Initial revision

Doc/ref/ref1.tex

+\chapter{Introduction}
+
+This reference manual describes the Python programming language.
+It is not intended as a tutorial.
+
+While I am trying to be as precise as possible, I chose to use English
+rather than formal specifications for everything except syntax and
+lexical analysis.  This should make the document better understandable
+to the average reader, but will leave room for ambiguities.
+Consequently, if you were coming from Mars and tried to re-implement
+Python from this document alone, you might have to guess things and in
+fact you would probably end up implementing quite a different language.
+On the other hand, if you are using
+Python and wonder what the precise rules about a particular area of
+the language are, you should definitely be able to find them here.
+
+It is dangerous to add too many implementation details to a language
+reference document --- the implementation may change, and other
+implementations of the same language may work differently.  On the
+other hand, there is currently only one Python implementation, and
+its particular quirks are sometimes worth being mentioned, especially
+where the implementation imposes additional limitations.  Therefore,
+you'll find short ``implementation notes'' sprinkled throughout the
+text.
+
+Every Python implementation comes with a number of built-in and
+standard modules.  These are not documented here, but in the separate
+{\em Python Library Reference} document.  A few built-in modules are
+mentioned when they interact in a significant way with the language
+definition.
+
+\section{Notation}
+
+The descriptions of lexical analysis and syntax use a modified BNF
+grammar notation.  This uses the following style of definition:
+\index{BNF}
+\index{grammar}
+\index{syntax}
+\index{notation}
+
+\begin{verbatim}
+name:           lc_letter (lc_letter | "_")*
+lc_letter:      "a"..."z"
+\end{verbatim}
+
+The first line says that a \verb\name\ is an \verb\lc_letter\ followed by
+a sequence of zero or more \verb\lc_letter\s and underscores.  An
+\verb\lc_letter\ in turn is any of the single characters `a' through `z'.
+(This rule is actually adhered to for the names defined in lexical and
+grammar rules in this document.)
+
+Each rule begins with a name (which is the name defined by the rule)
+and a colon.  A vertical bar (\verb\|\) is used to separate
+alternatives; it is the least binding operator in this notation.  A
+star (\verb\*\) means zero or more repetitions of the preceding item;
+likewise, a plus (\verb\+\) means one or more repetitions, and a
+phrase enclosed in square brackets (\verb\[ ]\) means zero or one
+occurrences (in other words, the enclosed phrase is optional).  The
+\verb\*\ and \verb\+\ operators bind as tightly as possible;
+parentheses are used for grouping.  Literal strings are enclosed in
+double quotes.  White space is only meaningful to separate tokens.
+Rules are normally contained on a single line; rules with many
+alternatives may be formatted alternatively with each line after the
+first beginning with a vertical bar.
+
+In lexical definitions (as the example above), two more conventions
+are used: Two literal characters separated by three dots mean a choice
+of any single character in the given (inclusive) range of ASCII
+characters.  A phrase between angular brackets (\verb\<...>\) gives an
+informal description of the symbol defined; e.g. this could be used
+to describe the notion of `control character' if needed.
+\index{lexical definitions}
+\index{ASCII}
+
+Even though the notation used is almost the same, there is a big
+difference between the meaning of lexical and syntactic definitions:
+a lexical definition operates on the individual characters of the
+input source, while a syntax definition operates on the stream of
+tokens generated by the lexical analysis.  All uses of BNF in the next
+chapter (``Lexical Analysis'') are lexical definitions; uses in
+subsequent chapters are syntactic definitions.

Doc/ref/ref2.tex

+\chapter{Lexical analysis}
+
+A Python program is read by a {\em parser}.  Input to the parser is a
+stream of {\em tokens}, generated by the {\em lexical analyzer}.  This
+chapter describes how the lexical analyzer breaks a file into tokens.
+\index{lexical analysis}
+\index{parser}
+\index{token}
+
+\section{Line structure}
+
+A Python program is divided in a number of logical lines.  The end of
+a logical line is represented by the token NEWLINE.  Statements cannot
+cross logical line boundaries except where NEWLINE is allowed by the
+syntax (e.g. between statements in compound statements).
+\index{line structure}
+\index{logical line}
+\index{NEWLINE token}
+
+\subsection{Comments}
+
+A comment starts with a hash character (\verb\#\) that is not part of
+a string literal, and ends at the end of the physical line.  A comment
+always signifies the end of the logical line.  Comments are ignored by
+the syntax.
+\index{comment}
+\index{logical line}
+\index{physical line}
+\index{hash character}
+
+\subsection{Line joining}
+
+Two or more physical lines may be joined into logical lines using
+backslash characters (\verb/\/), as follows: when a physical line ends
+in a backslash that is not part of a string literal or comment, it is
+joined with the following forming a single logical line, deleting the
+backslash and the following end-of-line character.  For example:
+\index{physical line}
+\index{line joining}
+\index{backslash character}
+%
+\begin{verbatim}
+month_names = ['Januari', 'Februari', 'Maart',     \
+               'April',   'Mei',      'Juni',      \
+               'Juli',    'Augustus', 'September', \
+               'Oktober', 'November', 'December']
+\end{verbatim}
+
+\subsection{Blank lines}
+
+A logical line that contains only spaces, tabs, and possibly a
+comment, is ignored (i.e., no NEWLINE token is generated), except that
+during interactive input of statements, an entirely blank logical line
+terminates a multi-line statement.
