add(1)
>>> if a.is_element(3): print '3 is in the set'
...
3 is in the set
>>> if not a.is_element(4): print '4 is not in the set'
...
4 is not in the set
>>> print 'a has', a.size(), 'elements'
a has 3 elements
>>> a.remove(1)
>>> print 'now a has', a.size(), 'elements'
>>>
now a has 2 elements
>>>
\end{verbatim}\ecode
From the example we learn in the first place that the functions defined
in the class (e.g.,
{\tt add})
can be called using the
{\em member}
notation for the object
{\tt a}.
The member function is called with one less argument than it is defined:
the object is implicitly passed as the first argument.
Thus, the call
{\tt a.add(2)}
is equivalent to
{\tt Set.add(a, 2)}.
XXX This section is not complete yet! Inheritance!
\section{XXX P.M.}
\begin{itemize}
\item The {\tt del} statement.
\item The {\tt dir()} function.
\item Tuples.
\item Dictionaries.
\item Objects and types in general.
\item Backquotes.
\item Output formatting.
\item And/Or/Not.
\item ``.pyc'' files.
\end{itemize}
\end{document} \end{document}
Doc/tut/tut.tex
View file @ 6fc178f4
% Format this file with latex. % Format this file with latex.
%\documentstyle[11pt,myformat]{article} \documentstyle[myformat]{report}
\documentstyle[palatino,11pt,myformat]{article}
\title{\bf \title{\bf
Python Tutorial \\ Python Tutorial
(DRAFT)
} }
\author{ \author{
...@@ -25,29 +23,30 @@ ...@@ -25,29 +23,30 @@
\noindent \noindent
Python is a simple, yet powerful programming language that bridges the Python is a simple, yet powerful programming language that bridges the
gap between C and shell programming, and is thus ideally suited for rapid gap between C and shell programming, and is thus ideally suited for
prototyping. ``throw-away programming''
Its syntax is put together from constructs borrowed from a variety of other and rapid prototyping. Its syntax is put
languages; most prominent are influences from ABC, C, Modula-3 and Icon. together from constructs borrowed from a variety of other languages;
most prominent are influences from ABC, C, Modula-3 and Icon.
The Python interpreter is easily extended with new functions and data The Python interpreter is easily extended with new functions and data
types implemented in C. types implemented in C. Python is also suitable as an extension
Python is also suitable as an extension language for highly language for highly customizable C applications such as editors or
customizable C applications such as editors or window managers. window managers.
Python is available for various operating systems, amongst which Python is available for various operating systems, amongst which
several flavors of \UNIX, Amoeba, and the Apple Macintosh O.S. several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S.,
and MS-DOS.
This tutorial introduces the reader informally to the basic concepts and This tutorial introduces the reader informally to the basic concepts
features of the Python language and system. and features of the Python language and system. It helps to have a
It helps to have a Python interpreter handy for hands-on experience, Python interpreter handy for hands-on experience, but as the examples
but as the examples are self-contained, the tutorial can be read are self-contained, the tutorial can be read off-line as well.
off-line as well.
For a description of standard objects and modules, see the Library For a description of standard objects and modules, see the {\em
Reference document. Library Reference} document. The {\em Language Reference} document
The Language Reference document (XXX not yet existing) (when it is ever written)
gives a more formal definition of the language. will give a more formal definition of the language.
\end{abstract} \end{abstract}
...@@ -59,156 +58,148 @@ gives a more formal definition of the language. ...@@ -59,156 +58,148 @@ gives a more formal definition of the language.
\pagenumbering{arabic} \pagenumbering{arabic}
\section{Whetting Your Appetite} \chapter{Whetting Your Appetite}
If you ever wrote a large shell script, you probably know this feeling: If you ever wrote a large shell script, you probably know this
you'd love to add yet another feature, but it's already so slow, and so feeling: you'd love to add yet another feature, but it's already so
big, and so complicated; or the feature involves a system call or other slow, and so big, and so complicated; or the feature involves a system
funcion that is only accessible from C \ldots call or other funcion that is only accessible from C \ldots Usually
Usually the problem at hand isn't serious enough to warrant rewriting the problem at hand isn't serious enough to warrant rewriting the
the script in C; perhaps because the problem requires variable-length script in C; perhaps because the problem requires variable-length
strings or other data types (like sorted lists of file names) that strings or other data types (like sorted lists of file names) that are
are easy in the shell but lots of work to implement in C; or perhaps easy in the shell but lots of work to implement in C; or perhaps just
just because you're not sufficiently familiar with C. because you're not sufficiently familiar with C.
In such cases, Python may be just the language for you. In such cases, Python may be just the language for you. Python is
Python is simple to use, but it is a real programming language, offering simple to use, but it is a real programming language, offering much
much more structure and support for large programs than the shell has. more structure and support for large programs than the shell has. On
On the other hand, it also offers much more error checking than C, and, the other hand, it also offers much more error checking than C, and,
being a being a {\em very-high-level language}, it has high-level data types
{\em very-high-level language}, built in, such as flexible arrays and dictionaries that would cost you
it has high-level data types built in, such as flexible arrays and days to implement efficiently in C. Because of its more general data
dictionaries that would cost you days to implement efficiently in C. types Python is applicable to a much larger problem domain than {\em
Because of its more general data types Python is applicable to a Awk} or even {\em Perl}, yet most simple things are at least as easy
much larger problem domain than in Python as in those languages.
{\em Awk}
or even Python allows you to split up your program in modules that can be
{\em Perl}, reused in other Python programs. It comes with a large collection of
yet most simple things are at least as easy in Python as in those standard modules that you can use as the basis of your programs ---
languages. or as examples to start learning to program in Python. There are also
built-in modules that provide things like file I/O, system calls, and
Python allows you to split up your program in modules that can be reused even a generic interface to window systems (STDWIN).
in other Python programs.
It comes with a large collection of standard modules that you can use as
the basis for your programs --- or as examples to start learning to
program in Python.
There are also built-in modules that provide things like file I/O,
system calls, and even a generic interface to window systems (STDWIN).
Python is an interpreted language, which saves you considerable time Python is an interpreted language, which saves you considerable time
during program development because no compilation and linking is during program development because no compilation and linking is
necessary. necessary. The interpreter can be used interactively, which makes it
The interpreter can be used interactively, which makes it easy to easy to experiment with features of the language, to write throw-away
experiment with features of the language, to write throw-away programs, programs, or to test functions during bottom-up program development.
or to test functions during bottom-up program development.
It is also a handy desk calculator. It is also a handy desk calculator.
Python allows writing very compact and readable programs. Python allows writing very compact and readable programs. Programs
Programs written in Python are typically much shorter than equivalent C written in Python are typically much shorter than equivalent C
programs: programs, for several reasons:
No declarations are necessary (all type checking is \begin{itemize}
dynamic); statement grouping is done by indentation instead of begin/end \item
brackets; and the high-level data types allow you to express complex the high-level data types allow you to express complex operations in a
operations in a single statement. single statement;
\item
Python is statement grouping is done by indentation instead of begin/end
{\em extensible}: brackets;
if you know how to program in C it is easy to add a new built-in module \item
to the interpreter, either to perform critical operations at maximum no variable or argument declarations are necessary.
speed, or to link Python programs to libraries that may be only available \end{itemize}
in binary form (such as a vendor-specific graphics library).
Once you are really hooked, you can link the Python interpreter into an Python is {\em extensible}: if you know how to program in C it is easy
application written in C and use it as an extension or command language. to add a new built-in
function or
\subsection{Where From Here} module to the interpreter, either to
perform critical operations at maximum speed, or to link Python
Now that you are all excited about Python, you'll want to examine it in programs to libraries that may only be available in binary form (such
some more detail. as a vendor-specific graphics library). Once you are really hooked,
Since the best introduction to a language is using it, you are invited you can link the Python interpreter into an application written in C
here to do so. and use it as an extension or command language.
In the next section, the mechanics of using the interpreter are \section{Where From Here}
explained.
This is rather mundane information, but essential for trying out the Now that you are all excited about Python, you'll want to examine it
examples shown later. in some more detail. Since the best introduction to a language is
using it, you are invited here to do so.
In the next chapter, the mechanics of using the interpreter are
explained. This is rather mundane information, but essential for
trying out the examples shown later.
The rest of the tutorial introduces various features of the Python The rest of the tutorial introduces various features of the Python
language and system though examples, beginning with simple expressions, language and system though examples, beginning with simple
statements and data types, through functions and modules, and finally expressions, statements and data types, through functions and modules,
touching upon advanced concepts like exceptions and classes. and finally touching upon advanced concepts like exceptions.
When you're through with the turtorial (or just getting bored), you
should read the Library Reference, which gives complete (though terse)
reference material about built-in and standard types, functions and
modules that can save you a lot of time when writing Python programs.
\section{Using the Python Interpreter} \chapter{Using the Python Interpreter}
The Python interpreter is usually installed as The Python interpreter is usually installed as {\tt /usr/local/python}
{\tt /usr/local/python} on those machines where it is available; putting {\tt /usr/local} in
on those machines where it is available; putting your {\UNIX} shell's search path makes it possible to start it by
{\tt /usr/local}
in your {\UNIX} shell's search path makes it possible to start it by
typing the command typing the command
\bcode\begin{verbatim} \bcode\begin{verbatim}
python python
\end{verbatim}\ecode \end{verbatim}\ecode
to the shell. to the shell. Since the choice of the directory where the interpreter
Since the choice of the directory where the interpreter lives is an lives is an installation option, other places are possible; check with
installation option, other places instead of your local Python guru or system administrator.