+\index{blank line}
+
+\subsection{Indentation}
+
+Leading whitespace (spaces and tabs) at the beginning of a logical
+line is used to compute the indentation level of the line, which in
+turn is used to determine the grouping of statements.
+\index{indentation}
+\index{whitespace}
+\index{leading whitespace}
+\index{space}
+\index{tab}
+\index{grouping}
+\index{statement grouping}
+
+First, tabs are replaced (from left to right) by one to eight spaces
+such that the total number of characters up to there is a multiple of
+eight (this is intended to be the same rule as used by {\UNIX}).  The
+total number of spaces preceding the first non-blank character then
+determines the line's indentation.  Indentation cannot be split over
+multiple physical lines using backslashes.
+
+The indentation levels of consecutive lines are used to generate
+INDENT and DEDENT tokens, using a stack, as follows.
+\index{INDENT token}
+\index{DEDENT token}
+
+Before the first line of the file is read, a single zero is pushed on
+the stack; this will never be popped off again.  The numbers pushed on
+the stack will always be strictly increasing from bottom to top.  At
+the beginning of each logical line, the line's indentation level is
+compared to the top of the stack.  If it is equal, nothing happens.
+If it is larger, it is pushed on the stack, and one INDENT token is
+generated.  If it is smaller, it {\em must} be one of the numbers
+occurring on the stack; all numbers on the stack that are larger are
+popped off, and for each number popped off a DEDENT token is
+generated.  At the end of the file, a DEDENT token is generated for
+each number remaining on the stack that is larger than zero.
+
+Here is an example of a correctly (though confusingly) indented piece
+of Python code:
+
+\begin{verbatim}
+def perm(l):
+        # Compute the list of all permutations of l
+
+    if len(l) <= 1:
+                  return [l]
+    r = []
+    for i in range(len(l)):
+             s = l[:i] + l[i+1:]
+             p = perm(s)
+             for x in p:
+              r.append(l[i:i+1] + x)
+    return r
+\end{verbatim}
+
+The following example shows various indentation errors:
+
+\begin{verbatim}
+    def perm(l):                        # error: first line indented
+    for i in range(len(l)):             # error: not indented
+        s = l[:i] + l[i+1:]
+            p = perm(l[:i] + l[i+1:])   # error: unexpected indent
+            for x in p:
+                    r.append(l[i:i+1] + x)
+                return r                # error: inconsistent dedent
+\end{verbatim}
+
+(Actually, the first three errors are detected by the parser; only the
+last error is found by the lexical analyzer --- the indentation of
+\verb\return r\ does not match a level popped off the stack.)
+
+\section{Other tokens}
+
+Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
+exist: identifiers, keywords, literals, operators, and delimiters.
+Spaces and tabs are not tokens, but serve to delimit tokens.  Where
+ambiguity exists, a token comprises the longest possible string that
+forms a legal token, when read from left to right.
+
+\section{Identifiers}
+
+Identifiers (also referred to as names) are described by the following
+lexical definitions:
+\index{identifier}
+\index{name}
+
+\begin{verbatim}
+identifier:     (letter|"_") (letter|digit|"_")*
+letter:         lowercase | uppercase
+lowercase:      "a"..."z"
+uppercase:      "A"..."Z"
+digit:          "0"..."9"
+\end{verbatim}
+
+Identifiers are unlimited in length.  Case is significant.
+
+\subsection{Keywords}
+
+The following identifiers are used as reserved words, or {\em
+keywords} of the language, and cannot be used as ordinary
+identifiers.  They must be spelled exactly as written here:
+\index{keyword}
+\index{reserved word}
+
+\begin{verbatim}
+and        del        for        in         print
+break      elif       from       is         raise
+class      else       global     not        return
+continue   except     if         or         try
+def        finally    import     pass       while
+\end{verbatim}
+
+%	# This Python program sorts and formats the above table
+%	import string
+%	l = []
+%	try:
+%		while 1:
+%			l = l + string.split(raw_input())
+%	except EOFError:
+%		pass
+%	l.sort()
+%	for i in range((len(l)+4)/5):
+%		for j in range(i, len(l), 5):
+%			print string.ljust(l[j], 10),
+%		print
+
+\section{Literals} \label{literals}
+
+Literals are notations for constant values of some built-in types.
+\index{literal}
+\index{constant}
+
+\subsection{String literals}
+
+String literals are described by the following lexical definitions:
+\index{string literal}
+
+\begin{verbatim}
+stringliteral:  "'" stringitem* "'"
+stringitem:     stringchar | escapeseq
+stringchar:     <any ASCII character except newline or "\" or "'">
+escapeseq:      "'" <any ASCII character except newline>
+\end{verbatim}
+\index{ASCII}
+
+String literals cannot span physical line boundaries.  Escape
+sequences in strings are actually interpreted according to rules
+similar to those used by Standard C.  The recognized escape sequences
+are:
+\index{physical line}
+\index{escape sequence}
+\index{Standard C}
+\index{C}
+
+\begin{center}
+\begin{tabular}{|l|l|}
+\hline
+\verb/\\/	& Backslash (\verb/\/) \\
+\verb/\'/	& Single quote (\verb/'/) \\
+\verb/\a/	& ASCII Bell (BEL) \\
+\verb/\b/	& ASCII Backspace (BS) \\
+%\verb/\E/	& ASCII Escape (ESC) \\
+\verb/\f/	& ASCII Formfeed (FF) \\
+\verb/\n/	& ASCII Linefeed (LF) \\
+\verb/\r/	& ASCII Carriage Return (CR) \\
+\verb/\t/	& ASCII Horizontal Tab (TAB) \\
+\verb/\v/	& ASCII Vertical Tab (VT) \\
+\verb/\/{\em ooo}	& ASCII character with octal value {\em ooo} \\
+\verb/\x/{\em xx...}	& ASCII character with hex value {\em xx...} \\
+\hline
+\end{tabular}
+\end{center}
+\index{ASCII}
+
+In strict compatibility with Standard C, up to three octal digits are
+accepted, but an unlimited number of hex digits is taken to be part of
+the hex escape (and then the lower 8 bits of the resulting hex number
+are used in all current implementations...).
+
+All unrecognized escape sequences are left in the string unchanged,
+i.e., {\em the backslash is left in the string.}  (This behavior is
+useful when debugging: if an escape sequence is mistyped, the
+resulting output is more easily recognized as broken.  It also helps a
+great deal for string literals used as regular expressions or
+otherwise passed to other modules that do their own escape handling.)
+\index{unrecognized escape sequence}
+
+\subsection{Numeric literals}
+
+There are three types of numeric literals: plain integers, long
+integers, and floating point numbers.
+\index{number}
+\index{numeric literal}
+\index{integer literal}
+\index{plain integer literal}
+\index{long integer literal}
+\index{floating point literal}
+\index{hexadecimal literal}
+\index{octal literal}
+\index{decimal literal}
+
+Integer and long integer literals are described by the following
+lexical definitions:
+
+\begin{verbatim}
+longinteger:    integer ("l"|"L")
+integer:        decimalinteger | octinteger | hexinteger
+decimalinteger: nonzerodigit digit* | "0"
+octinteger:     "0" octdigit+
+hexinteger:     "0" ("x"|"X") hexdigit+
+
+nonzerodigit:   "1"..."9"
+octdigit:       "0"..."7"
+hexdigit:        digit|"a"..."f"|"A"..."F"
+\end{verbatim}
+
+Although both lower case `l' and upper case `L' are allowed as suffix
+for long integers, it is strongly recommended to always use `L', since
+the letter `l' looks too much like the digit `1'.
+
+Plain integer decimal literals must be at most $2^{31} - 1$ (i.e., the
+largest positive integer, assuming 32-bit arithmetic).  Plain octal and
+hexadecimal literals may be as large as $2^{32} - 1$, but values
+larger than $2^{31} - 1$ are converted to a negative value by
+subtracting $2^{32}$.  There is no limit for long integer literals.
+
+Some examples of plain and long integer literals:
+
+\begin{verbatim}
+7     2147483647                        0177    0x80000000
+3L    79228162514264337593543950336L    0377L   0x100000000L
+\end{verbatim}
+
+Floating point literals are described by the following lexical
+definitions:
+
+\begin{verbatim}
+floatnumber:    pointfloat | exponentfloat
+pointfloat:     [intpart] fraction | intpart "."
+exponentfloat:  (intpart | pointfloat) exponent
+intpart:        digit+
+fraction:       "." digit+
+exponent:       ("e"|"E") ["+"|"-"] digit+
+\end{verbatim}
+
+The allowed range of floating point literals is
+implementation-dependent.
+
+Some examples of floating point literals:
+
+\begin{verbatim}
+3.14    10.    .001    1e100    3.14e-10
+\end{verbatim}
+
+Note that numeric literals do not include a sign; a phrase like
+\verb\-1\ is actually an expression composed of the operator
+\verb\-\ and the literal \verb\1\.
+
+\section{Operators}
+
+The following tokens are operators:
+\index{operators}
+
+\begin{verbatim}
++       -       *       /       %
+<<      >>      &       |       ^       ~
+<       ==      >       <=      <>      !=      >=
+\end{verbatim}
+
+The comparison operators \verb\<>\ and \verb\!=\ are alternate
+spellings of the same operator.
+
+\section{Delimiters}
+
+The following tokens serve as delimiters or otherwise have a special
+meaning:
+\index{delimiters}
+
+\begin{verbatim}
+(       )       [       ]       {       }
+;       ,       :       .       `       =
+\end{verbatim}
+
+The following printing ASCII characters are not used in Python.  Their
+occurrence outside string literals and comments is an unconditional
+error:
+\index{ASCII}
+
+\begin{verbatim}
+@       $       "       ?
+\end{verbatim}
+
+They may be used by future versions of the language though!

Doc/ref/ref3.tex

+\chapter{Data model}
+
+\section{Objects, values and types}
+
+{\em Objects} are Python's abstraction for data.  All data in a Python
+program is represented by objects or by relations between objects.
+(In a sense, and in conformance to Von Neumann's model of a
+``stored program computer'', code is also represented by objects.)
+\index{object}
+\index{data}
+
+Every object has an identity, a type and a value.  An object's {\em
+identity} never changes once it has been created; you may think of it
+as the object's address in memory.  An object's {\em type} is also
+unchangeable.  It determines the operations that an object supports
+(e.g. ``does it have a length?'') and also defines the possible
+values for objects of that type.  The {\em value} of some objects can
+change.  Objects whose value can change are said to be {\em mutable};
+objects whose value is unchangeable once they are created are called
+{\em immutable}.  The type determines an object's (im)mutability.
+\index{identity of an object}
+\index{value of an object}
+\index{type of an object}
+\index{mutable object}
+\index{immutable object}
+
+Objects are never explicitly destroyed; however, when they become
+unreachable they may be garbage-collected.  An implementation is
+allowed to delay garbage collection or omit it altogether --- it is a
+matter of implementation quality how garbage collection is
+implemented, as long as no objects are collected that are still
+reachable.  (Implementation note: the current implementation uses a
+reference-counting scheme which collects most objects as soon as they
+become unreachable, but never collects garbage containing circular
+references.)