{\tt /usr/local}
are possible; check with your local Python guru or system The interpreter operates somewhat like the {\UNIX} shell: when called
administrator. with standard input connected to a tty device, it reads and executes
commands interactively; when called with a file name argument or with
The interpreter operates somewhat like the {\UNIX} shell: when called with a file as standard input, it reads and executes a {\em script} from
standard input connected to a tty device, it reads and executes commands that file.
interactively; when called with a file name argument or with a file as
standard input, it reads and executes a Note that there is a difference between ``{\tt python file}'' and
{\em script} ``{\tt python $<$file}''. In the latter case, input requests from the
from that file.% program, such as calls to {\tt input()} and {\tt raw\_input()}, are
\footnote{ satisfied from {\em file}. Since this file has already been read
There is a difference between ``{\tt python file}'' and until the end by the parser before the program starts executing, the
``{\tt python $<$file}''. In the latter case {\tt input()} and program will encounter EOF immediately. In the former case (which is
{\tt raw\_input()} are satisfied from {\em file}, which has usually what you want) they are satisfied from whatever file or device
already been read until the end by the parser, so they will read is connected to standard input of the Python interpreter.
EOF immediately. In the former case (which is usually what
you want) they are satisfied from whatever file or device is
connected to standard input of the Python interpreter.
}
A third possibility is ``{\tt python -c command [arg] ...}'', which A third possibility is ``{\tt python -c command [arg] ...}'', which
executes the statement(s) in {\tt command}, in analogy of the shell's executes the statement(s) in {\tt command}, analogous to the shell's
{\tt -c} option. {\tt -c} option. Usually {\tt command} will contain spaces or other
When available, the script name and additional arguments thereafter are characters that are special to the shell, so it is best to quote it.
passed to the script in the variable
{\tt sys.argv}, When available, the script name and additional arguments thereafter
which is a list of strings. are passed to the script in the variable {\tt sys.argv}, which is a
list of strings.
When {\tt -c command} is used, {\tt sys.argv} is set to {\tt '-c'}. When {\tt -c command} is used, {\tt sys.argv} is set to {\tt '-c'}.
When commands are read from a tty, the interpreter is said to be in When commands are read from a tty, the interpreter is said to be in
{\em interactive\ mode}. {\em interactive\ mode}. In this mode it prompts for the next command
In this mode it prompts for the next command with the with the {\em primary\ prompt}, usually three greater-than signs ({\tt
{\em primary\ prompt}, >>>}); for continuation lines it prompts with the {\em secondary\
usually three greater-than signs ({\tt >>>}); for continuation lines prompt}, by default three dots ({\tt ...}). Typing an EOF (Control-D)
it prompts with the at the primary prompt causes the interpreter to exit with a zero exit
{\em secondary\ prompt}, status.
by default three dots ({\tt ...}).
Typing an EOF (Control-D) at the primary prompt causes the interpreter
to exit with a zero exit status.
When an error occurs in interactive mode, the interpreter prints a When an error occurs in interactive mode, the interpreter prints a
message and a stack trace and returns to the primary prompt; with input message and a stack trace and returns to the primary prompt; with
from a file, it exits with a nonzero exit status. input from a file, it exits with a nonzero exit status after printing
(Exceptions handled by an the stack trace. (Exceptions handled by an {\tt except} clause in a
{\tt except} {\tt try} statement are not errors in this context.) Some errors are
clause in a unconditionally fatal and cause an exit with a nonzero exit; this
{\tt try} applies to internal inconsistencies and some cases of running out of
statement are not errors in this context.) memory. All error messages are written to the standard error stream;
Some errors are unconditionally fatal and cause an exit with a nonzero normal output from the executed commands is written to standard
exit; this applies to internal inconsistencies and some cases of running output.
out of memory.
All error messages are written to the standard error stream; normal
output from the executed commands is written to standard output.
Typing an interrupt (normally Control-C or DEL) to the primary or Typing an interrupt (normally Control-C or DEL) to the primary or
secondary prompt cancels the input and returns to the primary prompt. secondary prompt cancels the input and returns to the primary prompt.
Typing an interrupt while a command is being executed raises the Typing an interrupt while a command is being executed raises the {\tt
{\tt KeyboardInterrupt} KeyboardInterrupt} exception, which may be handled by a {\tt try}
exception, which may be handled by a
{\tt try}
statement. statement.
When a module named When a module named
...@@ -223,79 +214,67 @@ i.e., a list of colon-separated directory names. ...@@ -223,79 +214,67 @@ i.e., a list of colon-separated directory names.
When When
{\tt PYTHONPATH} {\tt PYTHONPATH}
is not set, an installation-dependent default path is used, usually is not set, an installation-dependent default path is used, usually
{\tt .:/usr/local/lib/python}.% {\tt .:/usr/local/lib/python}.
\footnote{ (Modules are really searched in the list of directories given by the
Modules are really searched in the list of directories given by variable {\tt sys.path} which is initialized from {\tt PYTHONPATH} or
the variable {\tt sys.path} which is initialized from from the installation-dependent default. See the section on Standard
{\tt PYTHONPATH} or from the installation-dependent default. Modules later.)
See the section on Standard Modules later.
}
As an important speed-up of the start-up time of short programs, if a As an important speed-up of the start-up time for short programs, if a
file called {\tt foo.pyc} exists in the directory where {\tt foo.py} file called {\tt foo.pyc} exists in the directory where {\tt foo.py}
is found, this is assumed to contain an already-``compiled'' version is found, this is assumed to contain an already-``compiled'' version
of the module {\tt foo}. The last modification time of {\tt foo.py} of the module {\tt foo}. The last modification time of {\tt foo.py}
is recorded in {\tt foo.pyc}, and if these don't match, {\tt foo.pyc} is recorded in {\tt foo.pyc}, and the file is ignored if these don't
is ignored. Whenever {\tt foo.py} is successfully compiled, an match. Whenever {\tt foo.py} is successfully compiled, an attempt is
attempt is made to write the compiled version to {\tt foo.pyc}. made to write the compiled version to {\tt foo.pyc}.
On BSD'ish {\UNIX} systems, Python scripts can be made directly executable, On BSD'ish {\UNIX} systems, Python scripts can be made directly
like shell scripts, by putting the line executable, like shell scripts, by putting the line
\bcode\begin{verbatim} \bcode\begin{verbatim}
#! /usr/local/python #! /usr/local/python
\end{verbatim}\ecode \end{verbatim}\ecode
(assuming that's the name of the interpreter) at the beginning of the (assuming that's the name of the interpreter) at the beginning of the
script and giving the file an executable mode. script and giving the file an executable mode. (The {\tt \#!} must be
(The the first two characters of the file.)
{\tt \#!}
must be the first two characters of the file.)
\subsection{Interactive Input Editing and History Substitution} \section{Interactive Input Editing and History Substitution}
Some versions of the Python interpreter support editing of the current Some versions of the Python interpreter support editing of the current
input line and history substitution, similar to facilities found in the input line and history substitution, similar to facilities found in
Korn shell and the GNU Bash shell. the Korn shell and the GNU Bash shell. This is implemented using the
This is implemented using the {\em GNU\ Readline} library, which supports Emacs-style and vi-style
{\em GNU\ Readline} editing. This library has its own documentation which I won't
library, which supports Emacs-style and vi-style editing. duplicate here; however, the basics are easily explained.
This library has its own documentation which I won't duplicate here;
however, the basics are easily explained. Perhaps the quickest check to see whether command line editing is
supported is typing Control-P to the first Python prompt you get. If
If supported,% it beeps, you have command line editing. If nothing appears to
\footnote{ happen, or if \verb/^P/ is echoed, you can skip the rest of this
Perhaps the quickest check to see whether command line editing section.
is supported is typing Control-P to the first Python prompt
you get. If it beeps, you have command line editing. If supported, input line editing is active whenever the interpreter
If not, you can skip the rest of this section. prints a primary or secondary prompt. The current line can be edited
} using the conventional Emacs control characters. The most important
input line editing is active whenever the interpreter prints a primary of these are: C-A (Control-A) moves the cursor to the beginning of the
or secondary prompt. line, C-E to the end, C-B moves it one position to the left, C-F to
The current line can be edited using the conventional Emacs control the right. Backspace erases the character to the left of the cursor,
characters. C-D the character to its right. C-K kills (erases) the rest of the
The most important of these are: line to the right of the cursor, C-Y yanks back the last killed
C-A (Control-A) moves the cursor to the beginning of the line, C-E to string. C-underscore undoes the last change you made; it can be
the end, C-B moves it one position to the left, C-F to the right. repeated for cumulative effect.
Backspace erases the character to the left of the cursor, C-D the
character to its right. History substitution works as follows. All non-empty input lines
C-K kills (erases) the rest of the line to the right of the cursor, C-Y issued are saved in a history buffer, and when a new prompt is given
yanks back the last killed string. you are positioned on a new line at the bottom of this buffer. C-P
C-underscore undoes the last change you made; it can be repeated for moves one line up (back) in the history buffer, C-N moves one down.
cumulative effect.
History substitution works as follows.
All non-empty input lines issued are saved in a history buffer,
and when a new prompt is given you are positioned on a new line at the
bottom of this buffer.
C-P moves one line up (back) in the history buffer, C-N moves one down.
Any line in the history buffer can be edited; an asterisk appears in Any line in the history buffer can be edited; an asterisk appears in
front of the prompt to mark a line as modified. front of the prompt to mark a line as modified. Pressing the Return
Pressing the Return key passes the current line to the interpreter. key passes the current line to the interpreter. C-R starts an
C-R starts an incremental reverse search; C-S starts a forward search. incremental reverse search; C-S starts a forward search.