+\index{garbage collection}
+\index{reference counting}
+\index{unreachable object}
+
+Note that the use of the implementation's tracing or debugging
+facilities may keep objects alive that would normally be collectable.
+
+Some objects contain references to ``external'' resources such as open
+files or windows.  It is understood that these resources are freed
+when the object is garbage-collected, but since garbage collection is
+not guaranteed to happen, such objects also provide an explicit way to
+release the external resource, usually a \verb\close\ method.
+Programs are strongly recommended to always explicitly close such
+objects.
+
+Some objects contain references to other objects; these are called
+{\em containers}.  Examples of containers are tuples, lists and
+dictionaries.  The references are part of a container's value.  In
+most cases, when we talk about the value of a container, we imply the
+values, not the identities of the contained objects; however, when we
+talk about the (im)mutability of a container, only the identities of
+the immediately contained objects are implied.  (So, if an immutable
+container contains a reference to a mutable object, its value changes
+if that mutable object is changed.)
+\index{container}
+
+Types affect almost all aspects of objects' lives.  Even the meaning
+of object identity is affected in some sense: for immutable types,
+operations that compute new values may actually return a reference to
+any existing object with the same type and value, while for mutable
+objects this is not allowed.  E.g. after
+
+\begin{verbatim}
+a = 1; b = 1; c = []; d = []
+\end{verbatim}
+
+\verb\a\ and \verb\b\ may or may not refer to the same object with the
+value one, depending on the implementation, but \verb\c\ and \verb\d\
+are guaranteed to refer to two different, unique, newly created empty
+lists.
+
+\section{The standard type hierarchy} \label{types}
+
+Below is a list of the types that are built into Python.  Extension
+modules written in C can define additional types.  Future versions of
+Python may add types to the type hierarchy (e.g. rational or complex
+numbers, efficiently stored arrays of integers, etc.).
+\index{type}
+\indexii{data}{type}
+\indexii{type}{hierarchy}
+\indexii{extension}{module}
+\index{C}
+
+Some of the type descriptions below contain a paragraph listing
+`special attributes'.  These are attributes that provide access to the
+implementation and are not intended for general use.  Their definition
+may change in the future.  There are also some `generic' special
+attributes, not listed with the individual objects: \verb\__methods__\
+is a list of the method names of a built-in object, if it has any;
+\verb\__members__\ is a list of the data attribute names of a built-in
+object, if it has any.
+\index{attribute}
+\indexii{special}{attribute}
+\indexiii{generic}{special}{attribute}
+\ttindex{__methods__}
+\ttindex{__members__}
+
+\begin{description}
+
+\item[None]
+This type has a single value.  There is a single object with this value.
+This object is accessed through the built-in name \verb\None\.
+It is returned from functions that don't explicitly return an object.
+\ttindex{None}
+\obindex{None@{\tt None}}
+
+\item[Numbers]
+These are created by numeric literals and returned as results by
+arithmetic operators and arithmetic built-in functions.  Numeric
+objects are immutable; once created their value never changes.  Python
+numbers are of course strongly related to mathematical numbers, but
+subject to the limitations of numerical representation in computers.
+\obindex{number}
+\obindex{numeric}
+
+Python distinguishes between integers and floating point numbers:
+
+\begin{description}
+\item[Integers]
+These represent elements from the mathematical set of whole numbers.
+\obindex{integer}
+
+There are two types of integers:
+
+\begin{description}
+
+\item[Plain integers]
+These represent numbers in the range $-2^{31}$ through $2^{31}-1$.
+(The range may be larger on machines with a larger natural word
+size, but not smaller.)
+When the result of an operation falls outside this range, the
+exception \verb\OverflowError\ is raised.
+For the purpose of shift and mask operations, integers are assumed to
+have a binary, 2's complement notation using 32 or more bits, and
+hiding no bits from the user (i.e., all $2^{32}$ different bit
+patterns correspond to different values).
+\obindex{plain integer}
+
+\item[Long integers]
+These represent numbers in an unlimited range, subject to available
+(virtual) memory only.  For the purpose of shift and mask operations,
+a binary representation is assumed, and negative numbers are
+represented in a variant of 2's complement which gives the illusion of
+an infinite string of sign bits extending to the left.
+\obindex{long integer}
+
+\end{description} % Integers
+
+The rules for integer representation are intended to give the most
+meaningful interpretation of shift and mask operations involving
+negative integers and the least surprises when switching between the
+plain and long integer domains.  For any operation except left shift,
+if it yields a result in the plain integer domain without causing
+overflow, it will yield the same result in the long integer domain or
+when using mixed operands.
+\indexii{integer}{representation}
+
+\item[Floating point numbers]
+These represent machine-level double precision floating point numbers.  
+You are at the mercy of the underlying machine architecture and
+C implementation for the accepted range and handling of overflow.
+\obindex{floating point}
+\indexii{floating point}{number}
+\index{C}
+
+\end{description} % Numbers
+
+\item[Sequences]
+These represent finite ordered sets indexed by natural numbers.
+The built-in function \verb\len()\ returns the number of elements
+of a sequence.  When this number is $n$, the index set contains
+the numbers $0, 1, \ldots, n-1$.  Element \verb\i\ of sequence
+\verb\a\ is selected by \verb\a[i]\.