The key bindings and some other parameters of the Readline library can The key bindings and some other parameters of the Readline library can
be customized by placing commands in an initialization file called be customized by placing commands in an initialization file called
{\tt \$HOME/.initrc}. {\tt \$HOME/.inputrc}. Key bindings have the form
Key bindings have the form
\bcode\begin{verbatim} \bcode\begin{verbatim}
key-name: function-name key-name: function-name
\end{verbatim}\ecode \end{verbatim}\ecode
...@@ -314,50 +293,42 @@ Meta-h: backward-kill-word ...@@ -314,50 +293,42 @@ Meta-h: backward-kill-word
Control-u: universal-argument Control-u: universal-argument
\end{verbatim}\ecode \end{verbatim}\ecode
Note that the default binding for TAB in Python is to insert a TAB Note that the default binding for TAB in Python is to insert a TAB
instead of Readline's default filename completion function. instead of Readline's default filename completion function. If you
If you insist, you can override this by putting insist, you can override this by putting
\bcode\begin{verbatim} \bcode\begin{verbatim}
TAB: complete TAB: complete
\end{verbatim}\ecode \end{verbatim}\ecode
in your in your {\tt \$HOME/.inputrc}. (Of course, this makes it hard to type
{\tt \$HOME/.inputrc}. indented continuation lines.)
(Of course, this makes it hard to type indented continuation lines.)
This facility is an enormous step forward compared to previous versions of This facility is an enormous step forward compared to previous
the interpreter; however, some wishes are left: versions of the interpreter; however, some wishes are left: It would
It would be nice if the proper indentation were suggested on be nice if the proper indentation were suggested on continuation lines
continuation lines (the parser knows if an indent token is required (the parser knows if an indent token is required next). The
next). completion mechanism might use the interpreter's symbol table. A
The completion mechanism might use the interpreter's symbol table. function to check (or even suggest) matching parentheses, quotes etc.
A function to check (or even suggest) matching parentheses, quotes would also be useful.
etc. would also be useful.
\section{An Informal Introduction to Python} \chapter{An Informal Introduction to Python}
In the following examples, input and output are distinguished by the In the following examples, input and output are distinguished by the
presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat the presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat the
example, you must type everything after the prompt, when the prompt example, you must type everything after the prompt, when the prompt
appears; everything on lines that do not begin with a prompt is output appears;
from the interpreter. lines that do not begin with a prompt are output from the interpreter.
Note that a secondary prompt on a line by itself in an example means you Note that a secondary prompt on a line by itself in an example means
must type a blank line; this is used to end a multi-line command. you must type a blank line; this is used to end a multi-line command.
\subsection{Using Python as a Calculator} \section{Using Python as a Calculator}
Let's try some simple Python commands. Let's try some simple Python commands. Start the interpreter and wait
Start the interpreter and wait for the primary prompt, for the primary prompt, {\tt >>>}.
{\tt >>>}.
The interpreter acts as a simple calculator: you can type an expression The interpreter acts as a simple calculator: you can type an
at it and it will write the value. expression at it and it will write the value. Expression syntax is
Expression syntax is straightforward: the operators straightforward: the operators {\tt +}, {\tt -}, {\tt *} and {\tt /}
{\tt +},
{\tt -},
{\tt *}
and
{\tt /}
work just as in most other languages (e.g., Pascal or C); parentheses work just as in most other languages (e.g., Pascal or C); parentheses
can be used for grouping. can be used for grouping. For example:
For example:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> # This is a comment >>> # This is a comment
>>> 2+2 >>> 2+2
...@@ -370,8 +341,8 @@ For example: ...@@ -370,8 +341,8 @@ For example:
2 2
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
As in C, the equal sign ({\tt =}) is used to assign a value to a variable. As in C, the equal sign ({\tt =}) is used to assign a value to a
The value of an assignment is not written: variable. The value of an assignment is not written:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> width = 20 >>> width = 20
>>> height = 5*9 >>> height = 5*9
...@@ -379,15 +350,21 @@ The value of an assignment is not written: ...@@ -379,15 +350,21 @@ The value of an assignment is not written:
900 900
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
There is some support for floating point, but you can't mix floating A value can be assigned to several variables simultaneously:
point and integral numbers in expression (yet): \bcode\begin{verbatim}
>>> # Zero x, y and z
>>> x = y = z = 0
>>>
\end{verbatim}\ecode
There is full support for floating point; operators with mixed type
operands convert the integer operand to floating point:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> 10.0 / 3.3 >>> 4 * 2.5 / 3.3
3.0303030303 3.0303030303
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Besides numbers, Python can also manipulate strings, enclosed in single Besides numbers, Python can also manipulate strings, enclosed in
quotes: single quotes:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> 'foo bar' >>> 'foo bar'
'foo bar' 'foo bar'
...@@ -395,12 +372,14 @@ quotes: ...@@ -395,12 +372,14 @@ quotes:
'doesn\'t' 'doesn\'t'
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Strings are written inside quotes and with quotes and other funny Strings are written
characters escaped by backslashes, to show the precise value. the same way as they are typed for input:
(There is also a way to write strings without quotes and escapes.) inside quotes and with quotes and other funny characters escaped by
Strings can be concatenated (glued together) with the backslashes, to show the precise value. (There is also a way to write
{\tt +} strings without quotes and escapes.)
operator, and repeated with~{\tt *}:
Strings can be concatenated (glued together) with the {\tt +}
operator, and repeated with {\tt *}:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word = 'Help' + 'A' >>> word = 'Help' + 'A'
>>> word >>> word
...@@ -409,13 +388,12 @@ operator, and repeated with~{\tt *}: ...@@ -409,13 +388,12 @@ operator, and repeated with~{\tt *}:
'<HelpAHelpAHelpAHelpAHelpA>' '<HelpAHelpAHelpAHelpAHelpA>'
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Strings can be subscripted; as in C, the first character of a string has Strings can be subscripted; as in C, the first character of a string
subscript 0. has subscript 0.
There is no separate character type; a character is simply a string of There is no separate character type; a character is simply a string of
size one. size one. As in Icon, substrings can be specified with the {\em
As in Icon, substrings can be specified with the slice} notation: two subscripts (indices) separated by a colon.
{\em slice}
notation: two subscripts (indices) separated by a colon.
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word[4] >>> word[4]
'A' 'A'
...@@ -434,8 +412,8 @@ notation: two subscripts (indices) separated by a colon. ...@@ -434,8 +412,8 @@ notation: two subscripts (indices) separated by a colon.
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Degenerate cases are handled gracefully: an index that is too large is Degenerate cases are handled gracefully: an index that is too large is
replaced by the string size, an upper bound smaller than the lower bound replaced by the string size, an upper bound smaller than the lower
returns an empty string. bound returns an empty string.
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word[1:100] >>> word[1:100]
'elpA' 'elpA'
...@@ -446,8 +424,7 @@ returns an empty string. ...@@ -446,8 +424,7 @@ returns an empty string.
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Slice indices (but not simple subscripts) may be negative numbers, to Slice indices (but not simple subscripts) may be negative numbers, to
start counting from the right. start counting from the right. For example:
For example:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> word[-2:] # Take last two characters >>> word[-2:] # Take last two characters
'pA' 'pA'
...@@ -459,14 +436,9 @@ For example: ...@@ -459,14 +436,9 @@ For example:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
The best way to remember how slices work is to think of the indices as The best way to remember how slices work is to think of the indices as
pointing pointing {\em between} characters, with the left edge of the first
{\em between} character numbered 0. Then the right edge of the last character of a
characters, with the left edge of the first character numbered 0. string of {\tt n} characters has index {\tt n}, for example:
Then the right edge of the last character of a string of
{\tt n}
characters has index
{\tt n},
for example:
\bcode\begin{verbatim} \bcode\begin{verbatim}
+---+---+---+---+---+ +---+---+---+---+---+
| H | e | l | p | A | | H | e | l | p | A |
...@@ -474,29 +446,22 @@ for example: ...@@ -474,29 +446,22 @@ for example:
0 1 2 3 4 5 0 1 2 3 4 5
-5 -4 -3 -2 -1 -5 -4 -3 -2 -1
\end{verbatim}\ecode \end{verbatim}\ecode
The first row of numbers gives the position of the indices 0...5 in the The first row of numbers gives the position of the indices 0...5 in
string; the second row gives the corresponding negative indices. the string; the second row gives the corresponding negative indices.
For nonnegative indices, the length of a slice is the difference of the For nonnegative indices, the length of a slice is the difference of
indices, if both are within bounds, the indices, if both are within bounds, e.g., the length of {\tt
e.g., word[1:3]} is 3--1 = 2.
the length of
{\tt word[1:3]}
is 3--1 = 2.
Finally, the built-in function {\tt len()} computes the length of a The built-in function {\tt len()} computes the length of a string:
string:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> s = 'supercalifragilisticexpialidocious' >>> s = 'supercalifragilisticexpialidocious'
>>> len(s) >>> len(s)
34 34
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Python knows a number of Python knows a number of {\em compound} data types, used to group
{\em compound} together other values. The most versatile is the {\em list}, which
data types, used to group together other values. can be written as a list of comma-separated values between square
The most versatile is the
{\em list},
which can be written as a list of comma-separated values between square
brackets: brackets:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a = ['foo', 'bar', 100, 1234] >>> a = ['foo', 'bar', 100, 1234]
...@@ -522,9 +487,8 @@ Lists can be sliced, concatenated and so on, like strings: ...@@ -522,9 +487,8 @@ Lists can be sliced, concatenated and so on, like strings:
['foo', 'bar', 100, 'foo', 'bar', 100, 'foo', 'bar', 100, 'Boe!'] ['foo', 'bar', 100, 'foo', 'bar', 100, 'foo', 'bar', 100, 'Boe!']
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Unlike strings, which are Unlike strings, which are {\em immutable}, it is possible to change
{\em immutable}, individual elements of a list:
it is possible to change individual elements of a list:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a >>> a
['foo', 'bar', 100, 1234] ['foo', 'bar', 100, 1234]
...@@ -556,23 +520,32 @@ The built-in function {\tt len()} also applies to lists: ...@@ -556,23 +520,32 @@ The built-in function {\tt len()} also applies to lists:
4 4
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
It is possible to nest lists (create lists containing other lists),
for example:
\bcode\begin{verbatim}
>>> p = [1, [2, 3], 4]
>>> len(p)
3
>>> p[1]
[2, 3]
>>> p[1][0]
2
>>> p[1].append('xtra')
>>> p
[1, [2, 3, 'xtra'], 4]
>>>
\end{verbatim}\ecode
\subsection{Tuples and Sequences} \section{First Steps Towards Programming}
XXX To Be Done.
\subsection{First Steps Towards Programming}
Of course, we can use Python for more complicated tasks than adding two Of course, we can use Python for more complicated tasks than adding
and two together. two and two together. For instance, we can write an initial
For instance, we can write an initial subsequence of the subsequence of the {\em Fibonacci} series as follows:
{\em Fibonacci}
series as follows:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> # Fibonacci series: >>> # Fibonacci series:
>>> # the sum of two elements defines the next >>> # the sum of two elements defines the next
>>> a, b = 0, 1 >>> a, b = 0, 1
>>> while b < 100: >>> while b < 10:
... print b ... print b
... a, b = b, a+b ... a, b = b, a+b
... ...
...@@ -582,68 +555,53 @@ series as follows: ...@@ -582,68 +555,53 @@ series as follows:
3 3
5 5
8 8
13
21
34
55
89
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
This example introduces several new features. This example introduces several new features.
\begin{itemize} \begin{itemize}
\item \item
The first line contains a The first line contains a {\em multiple assignment}: the variables
{\em multiple\ assignment}: {\tt a} and {\tt b} simultaneously get the new values 0 and 1. On the
the variables last line this is used again, demonstrating that the expressions on
{\tt a} the right-hand side are all evaluated first before any of the
and
{\tt b}
simultaneously get the new values 0 and 1.
On the last line this is used again, demonstrating that the expressions
on the right-hand side are all evaluated first before any of the
assignments take place. assignments take place.
\item \item
The The {\tt while} loop executes as long as the condition (here: {\tt b <
{\tt while} 100}) remains true. In Python, as in C, any non-zero integer value is
loop executes as long as the condition (here: $b < 100$) remains true. true; zero is false. The condition may also be a string or list value,
In Python, as in C, any non-zero integer value is true; zero is false. in fact any sequence; anything with a non-zero length is true, empty
The condition may also be a string or list value, in fact any sequence; sequences are false. The test used in the example is a simple
anything with a non-zero length is true, empty sequences are false. comparison. The standard comparison operators are written as {\tt <},
The test used in the example is a simple comparison. {\tt >}, {\tt =}, {\tt <=}, {\tt >=} and {\tt <>}.%
The standard comparison operators are written as
{\tt <},
{\tt >},
{\tt =},
{\tt <=},
{\tt >=}
and
{\tt <>}.%
\footnote{ \footnote{
The ambiguity of using {\tt =} The ambiguity of using {\tt =}
for both assignment and equality is resolved by disallowing for both assignment and equality is resolved by disallowing
unparenthesized conditions at the right hand side of assignments. unparenthesized conditions on the right hand side of assignments.
Parenthesized assignment is also disallowed; instead it is
interpreted as an equality test.
} }
\item \item
The The {\em body} of the loop is {\em indented}: indentation is Python's
{\em body} way of grouping statements. Python does not (yet!) provide an
of the loop is intelligent input line editing facility, so you have to type a tab or
{\em indented}: indentation is Python's way of grouping statements. space(s) for each indented line. In practice you will prepare more
Python does not (yet!) provide an intelligent input line editing complicated input for Python with a text editor; most text editors have
facility, so you have to type a tab or space(s) for each indented line. an auto-indent facility. When a compound statement is entered
In practice you will prepare more complicated input for Python with a interactively, it must be followed by a blank line to indicate
text editor; most text editors have an auto-indent facility. completion (since the parser cannot guess when you have typed the last
When a compound statement is entered interactively, it must be line).
followed by a blank line to indicate completion (since the parser
cannot guess when you have typed the last line).
\item \item
The The {\tt print} statement writes the value of the expression(s) it is
{\tt print} given. It differs from just writing the expression you want to write
statement writes the value of the expression(s) it is given. (as we did earlier in the calculator examples) in the way it handles
It differs from just writing the expression you want to write (as we did multiple expressions and strings. Strings are written without quotes,
earlier in the calculator examples) in the way it handles multiple and a space is inserted between items, so you can format things nicely,
expressions and strings. like this:
Strings are written without quotes and a space is inserted between
items, so you can format things nicely, like this:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> i = 256*256 >>> i = 256*256
>>> print 'The value of i is', i >>> print 'The value of i is', i
...@@ -662,15 +620,16 @@ A trailing comma avoids the newline after the output: ...@@ -662,15 +620,16 @@ A trailing comma avoids the newline after the output:
\end{verbatim}\ecode \end{verbatim}\ecode
Note that the interpreter inserts a newline before it prints the next Note that the interpreter inserts a newline before it prints the next
prompt if the last line was not completed. prompt if the last line was not completed.
\end{itemize} \end{itemize}
\subsection{More Control Flow Tools} \chapter{More Control Flow Tools}
Besides the {\tt while} statement just introduced, Python knows the Besides the {\tt while} statement just introduced, Python knows the
usual control flow statements known from other languages, with some usual control flow statements known from other languages, with some
twists. twists.
\subsubsection{If Statements} \section{If Statements}
Perhaps the most well-known statement type is the {\tt if} statement. Perhaps the most well-known statement type is the {\tt if} statement.
For example: For example:
...@@ -687,21 +646,20 @@ For example: ...@@ -687,21 +646,20 @@ For example:
... ...
\end{verbatim}\ecode \end{verbatim}\ecode
There can be zero or more {\tt elif} parts, and the {\tt else} part is There can be zero or more {\tt elif} parts, and the {\tt else} part is
optional. optional. The keyword `{\tt elif}' is short for `{\tt else if}', and is
The keyword `{\tt elif}' is short for `{\tt else if}', and is useful to useful to avoid excessive indentation. An {\tt if...elif...elif...}
avoid excessive indentation. sequence is a substitute for the {\em switch} or {\em case} statements
An {\tt if...elif...elif...} sequence is a substitute for the found in other languages.
{\em switch} or {\em case} statements found in other languages.
\subsubsection{For Statements} \section{For Statements}
The {\tt for} statement in Python differs a bit from what you may be The {\tt for} statement in Python differs a bit from what you may be
used to in C or Pascal. used to in C or Pascal. Rather than always iterating over an
Rather than always iterating over an arithmetic progression of numbers arithmetic progression of numbers (as in Pascal), or leaving the user
(as Pascal), or leaving the user completely free in the iteration test completely free in the iteration test and step (as C), Python's {\tt
and step (as C), Python's {\tt for} statement iterates over the items for} statement iterates over the items of any sequence (e.g., a list
of any sequence (e.g., a list or a string). or a string), in the order that they appear in the sequence. For
For example (no pun intended): example (no pun intended):
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> # Measure some strings: >>> # Measure some strings:
>>> a = ['cat', 'window', 'defenestrate'] >>> a = ['cat', 'window', 'defenestrate']
...@@ -713,23 +671,34 @@ window 6 ...@@ -713,23 +671,34 @@ window 6
defenestrate 12 defenestrate 12
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
It is not safe to modify the sequence being iterated over in the loop
(this can only happen for mutable sequence types, i.e., lists). If
you need to modify the list you are iterating over, e.g., duplicate
selected items, you must iterate over a copy. The slice notation
makes this particularly convenient:
\bcode\begin{verbatim}
>>> for x in a[:]: # make a slice copy of the entire list
... if len(x) > 6: a.insert(0, x)
...
>>> a
['defenestrate', 'cat', 'window', 'defenestrate']
>>>
\end{verbatim}\ecode
\subsubsection{The {\tt range()} Function} \section{The {\tt range()} Function}
If you do need to iterate over a sequence of numbers, the built-in If you do need to iterate over a sequence of numbers, the built-in
function {\tt range()} comes in handy. function {\tt range()} comes in handy. It generates lists containing
It generates lists containing arithmetic progressions, arithmetic progressions, e.g.:
e.g.:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> range(10) >>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
The given end point is never part of the generated list; The given end point is never part of the generated list; {\tt range(10)}
{\tt range(10)} generates a list of 10 values, generates a list of 10 values, exactly the legal indices for items of a
exactly the legal indices for items of a sequence of length 10. sequence of length 10. It is possible to let the range start at another
It is possible to let the range start at another number, or to specify a number, or to specify a different increment (even negative):
different increment (even negative):
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> range(5, 10) >>> range(5, 10)
[5, 6, 7, 8, 9] [5, 6, 7, 8, 9]
...@@ -739,10 +708,10 @@ different increment (even negative): ...@@ -739,10 +708,10 @@ different increment (even negative):
[-10, -40, -70] [-10, -40, -70]
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
To iterate over the indices of a sequence, combine {\tt range()} To iterate over the indices of a sequence, combine {\tt range()} and
and {\tt len()} as follows: {\tt len()} as follows:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a = ['Mary', 'had', 'a', 'little', 'boy'] >>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)): >>> for i in range(len(a)):
... print i, a[i] ... print i, a[i]
... ...