+\obindex{seqence}
+\bifuncindex{len}
+\index{index operation}
+\index{item selection}
+\index{subscription}
+
+Sequences also support slicing: \verb\a[i:j]\ selects all elements
+with index $k$ such that $i <= k < j$.  When used as an expression,
+a slice is a sequence of the same type --- this implies that the
+index set is renumbered so that it starts at 0 again.
+\index{slicing}
+
+Sequences are distinguished according to their mutability:
+
+\begin{description}
+%
+\item[Immutable sequences]
+An object of an immutable sequence type cannot change once it is
+created.  (If the object contains references to other objects,
+these other objects may be mutable and may be changed; however
+the collection of objects directly referenced by an immutable object
+cannot change.)
+\obindex{immutable sequence}
+\obindex{immutable}
+
+The following types are immutable sequences:
+
+\begin{description}
+
+\item[Strings]
+The elements of a string are characters.  There is no separate
+character type; a character is represented by a string of one element.
+Characters represent (at least) 8-bit bytes.  The built-in
+functions \verb\chr()\ and \verb\ord()\ convert between characters
+and nonnegative integers representing the byte values.
+Bytes with the values 0-127 represent the corresponding ASCII values.
+The string data type is also used to represent arrays of bytes, e.g.
+to hold data read from a file.
+\obindex{string}
+\index{character}
+\index{byte}
+\index{ASCII}
+\bifuncindex{chr}
+\bifuncindex{ord}
+
+(On systems whose native character set is not ASCII, strings may use
+EBCDIC in their internal representation, provided the functions
+\verb\chr()\ and \verb\ord()\ implement a mapping between ASCII and
+EBCDIC, and string comparison preserves the ASCII order.
+Or perhaps someone can propose a better rule?)
+\index{ASCII}
+\index{EBCDIC}
+\index{character set}
+\indexii{string}{comparison}
+\bifuncindex{chr}
+\bifuncindex{ord}
+
+\item[Tuples]
+The elements of a tuple are arbitrary Python objects.
+Tuples of two or more elements are formed by comma-separated lists
+of expressions.  A tuple of one element (a `singleton') can be formed
+by affixing a comma to an expression (an expression by itself does
+not create a tuple, since parentheses must be usable for grouping of
+expressions).  An empty tuple can be formed by enclosing `nothing' in
+parentheses.
+\obindex{tuple}
+\indexii{singleton}{tuple}
+\indexii{empty}{tuple}
+
+\end{description} % Immutable sequences
+
+\item[Mutable sequences]
+Mutable sequences can be changed after they are created.  The
+subscription and slicing notations can be used as the target of
+assignment and \verb\del\ (delete) statements.
+\obindex{mutable sequece}
+\obindex{mutable}
+\indexii{assignment}{statement}
+\index{delete}
+\stindex{del}
+\index{subscription}
+\index{slicing}
+
+There is currently a single mutable sequence type:
+
+\begin{description}
+
+\item[Lists]
+The elements of a list are arbitrary Python objects.  Lists are formed
+by placing a comma-separated list of expressions in square brackets.
+(Note that there are no special cases needed to form lists of length 0
+or 1.)
+\obindex{list}
+
+\end{description} % Mutable sequences
+
+\end{description} % Sequences
+
+\item[Mapping types]
+These represent finite sets of objects indexed by arbitrary index sets.
+The subscript notation \verb\a[k]\ selects the element indexed
+by \verb\k\ from the mapping \verb\a\; this can be used in
+expressions and as the target of assignments or \verb\del\ statements.
+The built-in function \verb\len()\ returns the number of elements
+in a mapping.
+\bifuncindex{len}
+\index{subscription}
+\obindex{mapping}
+
+There is currently a single mapping type:
+
+\begin{description}
+
+\item[Dictionaries]
+These represent finite sets of objects indexed by strings.
+Dictionaries are mutable; they are created by the \verb\{...}\
+notation (see section \ref{dict}).  (Implementation note: the strings
+used for indexing must not contain null bytes.)
+\obindex{dictionary}
+\obindex{mutable}
+
+\end{description} % Mapping types
+
+\item[Callable types]
+These are the types to which the function call (invocation) operation,
+written as \verb\function(argument, argument, ...)\, can be applied:
+\indexii{function}{call}
+\index{invocation}
+\indexii{function}{argument}
+\obindex{callable}
+
+\begin{description}
+
+\item[User-defined functions]
+A user-defined function object is created by a function definition
+(see section \ref{function}).  It should be called with an argument
+list containing the same number of items as the function's formal
+parameter list.
+\indexii{user-defined}{function}
+\obindex{function}
+\obindex{user-defined function}
+
+Special read-only attributes: \verb\func_code\ is the code object
+representing the compiled function body, and \verb\func_globals\ is (a
+reference to) the dictionary that holds the function's global
+variables --- it implements the global name space of the module in
+which the function was defined.
+\ttindex{func_code}
+\ttindex{func_globals}
+\indexii{global}{name space}
+
+\item[User-defined methods]
+A user-defined method (a.k.a. {\em object closure}) is a pair of a
+class instance object and a user-defined function.  It should be
+called with an argument list containing one item less than the number
+of items in the function's formal parameter list.  When called, the
+class instance becomes the first argument, and the call arguments are
+shifted one to the right.
+\obindex{method}
+\obindex{user-defined method}
+\indexii{user-defined}{method}
+\index{object closure}
+
+Special read-only attributes: \verb\im_self\ is the class instance
+object, \verb\im_func\ is the function object.