...@@ -750,20 +719,23 @@ and {\tt len()} as follows: ...@@ -750,20 +719,23 @@ and {\tt len()} as follows:
1 had 1 had
2 a 2 a
3 little 3 little
4 boy 4 lamb
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Break Statements and Else Clauses on Loops} \section{Break and Continue Statements, and Else Clauses on Loops}
The {\tt break} statement, like in C, breaks out of the smallest
enclosing {\tt for} or {\tt while} loop.
The {\tt continue} statement, also borrowed from C, continues with the
next iteration of the loop.
The {\tt break} statement breaks out of the smallest enclosing {\tt for}
or {\tt while} loop.
Loop statements may have an {\tt else} clause; it is executed when the Loop statements may have an {\tt else} clause; it is executed when the
loop terminates through exhaustion of the list (with {\tt for}) or when loop terminates through exhaustion of the list (with {\tt for}) or when
the condition becomes false (with {\tt while}) but not when the loop is the condition becomes false (with {\tt while}), but not when the loop is
terminated by a {\tt break} statement. terminated by a {\tt break} statement. This is exemplified by the
This is exemplified by the following loop, which searches for a list following loop, which searches for a list item of value 0:
item of value 0:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> for n in range(2, 10): >>> for n in range(2, 10):
... for x in range(2, n): ... for x in range(2, n):
...@@ -784,7 +756,7 @@ item of value 0: ...@@ -784,7 +756,7 @@ item of value 0:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Pass Statements} \section{Pass Statements}
The {\tt pass} statement does nothing. The {\tt pass} statement does nothing.
It can be used when a statement is required syntactically but the It can be used when a statement is required syntactically but the
...@@ -796,11 +768,7 @@ For example: ...@@ -796,11 +768,7 @@ For example:
... ...
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Conditions Revisited} \section{Defining Functions}
XXX To Be Done.
\subsection{Defining Functions}
We can create a function that writes the Fibonacci series to an We can create a function that writes the Fibonacci series to an
arbitrary boundary: arbitrary boundary:
...@@ -816,29 +784,24 @@ arbitrary boundary: ...@@ -816,29 +784,24 @@ arbitrary boundary:
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
The keyword The keyword {\tt def} introduces a function {\em definition}. It must
{\tt def} be followed by the function name and the parenthesized list of formal
introduces a function parameters. The statements that form the body of the function starts at
{\em definition}. the next line, indented by a tab stop.
It must be followed by the function name and the parenthesized list of
formal parameters. The {\em execution} of a function introduces a new symbol table used
The statements that form the body of the function starts at the next for the local variables of the function. More precisely, all variable
line, indented by a tab stop. assignments in a function store the value in the local symbol table;
The whereas
{\em execution} variable references first look in the local symbol table, then
of a function introduces a new symbol table used for the local variables in the global symbol table, and then in the table of built-in names.
of the function. Thus,
More precisely, all variable assignments in a function store the value global variables cannot be directly assigned to from within a
in the local symbol table; variable references first look in the local function, although they may be referenced.
symbol table, then in the global symbol table, and then in the table of
built-in names.
Thus, the global symbol table is
{\em read-only}
within a function.
The actual parameters (arguments) to a function call are introduced in The actual parameters (arguments) to a function call are introduced in
the local symbol table of the called function when it is called; the local symbol table of the called function when it is called; thus,
thus, arguments are passed using arguments are passed using {\em call\ by\ value}.%
{\em call\ by\ value}.%
\footnote{ \footnote{
Actually, {\em call by object reference} would be a better Actually, {\em call by object reference} would be a better
description, since if a mutable object is passed, the caller description, since if a mutable object is passed, the caller
...@@ -848,13 +811,14 @@ thus, arguments are passed using ...@@ -848,13 +811,14 @@ thus, arguments are passed using
When a function calls another function, a new local symbol table is When a function calls another function, a new local symbol table is
created for that call. created for that call.
A function definition introduces the function name in the global symbol A function definition introduces the function name in the
table. current
The value has a type that is recognized by the interpreter as a symbol table. The value
user-defined function. of the function name
This value can be assigned to another name which can then also be used has a type that is recognized by the interpreter as a user-defined
as a function. function. This value can be assigned to another name which can then
This serves as a general renaming mechanism: also be used as a function. This serves as a general renaming
mechanism:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> fib >>> fib
<function object at 10042ed0> <function object at 10042ed0>
...@@ -863,17 +827,13 @@ This serves as a general renaming mechanism: ...@@ -863,17 +827,13 @@ This serves as a general renaming mechanism:
1 1 2 3 5 8 13 21 34 55 89 1 1 2 3 5 8 13 21 34 55 89
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
You might object that You might object that {\tt fib} is not a function but a procedure. In
{\tt fib} Python, as in C, procedures are just functions that don't return a
is not a function but a procedure. value. In fact, technically speaking, procedures do return a value,
In Python, as in C, procedures are just functions that don't return a albeit a rather boring one. This value is called {\tt None} (it's a
value. built-in name). Writing the value {\tt None} is normally suppressed by
In fact, technically speaking, procedures do return a value, albeit a the interpreter if it would be the only value written. You can see it
rather boring one. if you really want to:
This value is called {\tt None} (it's a built-in name).
Writing the value {\tt None} is normally suppressed by the interpreter
if it would be the only value written.
You can see it if you really want to:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> print fib(0) >>> print fib(0)
None None
...@@ -896,103 +856,339 @@ the Fibonacci series, instead of printing it: ...@@ -896,103 +856,339 @@ the Fibonacci series, instead of printing it:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
This example, as usual, demonstrates some new Python features: This example, as usual, demonstrates some new Python features:
\begin{itemize} \begin{itemize}
\item \item
The The {\tt return} statement returns with a value from a function. {\tt
{\tt return} return} without an expression argument is used to return from the middle
statement returns with a value from a function. of a procedure (falling off the end also returns from a proceduce), in
{\tt return} which case the {\tt None} value is returned.
without an expression argument is used to return from the middle of a
procedure (falling off the end also returns from a proceduce).
\item \item
The statement The statement {\tt result.append(b)} calls a {\em method} of the list
{\tt ret.append(b)} object {\tt result}. A method is a function that `belongs' to an
calls a object and is named {\tt obj.methodname}, where {\tt obj} is some
{\em method} object (this may be an expression), and {\tt methodname} is the name
of the list object of a method that is defined by the object's type. Different types
{\tt ret}. define different methods. Methods of different types may have the
A method is a function that `belongs' to an object and is named same name without causing ambiguity. (It is possible to define your
{\tt obj.methodname}, own object types and methods, using {\em classes}. This is an
where advanced feature that is not discussed in this tutorial.)
{\tt obj} The method {\tt append} shown in the example, is defined for
is some object (this may be an expression), and list objects; it adds a new element at the end of the list. In this
{\tt methodname} example
is the name of a method that is defined by the object's type. it is equivalent to {\tt result = result + [b]}, but more efficient.
Different types define different methods.
Methods of different types may have the same name without causing
ambiguity.
See the section on classes, later, to find out how you can define your
own object types and methods.
The method
{\tt append}
shown in the example, is defined for list objects; it adds a new element
at the end of the list.
In this case it is equivalent to
{\tt ret = ret + [b]},
but more efficient.%
\footnote{
There is a subtle semantic difference if the object
is referenced from more than one place.
}
\end{itemize} \end{itemize}
The list object type has two more methods:
\chapter{Odds and Ends}
This chapter describes some things you've learned about already in
more detail, and adds some new things as well.
\section{More on Lists}
The list data type has some more methods. Here are all of the methods
of lists objects:
\begin{description} \begin{description}
\item[{\tt insert(i, x)}] \item[{\tt insert(i, x)}]
Inserts an item at a given position. Insert an item at a given position. The first argument is the index of
The first argument is the index of the element before which to insert, the element before which to insert, so {\tt a.insert(0, x)} inserts at
so {\tt a.insert(0, x)} inserts at the front of the list, and the front of the list, and {\tt a.insert(len(a), x)} is equivalent to
{\tt a.insert(len(a), x)} is equivalent to {\tt a.append(x)}. {\tt a.append(x)}.
\item[{\tt append(x)}]
Equivalent to {\tt a.insert(len(a), x)}.
\item[{\tt index(x)}]
Return the index in the list of the first item whose value is {\tt x}.
It is an error if there is no such item.
\item[{\tt remove(x)}]
Remove the first item from the list whose value is {\tt x}.
It is an error if there is no such item.
\item[{\tt sort()}] \item[{\tt sort()}]
Sorts the elements of the list. Sort the items of the list, in place.
\item[{\tt reverse()}]
Reverse the elements of the list, in place.
\end{description} \end{description}
For example:
An example that uses all list methods:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> a = [10, 100, 1, 1000] >>> a = [66.6, 333, 333, 1, 1234.5]
>>> a.insert(2, -1) >>> a.insert(2, -1)
>>> a.append(333)
>>> a >>> a
[10, 100, -1, 1, 1000] [66.6, 333, -1, 333, 1, 1234.5, 333]
>>> a.index(333)
1
>>> a.remove(333)
>>> a
[66.6, -1, 333, 1, 1234.5, 333]
>>> a.reverse()
>>> a
[333, 1234.5, 1, 333, -1, 66.6]
>>> a.sort() >>> a.sort()
>>> a >>> a
[-1, 1, 10, 100, 1000] [-1, 1, 66.6, 333, 333, 1234.5]
>>> # Strings are sorted according to ASCII:
>>> b = ['Mary', 'had', 'a', 'little', 'boy']
>>> b.sort()
>>> b
['Mary', 'a', 'boy', 'had', 'little']
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsection{Modules} \section{The {\tt del} statement}
There is a way to remove an item from a list given its index instead
of its value: the {\tt del} statement. This can also be used to
remove slices from a list (which we did earlier by assignment of an
empty list to the slice). For example:
\bcode\begin{verbatim}
>>> a
[-1, 1, 66.6, 333, 333, 1234.5]
>>> del a[0]
>>> a
[1, 66.6, 333, 333, 1234.5]
>>> del a[2:4]
>>> a
[1, 66.6, 1234.5]
>>>
\end{verbatim}\ecode
{\tt del} can also be used to delete entire variables:
\bcode\begin{verbatim}
>>> del a
>>>
\end{verbatim}\ecode
Referencing the name {\tt a} hereafter is an error (at least until
another value is assigned to it). We'll find other uses for {\tt del}
later.
\section{Tuples and Sequences}
We saw that lists and strings have many common properties, e.g.,
subscripting and slicing operations. They are two examples of {\em
sequence} data types. As Python is an evolving language, other
sequence data types may be added. There is also another standard
sequence data type: the {\em tuple}.
A tuple consists of a number of values separated by commas, for
instance:
\bcode\begin{verbatim}
>>> t = 12345, 54321, 'hello!'
>>> t[0]
12345
>>> t
(12345, 54321, 'hello!')
>>> # Tuples may be nested:
>>> u = t, (1, 2, 3, 4, 5)
>>> u
((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
>>>
\end{verbatim}\ecode
As you see, on output tuples are alway enclosed in parentheses, so
that nested tuples are interpreted correctly; they may be input with
or without surrounding parentheses, although often parentheses are
necessary anyway (if the tuple is part of a larger expression).
Tuples have many uses, e.g., (x, y) coordinate pairs, employee records
from a database, etc. Tuples, like strings, are immutable: it is not
possible to assign to the individual items of a tuple (you can
simulate much of the same effect with slicing and concatenation,
though).
A special problem is the construction of tuples containing 0 or 1
items: the syntax has some extra quirks to accomodate these. Empty
tuples are constructed by an empty pair of parentheses; a tuple with
one item is constructed by following a value with a comma
(it is not sufficient to enclose a single value in parentheses).
Ugly, but effective. For example:
\bcode\begin{verbatim}
>>> empty = ()
>>> singleton = 'hello', # <-- note trailing comma
>>> len(empty)
0
>>> len(singleton)
1
>>> singleton
('hello',)
>>>
\end{verbatim}\ecode
The statement {\tt t = 12345, 54321, 'hello!'} is an example of {\em
tuple packing}: the values {\tt 12345}, {\tt 54321} and {\tt 'hello!'}
are packed together in a tuple. The reverse operation is also
possible, e.g.:
\bcode\begin{verbatim}
>>> x, y, z = t
>>>
\end{verbatim}\ecode
This is called, appropriately enough, {\em tuple unpacking}. Tuple
unpacking requires that the list of variables on the left has the same
number of elements as the length of the tuple. Note that multiple
assignment is really just a combination of tuple packing and tuple
unpacking!
Occasionally, the corresponding operation on lists is useful: {\em list
unpacking}. This is supported by enclosing the list of variables in
square brackets:
\bcode\begin{verbatim}
>>> a = ['foo', 'bar', 100, 1234]
>>> [a1, a2, a3, a4] = a
>>>
\end{verbatim}\ecode
\section{Dictionaries}
Another useful data type built into Python is the {\em dictionary}.
Dictionaries are sometimes found in other languages as ``associative
memories'' or ``associative arrays''. Unlike sequences, which are
indexed by a range of numbers, dictionaries are indexed by {\em keys},
which are strings. It is best to think of a dictionary as an unordered set of
{\em key:value} pairs, with the requirement that the keys are unique
(within one dictionary).
A pair of braces creates an empty dictionary: \verb/{}/.
Placing a comma-separated list of key:value pairs within the
braces adds initial key:value pairs to the dictionary; this is also the
way dictionaries are written on output.
The main operations on a dictionary are storing a value with some key
and extracting the value given the key. It is also possible to delete
a key:value pair
with {\tt del}.
If you store using a key that is already in use, the old value
associated with that key is forgotten. It is an error to extract a
value using a non-existant key.
The {\tt keys()} method of a dictionary object returns a list of all the
keys used in the dictionary, in random order (if you want it sorted,
just apply the {\tt sort()} method to the list of keys). To check
whether a single key is in the dictionary, use the \verb/has_key()/
method of the dictionary.
Here is a small example using a dictionary:
\bcode\begin{verbatim}
>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139; 'guido': 4127; 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127; 'irv': 4127; 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> tel.has_key('guido')
1
>>>
\end{verbatim}\ecode
\section{More on Conditions}
The conditions used in {\tt while} and {\tt if} statements above can
contain other operators besides comparisons.
The comparison operators {\tt in} and {\tt not in} check whether a value
occurs (does not occur) in a sequence. The operators {\tt is} and {\tt
is not} compare whether two objects are really the same object; this
only matters for mutable objects like lists. All comparison operators
have the same priority, which is lower than that of all numerical
operators.
Comparisons can be chained: e.g., {\tt a < b = c} tests whether {\tt a}
is less than {\tt b} and moreover {\tt b} equals {\tt c}.
Comparisons may be combined by the Boolean operators {\tt and} and {\tt
or}, and the outcome of a comparison (or of any other Boolean
expression) may be negated with {\tt not}. These all have lower
priorities than comparison operators again; between them, {\tt not} has
the highest priority, and {\tt or} the lowest, so that
{\tt A and not B or C} is equivalent to {\tt (A and (not B)) or C}. Of
course, parentheses can be used to express the desired composition.
The Boolean operators {\tt and} and {\tt or} are so-called {\em
shortcut} operators: their arguments are evaluated from left to right,
and evaluation stops as soon as the outcome is determined. E.g., if
{\tt A} and {\tt C} are true but {\tt B} is false, {\tt A and B and C}
does not evaluate the expression C. In general, the return value of a
shortcut operator, when used as a general value and not as a Boolean, is
the last evaluated argument.
It is possible to assign the result of a comparison or other Boolean
expression to a variable, but you must enclose the entire Boolean
expression in parentheses. This is necessary because otherwise an
assignment like \verb/a = b = c/ would be ambiguous: does it assign the
value of {\tt c} to {\tt a} and {\tt b}, or does it compare {\tt b} to
{\tt c} and assign the outcome (0 or 1) to {\tt a}? As it is, the first
meaning is what you get, and to get the latter you have to write
\verb/a = (b = c)/. (In Python, unlike C, assignment cannot occur
inside expressions.)
\section{Comparing Sequences and Other Types}
Sequence objects may be compared to other objects with the same
sequence type. The comparison uses {\em lexicographical} ordering:
first the first two items are compared, and if they differ this
determines the outcome of the comparison; if they are equal, the next
two items are compared, and so on, until either sequence is exhausted.
If two items to be compared are themselves sequences of the same type,
the lexiographical comparison is carried out recursively. If all
items of two sequences compare equal, the sequences are considered
equal. If one sequence is an initial subsequence of the other, the
shorted sequence is the smaller one. Lexicographical ordering for
strings uses the ASCII ordering for individual characters. Some
examples of comparisons between sequences with the same types:
\bcode\begin{verbatim}
(1, 2, 3) < (1, 2, 4)
[1, 2, 3] < [1, 2, 4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1, 2, 3, 4) < (1, 2, 4)
(1, 2) < (1, 2, -1)
(1, 2, 3) = (1.0, 2.0, 3.0)
(1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
\end{verbatim}\ecode
Note that comparing objects of different types is legal. The outcome
is deterministic but arbitrary: the types are ordered by their name.
Thus, a list is always smaller than a string, a string is always
smaller than a tuple, etc. Mixed numeric types are compared according
to their numeric value, so 0 equals 0.0, etc.%
\footnote{
The rules for comparing objects of different types should
not be relied upon; they may change in a future version of
the language.
}
\chapter{Modules}
If you quit from the Python interpreter and enter it again, the If you quit from the Python interpreter and enter it again, the
definitions you have made (functions and variables) are lost. definitions you have made (functions and variables) are lost.
Therefore, if you want to write a somewhat longer program, you are Therefore, if you want to write a somewhat longer program, you are
better off using a text editor to prepare the input for the interpreter better off using a text editor to prepare the input for the interpreter
and run it with that file as input instead. and run it with that file as input instead. This is known as creating a
This is known as creating a {\em script}. As your program gets longer, you may want to split it
{\em script}. into several files for easier maintenance. You may also want to use a
As your program gets longer, you may want to split it into several files handy function that you've written in several programs without copying
for easier maintenance. its definition into each program.
You may also want to use a handy function that you've written in several
programs without copying its definition into each program.
To support this, Python has a way to put definitions in a file and use To support this, Python has a way to put definitions in a file and use
them in a script or in an interactive instance of the interpreter. them in a script or in an interactive instance of the interpreter.
Such a file is called a Such a file is called a {\em module}; definitions from a module can be
{\em module}; {\em imported} into other modules or into the {\em main} module (the
definitions from a module can be collection of variables that you have access to in a script
{\em imported} executed at the top level
into other modules or into the and in calculator mode).
{\em main}
module (the collection of variables that you have access to in A module is a file containing Python definitions and statements. The
a script and in calculator mode). file name is the module name with the suffix {\tt .py} appended. For
instance, use your favorite text editor to create a file called {\tt
A module is a file containing Python definitions and statements. fibo.py} in the current directory with the following contents:
The file name is the module name with the suffix
{\tt .py}
appended.