+\ttindex{im_func}
+\ttindex{im_self}
+
+\item[Built-in functions]
+A built-in function object is a wrapper around a C function.  Examples
+of built-in functions are \verb\len\ and \verb\math.sin\.  There
+are no special attributes.  The number and type of the arguments are
+determined by the C function.
+\obindex{built-in function}
+\obindex{function}
+\index{C}
+
+\item[Built-in methods]
+This is really a different disguise of a built-in function, this time
+containing an object passed to the C function as an implicit extra
+argument.  An example of a built-in method is \verb\list.append\ if
+\verb\list\ is a list object.
+\obindex{built-in method}
+\obindex{method}
+\indexii{built-in}{method}
+
+\item[Classes]
+Class objects are described below.  When a class object is called as a
+parameterless function, a new class instance (also described below) is
+created and returned.  The class's initialization function is not
+called --- this is the responsibility of the caller.  It is illegal to
+call a class object with one or more arguments.
+\obindex{class}
+\obindex{class instance}
+\obindex{instance}
+\indexii{class object}{call}
+
+\end{description}
+
+\item[Modules]
+Modules are imported by the \verb\import\ statement (see section
+\ref{import}).  A module object is a container for a module's name
+space, which is a dictionary (the same dictionary as referenced by the
+\verb\func_globals\ attribute of functions defined in the module).
+Module attribute references are translated to lookups in this
+dictionary.  A module object does not contain the code object used to
+initialize the module (since it isn't needed once the initialization
+is done).
+\stindex{import}
+\obindex{module}
+
+Attribute assignment update the module's name space dictionary.
+
+Special read-only attributes: \verb\__dict__\ yields the module's name
+space as a dictionary object; \verb\__name__\ yields the module's name
+as a string object.
+\ttindex{__dict__}
+\ttindex{__name__}
+\indexii{module}{name space}
+
+\item[Classes]
+Class objects are created by class definitions (see section
+\ref{class}).  A class is a container for a dictionary containing the
+class's name space.  Class attribute references are translated to
+lookups in this dictionary.  When an attribute name is not found
+there, the attribute search continues in the base classes.  The search
+is depth-first, left-to-right in the order of their occurrence in the
+base class list.
+\obindex{class}
+\obindex{class instance}
+\obindex{instance}
+\indexii{class object}{call}
+\index{container}
+\index{dictionary}
+\indexii{class}{attribute}
+
+Class attribute assignments update the class's dictionary, never the
+dictionary of a base class.
+\indexiii{class}{attribute}{assignment}
+
+A class can be called as a parameterless function to yield a class
+instance (see above).
+\indexii{class object}{call}
+
+Special read-only attributes: \verb\__dict__\ yields the dictionary
+containing the class's name space; \verb\__bases__\ yields a tuple
+(possibly empty or a singleton) containing the base classes, in the
+order of their occurrence in the base class list.
+\ttindex{__dict__}
+\ttindex{__bases__}
+
+\item[Class instances]
+A class instance is created by calling a class object as a
+parameterless function.  A class instance has a dictionary in which
+attribute references are searched.  When an attribute is not found
+there, and the instance's class has an attribute by that name, and
+that class attribute is a user-defined function (and in no other
+cases), the instance attribute reference yields a user-defined method
+object (see above) constructed from the instance and the function.
+\obindex{class instance}
+\obindex{instance}
+\indexii{class}{instance}
+\indexii{class instance}{attribute}
+
+Attribute assignments update the instance's dictionary.
+\indexiii{class instance}{attribute}{assignment}
+
+Class instances can pretend to be numbers, sequences, or mappings if
+they have methods with certain special names.  These are described in
+section \ref{specialnames}.
+\obindex{number}
+\obindex{sequence}
+\obindex{mapping}
+
+Special read-only attributes: \verb\__dict__\ yields the attribute
+dictionary; \verb\__class__\ yields the instance's class.
+\ttindex{__dict__}
+\ttindex{__class__}
+
+\item[Files]
+A file object represents an open file.  (It is a wrapper around a C
+{\tt stdio} file pointer.)  File objects are created by the
+\verb\open()\ built-in function, and also by \verb\posix.popen()\ and
+the \verb\makefile\ method of socket objects.  \verb\sys.stdin\,
+\verb\sys.stdout\ and \verb\sys.stderr\ are file objects corresponding
+the the interpreter's standard input, output and error streams.
+See the Python Library Reference for methods of file objects and other
+details.
+\obindex{file}
+\index{C}
+\index{stdio}
+\bifuncindex{open}
+\bifuncindex{popen}
+\bifuncindex{makefile}
+\ttindex{stdin}
+\ttindex{stdout}
+\ttindex{stderr}
+\ttindex{sys.stdin}
+\ttindex{sys.stdout}
+\ttindex{sys.stderr}
+
+\item[Internal types]
+A few types used internally by the interpreter are exposed to the user.
+Their definition may change with future versions of the interpreter,
+but they are mentioned here for completeness.
+\index{internal type}
+
+\begin{description}
+
+\item[Code objects]
+Code objects represent executable code.  The difference between a code
+object and a function object is that the function object contains an
+explicit reference to the function's context (the module in which it
+was defined) which a code object contains no context.  There is no way
+to execute a bare code object.