For instance, use your favorite text editor to create a file called
{\tt fibo.py}
in the current directory with the following contents:
\bcode\begin{verbatim} \bcode\begin{verbatim}
# Fibonacci numbers module # Fibonacci numbers module
...@@ -1003,12 +1199,12 @@ def fib(n): # write Fibonacci series up to n ...@@ -1003,12 +1199,12 @@ def fib(n): # write Fibonacci series up to n
a, b = b, a+b a, b = b, a+b
def fib2(n): # return Fibonacci series up to n def fib2(n): # return Fibonacci series up to n
ret = [] result = []
a, b = 0, 1 a, b = 0, 1
while b <= n: while b <= n:
ret.append(b) result.append(b)
a, b = b, a+b a, b = b, a+b
return ret return result
\end{verbatim}\ecode \end{verbatim}\ecode
Now enter the Python interpreter and import this module with the Now enter the Python interpreter and import this module with the
following command: following command:
...@@ -1018,7 +1214,7 @@ following command: ...@@ -1018,7 +1214,7 @@ following command:
\end{verbatim}\ecode \end{verbatim}\ecode
This does not enter the names of the functions defined in This does not enter the names of the functions defined in
{\tt fibo} {\tt fibo}
directly in the symbol table; it only enters the module name directly in the current symbol table; it only enters the module name
{\tt fibo} {\tt fibo}
there. there.
Using the module name you can access the functions: Using the module name you can access the functions:
...@@ -1037,7 +1233,7 @@ If you intend to use a function often you can assign it to a local name: ...@@ -1037,7 +1233,7 @@ If you intend to use a function often you can assign it to a local name:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{More on Modules} \section{More on Modules}
A module can contain executable statements as well as function A module can contain executable statements as well as function
definitions. definitions.
...@@ -1093,26 +1289,19 @@ There is even a variant to import all names that a module defines: ...@@ -1093,26 +1289,19 @@ There is even a variant to import all names that a module defines:
This imports all names except those beginning with an underscore This imports all names except those beginning with an underscore
({\tt \_}). ({\tt \_}).
\subsubsection{Standard Modules} \section{Standard Modules}
Python comes with a library of standard modules, described in a separate Python comes with a library of standard modules, described in a separate
document (Python Library and Module Reference). document (Python Library Reference). Some modules are built into the
Some modules are built into the interpreter; these provide access to interpreter; these provide access to operations that are not part of the
operations that are not part of the core of the language but are core of the language but are nevertheless built in, either for
nevertheless built in, either for efficiency or to provide access to efficiency or to provide access to operating system primitives such as
operating system primitives such as system calls. system calls. The set of such modules is a configuration option; e.g.,
The set of such modules is a configuration option; e.g., the the {\tt amoeba} module is only provided on systems that somehow support
{\tt amoeba} Amoeba primitives. One particular module deserves some attention: {\tt
module is only provided on systems that somehow support Amoeba sys}, which is built into every Python interpreter. The variables {\tt
primitives. sys.ps1} and {\tt sys.ps2} define the strings used as primary and
One particular module deserves some attention: secondary prompts:
{\tt sys},
which is built into every Python interpreter.
The variables
{\tt sys.ps1}
and
{\tt sys.ps2}
define the strings used as primary and secondary prompts:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> import sys >>> import sys
>>> sys.ps1 >>> sys.ps1
...@@ -1143,16 +1332,140 @@ You can modify it using standard list operations, e.g.: ...@@ -1143,16 +1332,140 @@ You can modify it using standard list operations, e.g.:
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsection{Errors and Exceptions} \section{The {\tt dir()} function}
Until now error messages haven't yet been mentioned, but if you have The built-in function {\tt dir} is used to find out which names a module
tried out the examples you have probably seen some. defines. It returns a sorted list of strings:
There are (at least) two distinguishable kinds of errors: \bcode\begin{verbatim}
{\em syntax\ errors} >>> import fibo, sys
and >>> dir(fibo)
{\em exceptions}. ['fib', 'fib2']
>>> dir(sys)
['argv', 'exit', 'modules', 'path', 'ps1', 'ps2', 'stderr', 'stdin', 'stdout']
>>>
\end{verbatim}\ecode
Without arguments, {\tt dir()} lists the names you have defined currently:
\bcode\begin{verbatim}
>>> a = [1, 2, 3, 4, 5]
>>> import fibo, sys
>>> fib = fibo.fib
>>> dir()
['a', 'fib', 'fibo', 'sys']
>>>
\end{verbatim}\ecode
Note that it lists all types of names: variables, modules, functions, etc.
{\tt dir()} does not list the names of built-in functions and variables.
If you want a list of those, they are defined in the standard module
{\tt builtin}:
\bcode\begin{verbatim}
>>> import builtin
>>> dir(builtin)
['EOFError', 'KeyboardInterrupt', 'MemoryError', 'NameError', 'None', 'Runti
meError', 'SystemError', 'TypeError', 'abs', 'chr', 'dir', 'divmod', 'eval',
'exec', 'float', 'input', 'int', 'len', 'long', 'max', 'min', 'open', 'ord'
, 'pow', 'range', 'raw_input', 'reload', 'type']
>>>
\end{verbatim}\ecode
\chapter{Output Formatting}
So far we've encountered two ways of writing values: {\em expression
statements} and the {\tt print} statement. (A third way is using the
{\tt write} method of file objects; the standard output file can be
referenced as {\tt sys.stdout}. See the Library Reference for more
information on this.)
Often you'll want more control over the formatting of your output than
simply printing space-separated values. The key to nice formatting in
Python is to do all the string handling yourself; using string slicing
and concatenation operations you can create any lay-out you can imagine.
The standard module {\tt string} contains some useful operations for
padding strings to a given column width; these will be discussed shortly.
One question remains, of course: how do you convert values to strings?
Luckily, Python has a way to convert any value to a string: just write
the value between reverse quotes (\verb/``/). Some examples:
\bcode\begin{verbatim}
>>> x = 10 * 3.14
>>> y = 200*200
>>> s = 'The value of x is ' + `x` + ', and y is ' + `y` + '...'
>>> print s
The value of x is 31.4, and y is 40000...
>>> # Reverse quotes work on other types besides numbers:
>>> p = [x, y]
>>> ps = `p`
>>> ps
'[31.4, 40000]'
>>> # Converting a string adds string quotes and backslashes:
>>> hello = 'hello, world\n'
>>> hellos = `hello`
>>> print hellos
'hello, world\012'
>>> # The argument of reverse quotes may be a tuple:
>>> `x, y, ('foo', 'bar')`
'(31.4, 40000, (\'foo\', \'bar\'))'
>>>
\end{verbatim}\ecode
Here is how you write a table of squares and cubes:
\bcode\begin{verbatim}
>>> import string
>>> for x in range(1, 11):
... print string.rjust(`x`, 2), string.rjust(`x*x`, 3),
... # Note trailing comma on previous line
... print string.rjust(`x*x*x`, 4)
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
>>>
\end{verbatim}\ecode
(Note that one space between each column was added by the way {\tt print}
works: it always adds spaces between its arguments.)
This example demonstrates the function {\tt string.rjust()}, which
right-justifies a string in a field of a given width by padding it with
spaces on the left. There are similar functions {\tt string.ljust()}
and {\tt string.center()}. These functions do not write anything, they
just return a new string. If the input string is too long, they don't
truncate it, but return it unchanged; this will mess up your column
lay-out but that's usually better than the alternative, which would be
lying about a value. (If you really want truncation you can always add
a slice operation, as in {\tt string.ljust(x,~n)[0:n]}.)
There is another function, {\tt string.zfill}, which pads a numeric
string on the left with zeros. It understands about plus and minus
signs:%
\footnote{
Better facilities for formatting floating point numbers are
lacking at this moment.
}
\bcode\begin{verbatim}
>>> string.zfill('12', 5)
'00012'
>>> string.zfill('-3.14', 7)
'-003.14'
>>> string.zfill('3.14159265359', 5)
'3.14159265359'
>>>
\end{verbatim}\ecode
\chapter{Errors and Exceptions}
Until now error messages haven't been more than mentioned, but if you
have tried out the examples you have probably seen some. There are
(at least) two distinguishable kinds of errors: {\em syntax\ errors}
and {\em exceptions}.
\subsubsection{Syntax Errors} \section{Syntax Errors}
Syntax errors, also known as parsing errors, are perhaps the most common Syntax errors, also known as parsing errors, are perhaps the most common
kind of complaint you get while you are still learning Python: kind of complaint you get while you are still learning Python:
...@@ -1173,10 +1486,14 @@ the arrow: in the example, the error is detected at the keyword ...@@ -1173,10 +1486,14 @@ the arrow: in the example, the error is detected at the keyword
File name and line number are printed so you know where to look in case File name and line number are printed so you know where to look in case
the input came from a script. the input came from a script.
\subsubsection{Exceptions} \section{Exceptions}
Even if a statement or expression is syntactically correct, it may cause Even if a statement or expression is syntactically correct, it may
an error when an attempt is made to execute it: cause an error when an attempt is made to execute it.
Errors detected during execution are called {\em exceptions} and are
not unconditionally fatal: you will soon learn how to handle them in
Python programs. Most exceptions are not handled by programs,
however, and result in error messages as shown here:
\bcode\small\begin{verbatim} \bcode\small\begin{verbatim}
>>> 10 * (1/0) >>> 10 * (1/0)
Unhandled exception: run-time error: integer division by zero Unhandled exception: run-time error: integer division by zero
...@@ -1192,12 +1509,6 @@ Stack backtrace (innermost last): ...@@ -1192,12 +1509,6 @@ Stack backtrace (innermost last):
File "<stdin>", line 1 File "<stdin>", line 1
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
Errors detected during execution are called
{\em exceptions}
and are not unconditionally fatal: you will soon learn how to handle
them in Python programs.
Most exceptions are not handled by programs, however, and result
in error messages as shown here.
The first line of the error message indicates what happened. The first line of the error message indicates what happened.
Exceptions come in different types, and the type is printed as part of Exceptions come in different types, and the type is printed as part of
...@@ -1227,25 +1538,24 @@ errors are more serious: these are usually caused by misspelled ...@@ -1227,25 +1538,24 @@ errors are more serious: these are usually caused by misspelled
identifiers.% identifiers.%
\footnote{ \footnote{
The parser does not check whether names used in a program are at The parser does not check whether names used in a program are at
all defined elsewhere in the program, so such checks are all defined elsewhere in the program; such checks are
postponed until run-time. The same holds for type checking. postponed until run-time. The same holds for type checking.
} }
The detail is the offending identifier. The detail is the offending identifier.
\item \item
{\em Type\ errors} {\em Type\ errors} are also pretty serious: this is another case of
are also pretty serious: this is another case of using wrong data (or using wrong data (or better, using data the wrong way), but here the
better, using data the wrong way), but here the error can be glanced error can be gleaned from the object type(s) alone. The detail shows
from the object type(s) alone. in what context the error was detected.
The detail shows in what context the error was detected.
\end{itemize} \end{itemize}
\subsubsection{Handling Exceptions} \section{Handling Exceptions}
It is possible to write programs that handle selected exceptions. It is possible to write programs that handle selected exceptions.
Look at the following example, which prints a table of inverses of Look at the following example, which prints a table of inverses of
some floating point numbers: some floating point numbers:
\bcode\begin{verbatim} \bcode\begin{verbatim}
>>> numbers = [0.3333, 2.5, 0.0, 10.0] >>> numbers = [0.3333, 2.5, 0, 10]
>>> for x in numbers: >>> for x in numbers:
... print x, ... print x,
... try: ... try:
...@@ -1271,10 +1581,11 @@ If no exception occurs, the ...@@ -1271,10 +1581,11 @@ If no exception occurs, the
{\em except\ clause} {\em except\ clause}
is skipped and execution of the {\tt try} statement is finished. is skipped and execution of the {\tt try} statement is finished.
\item \item
If an exception occurs during execution of the try clause, and its If an exception occurs during execution of the try clause,
type matches the exception named after the {\tt except} keyword, the the rest of the clause is skipped. Then if
rest of the try clause is skipped, the except clause is executed, and its type matches the exception named after the {\tt except} keyword,
then execution continues after the {\tt try} statement. the rest of the try clause is skipped, the except clause is executed,
and then execution continues after the {\tt try} statement.
\item \item
If an exception occurs which does not match the exception named in the If an exception occurs which does not match the exception named in the
except clause, it is passed on to outer try statements; if no handler is except clause, it is passed on to outer try statements; if no handler is
...@@ -1320,12 +1631,7 @@ Standard exception names are built-in identifiers (not reserved ...@@ -1320,12 +1631,7 @@ Standard exception names are built-in identifiers (not reserved
keywords). keywords).
These are in fact string objects whose These are in fact string objects whose
{\em object\ identity} {\em object\ identity}
(not their value!) identifies the exceptions.% (not their value!) identifies the exceptions.
\footnote{
There should really be a separate exception type; it is pure
laziness that exceptions are identified by strings, and this may
be fixed in the future.
}
The string is printed as the second part of the message for unhandled The string is printed as the second part of the message for unhandled
exceptions. exceptions.
Their names and values are: Their names and values are:
...@@ -1358,7 +1664,7 @@ Handling run-time error: integer division by zero ...@@ -1358,7 +1664,7 @@ Handling run-time error: integer division by zero
>>> >>>
\end{verbatim}\ecode \end{verbatim}\ecode
\subsubsection{Raising Exceptions} \section{Raising Exceptions}
The {\tt raise} statement allows the programmer to force a specified The {\tt raise} statement allows the programmer to force a specified
exception to occur. exception to occur.
...@@ -1373,7 +1679,7 @@ Stack backtrace (innermost last): ...@@ -1373,7 +1679,7 @@ Stack backtrace (innermost last):
The first argument to {\tt raise} names the exception to be raised. The first argument to {\tt raise} names the exception to be raised.
The optional second argument specifies the exception's argument. The optional second argument specifies the exception's argument.
\subsubsection{User-defined Exceptions} \section{User-defined Exceptions}
Programs may name their own exceptions by assigning a string to a Programs may name their own exceptions by assigning a string to a
variable. variable.
...@@ -1395,7 +1701,7 @@ Stack backtrace (innermost last): ...@@ -1395,7 +1701,7 @@ Stack backtrace (innermost last):
Many standard modules use this to report errors that may occur in Many standard modules use this to report errors that may occur in
functions they define. functions they define.
\subsubsection{Defining Clean-up Actions} \section{Defining Clean-up Actions}
The {\tt try} statement has another optional clause which is intended to The {\tt try} statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances. define clean-up actions that must be executed under all circumstances.
...@@ -1415,141 +1721,11 @@ Stack backtrace (innermost last): ...@@ -1415,141 +1721,11 @@ Stack backtrace (innermost last):
The The
{\em finally\ clause} {\em finally\ clause}
must follow the except clauses(s), if any. must follow the except clauses(s), if any.
It is executed whether or not an exception occurred. It is executed whether or not an exception occurred,
or whether or not an exception is handled.
If the exception is handled, the finally clause is executed after the If the exception is handled, the finally clause is executed after the
handler (and even if another exception occurred in the handler). handler (and even if another exception occurred in the handler).
It is also executed when the {\tt try} statement is left via a It is also executed when the {\tt try} statement is left via a
{\tt break} or {\tt return} statement. {\tt break} or {\tt return} statement.
\subsection{Classes}
Classes in Python make it possible to play the game of encapsulation in a
somewhat different way than it is played with modules.
Classes are an advanced topic and are probably best skipped on the first
encounter with Python.
\subsubsection{Prologue}
Python's class mechanism is not particularly elegant, but quite powerful.
It is a mixture of the class mechanisms found in C++ and Modula-3.
As is true for modules, classes in Python do not put an absolute barrier
between definition and user, but rather rely on the politeness of the
user not to ``break into the definition.''
The most important features of classes are retained with full power,
however: the class inheritance mechanism allows multiple base classes,
a derived class can override any method of its base class(es), a method
can call the method of a base class with the same name.
Objects can contain an arbitrary amount of private data.
In C++ terminology, all class members (including data members) are
{\em public},
and all member functions (methods) are
{\em virtual}.
There are no special constructors or destructors.
As in Modula-3, there are no shorthands for referencing the object's
members from its methods: the method function is declared with an
explicit first argument representing the object, which is provided
implicitly by the call.
As in Smalltalk, classes themselves are objects, albeit in the wider
sense of the word: in Python, all data types are objects.
This provides semantics for renaming or aliasing.
But, just like in C++ or Modula-3, the built-in types cannot be used as
base classes for extension by the user.
Also, like Modula-3 but unlike C++, the built-in operators with special
syntax (arithmetic operators, subscripting etc.) cannot be redefined for
class members.%
\footnote{
They can be redefined for new object types implemented in C in
extensions to the interpreter, however. It would require only a
naming convention and a relatively small change to the
interpreter to allow operator overloading for classes, so
perhaps someday...
}
\subsubsection{A Simple Example}
Consider the following example, which defines a class {\tt Set}
representing a (finite) mathematical set with operations to add and
remove elements, a membership test, and a request for the size of the
set.
\bcode\begin{verbatim}
class Set():
def new(self):
self.elements = []
return self
def add(self, e):
if e not in self.elements:
self.elements.append(e)
def remove(self, e):
if e in self.elements:
for i in range(len(self.elements)):
if self.elements[i] = e:
del self.elements[i]
break
def is_element(self, e):
return e in self.elements
def size(self):
return len(self.elements)
\end{verbatim}\ecode
Note that the class definition looks like a big compound statement,
with all the function definitons indented repective to the
{\tt class}
keyword.
Let's assume that this
{\em class\ definition}
is the only contents of the module file
{\tt SetClass.py}.
We can then use it in a Python program as follows:
\bcode\begin{verbatim}
>>> from SetClass import Set
>>> a = Set().new() # create a Set object
>>> a.add(2)
>>> a.add(3)
>>> a.add(1)
>>> a.add(1)
>>> if a.is_element(3): print '3 is in the set'
...
3 is in the set
>>> if not a.is_element(4): print '4 is not in the set'
...
4 is not in the set
>>> print 'a has', a.size(), 'elements'
a has 3 elements
>>> a.remove(1)
>>> print 'now a has', a.size(), 'elements'
>>>
now a has 2 elements
>>>
\end{verbatim}\ecode
From the example we learn in the first place that the functions defined
in the class (e.g.,
{\tt add})
can be called using the
{\em member}
notation for the object
{\tt a}.
The member function is called with one less argument than it is defined:
the object is implicitly passed as the first argument.
Thus, the call
{\tt a.add(2)}
is equivalent to
{\tt Set.add(a, 2)}.
XXX This section is not complete yet! Inheritance!
\section{XXX P.M.}
\begin{itemize}
\item The {\tt del} statement.
\item The {\tt dir()} function.
\item Tuples.
\item Dictionaries.
\item Objects and types in general.
\item Backquotes.
\item Output formatting.
\item And/Or/Not.
\item ``.pyc'' files.
\end{itemize}
\end{document} \end{document}
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