+\obindex{code}
+
+Special read-only attributes: \verb\co_code\ is a string representing
+the sequence of instructions; \verb\co_consts\ is a list of literals
+used by the code; \verb\co_names\ is a list of names (strings) used by
+the code; \verb\co_filename\ is the filename from which the code was
+compiled.  (To find out the line numbers, you would have to decode the
+instructions; the standard library module \verb\dis\ contains an
+example of how to do this.)
+\ttindex{co_code}
+\ttindex{co_consts}
+\ttindex{co_names}
+\ttindex{co_filename}
+
+\item[Frame objects]
+Frame objects represent execution frames.  They may occur in traceback
+objects (see below).
+\obindex{frame}
+
+Special read-only attributes: \verb\f_back\ is to the previous
+stack frame (towards the caller), or \verb\None\ if this is the bottom
+stack frame; \verb\f_code\ is the code object being executed in this
+frame; \verb\f_globals\ is the dictionary used to look up global
+variables; \verb\f_locals\ is used for local variables;
+\verb\f_lineno\ gives the line number and \verb\f_lasti\ gives the
+precise instruction (this is an index into the instruction string of
+the code object).
+\ttindex{f_back}
+\ttindex{f_code}
+\ttindex{f_globals}
+\ttindex{f_locals}
+\ttindex{f_lineno}
+\ttindex{f_lasti}
+
+\item[Traceback objects]
+Traceback objects represent a stack trace of an exception.  A
+traceback object is created when an exception occurs.  When the search
+for an exception handler unwinds the execution stack, at each unwound
+level a traceback object is inserted in front of the current
+traceback.  When an exception handler is entered, the stack trace is
+made available to the program as \verb\sys.exc_traceback\.  When the
+program contains no suitable handler, the stack trace is written
+(nicely formatted) to the standard error stream; if the interpreter is
+interactive, it is also made available to the user as
+\verb\sys.last_traceback\.
+\obindex{traceback}
+\indexii{stack}{trace}
+\indexii{exception}{handler}
+\indexii{execution}{stack}
+\ttindex{exc_traceback}
+\ttindex{last_traceback}
+\ttindex{sys.exc_traceback}
+\ttindex{sys.last_traceback}
+
+Special read-only attributes: \verb\tb_next\ is the next level in the
+stack trace (towards the frame where the exception occurred), or
+\verb\None\ if there is no next level; \verb\tb_frame\ points to the
+execution frame of the current level; \verb\tb_lineno\ gives the line
+number where the exception occurred; \verb\tb_lasti\ indicates the
+precise instruction.  The line number and last instruction in the
+traceback may differ from the line number of its frame object if the
+exception occurred in a \verb\try\ statement with no matching
+\verb\except\ clause or with a \verb\finally\ clause.
+\ttindex{tb_next}
+\ttindex{tb_frame}
+\ttindex{tb_lineno}
+\ttindex{tb_lasti}
+\stindex{try}
+
+\end{description} % Internal types
+
+\end{description} % Types
+
+
+\section{Special method names} \label{specialnames}
+
+A class can implement certain operations that are invoked by special
+syntax (such as subscription or arithmetic operations) by defining
+methods with special names.  For instance, if a class defines a
+method named \verb\__getitem__\, and \verb\x\ is an instance of this
+class, then \verb\x[i]\ is equivalent to \verb\x.__getitem__(i)\.
+(The reverse is not true --- if \verb\x\ is a list object,
+\verb\x.__getitem__(i)\ is not equivalent to \verb\x[i]\.)
+
+Except for \verb\__repr__\ and \verb\__cmp__\, attempts to execute an
+operation raise an exception when no appropriate method is defined.
+For \verb\__repr__\ and \verb\__cmp__\, the traditional
+interpretations are used in this case.
+
+
+\subsection{Special methods for any type}
+
+\begin{description}
+
+\item[\tt __repr__(self)]
+Called by the \verb\print\ statement and conversions (reverse quotes) to
+compute the string representation of an object.
+
+\item[\tt _cmp__(self, other)]
+Called by all comparison operations.  Should return -1 if
+\verb\self < other\,  0 if \verb\self == other\, +1 if
+\verb\self > other\.  (Implementation note: due to limitations in the
+interpreter, exceptions raised by comparisons are ignored, and the
+objects will be considered equal in this case.)
+
+\end{description}
+
+
+\subsection{Special methods for sequence and mapping types}
+
+\begin{description}
+
+\item[\tt __len__(self)]
+Called to implement the built-in function \verb\len()\.  Should return
+the length of the object, an integer \verb\>=\ 0.  Also, an object
+whose \verb\__len__()\ method returns 0 is considered to be false in a
+Boolean context.
+
+\item[\tt __getitem__(self, key)]
+Called to implement evaluation of \verb\self[key]\.  Note that the
+special interpretation of negative keys (if the class wishes to
+emulate a sequence type) is up to the \verb\__getitem__\ method.
+
+\item[\tt __setitem__(self, key, value)]
+Called to implement assignment to \verb\self[key]\.  Same note as for
+\verb\__getitem__\.
+
+\item[\tt __delitem__(self, key)]
+Called to implement deletion of \verb\self[key]\.  Same note as for
+\verb\__getitem__\.
+
+\end{description}
+
+
+\subsection{Special methods for sequence types}
+
+\begin{description}
+
+\item[\tt __getslice__(self, i, j)]
+Called to implement evaluation of \verb\self[i:j]\.  Note that missing
+\verb\i\ or \verb\j\ are replaced by 0 or \verb\len(self)\,
+respectively, and \verb\len(self)\ has been added (once) to originally
+negative \verb\i\ or \verb\j\ by the time this function is called
+(unlike for \verb\__getitem__\).
+
+\item[\tt __setslice__(self, i, j, sequence)]
+Called to implement assignment to \verb\self[i:j]\.  Same notes as for
+\verb\__getslice__\.
+
+\item[\tt __delslice__(self, i, j)]
+Called to implement deletion of \verb\self[i:j]\.  Same notes as for
+\verb\__getslice__\.
+
+\end{description}
+
+
+\subsection{Special methods for numeric types}
+
+\begin{description}
+
+\item[\tt __add__(self, other)]\itemjoin
+\item[\tt __sub__(self, other)]\itemjoin
+\item[\tt __mul__(self, other)]\itemjoin
+\item[\tt __div__(self, other)]\itemjoin
+\item[\tt __mod__(self, other)]\itemjoin
+\item[\tt __divmod__(self, other)]\itemjoin
+\item[\tt __pow__(self, other)]\itemjoin
+\item[\tt __lshift__(self, other)]\itemjoin
+\item[\tt __rshift__(self, other)]\itemjoin
+\item[\tt __and__(self, other)]\itemjoin
+\item[\tt __xor__(self, other)]\itemjoin
+\item[\tt __or__(self, other)]\itembreak
+Called to implement the binary arithmetic operations (\verb\+\,
+\verb\-\, \verb\*\, \verb\/\, \verb\%\, \verb\divmod()\, \verb\pow()\,
+\verb\<<\, \verb\>>\, \verb\&\, \verb\^\, \verb\|\).
+
+\item[\tt __neg__(self)]\itemjoin
+\item[\tt __pos__(self)]\itemjoin
+\item[\tt __abs__(self)]\itemjoin
+\item[\tt __invert__(self)]\itembreak
+Called to implement the unary arithmetic operations (\verb\-\, \verb\+\,
+\verb\abs()\ and \verb\~\).
+
+\item[\tt __nonzero__(self)]
+Called to implement boolean testing; should return 0 or 1.  An
+alternative name for this method is \verb\__len__\.
+
+\item[\tt __coerce__(self, other)]
+Called to implement ``mixed-mode'' numeric arithmetic.  Should either
+return a tuple containing self and other converted to a common numeric
+type, or None if no way of conversion is known.  When the common type
+would be the type of other, it is sufficient to return None, since the
+interpreter will also ask the other object to attempt a coercion (but
+sometimes, if the implementation of the other type cannot be changed,
+it is useful to do the conversion to the other type here).
+
+Note that this method is not called to coerce the arguments to \verb\+\
+and \verb\*\, because these are also used to implement sequence
+concatenation and repetition, respectively.  Also note that, for the
+same reason, in \verb\n*x\, where \verb\n\ is a built-in number and
+\verb\x\ is an instance, a call to \verb\x.__mul__(n)\ is made.%
+\footnote{The interpreter should really distinguish between
+user-defined classes implementing sequences, mappings or numbers, but
+currently it doesn't --- hence this strange exception.}
+
+\item[\tt __int__(self)]\itemjoin
+\item[\tt __long__(self)]\itemjoin
+\item[\tt __float__(self)]\itembreak
+Called to implement the built-in functions \verb\int()\, \verb\long()\
+and \verb\float()\.  Should return a value of the appropriate type.
+
+\end{description}

Doc/ref/ref4.tex

+\chapter{Execution model}
+\index{execution model}
+
+\section{Code blocks, execution frames, and name spaces} \label{execframes}
+\index{code block}
+\indexii{execution}{frame}
+\index{name space}
+
+A {\em code block} is a piece of Python program text that can be
+executed as a unit, such as a module, a class definition or a function
+body.  Some code blocks (like modules) are executed only once, others
+(like function bodies) may be executed many times.  Code block may
+textually contain other code blocks.  Code blocks may invoke other
+code blocks (that may or may not be textually contained in them) as
+part of their execution, e.g. by invoking (calling) a function.
+\index{code block}
+\indexii{code}{block}
+
+The following are code blocks:  A module is a code block.  A function
+body is a code block.  A class definition is a code block.  Each
+command typed interactively is a separate code block; a script file is
+a code block.  The string argument passed to the built-in functions
+\verb\eval\ and \verb\exec\ are code blocks.  And finally, the
+expression read and evaluated by the built-in function \verb\input\ is
+a code block.
+
+A code block is executed in an execution frame.  An {\em execution
+frame} contains some administrative information (used for debugging),
+determines where and how execution continues after the code block's
+execution has completed, and (perhaps most importantly) defines two
+name spaces, the local and the global name space, that affect
+execution of the code block.
+\indexii{execution}{frame}
+
+A {\em name space} is a mapping from names (identifiers) to objects.
+A particular name space may be referenced by more than one execution
+frame, and from other places as well.  Adding a name to a name space
+is called {\em binding} a name (to an object); changing the mapping of
+a name is called {\em rebinding}; removing a name is {\em unbinding}.
+Name spaces are functionally equivalent to dictionaries.
+\index{name space}
+\indexii{binding}{name}
+\indexii{rebinding}{name}
+\indexii{unbinding}{name}
+
+The {\em local name space} of an execution frame determines the default
+place where names are defined and searched.  The {\em global name
+space} determines the place where names listed in \verb\global\
+statements are defined and searched, and where names that are not
+explicitly bound in the current code block are searched.
+\indexii{local}{name space}
+\indexii{global}{name space}
+\stindex{global}
+
+Whether a name is local or global in a code block is determined by
+static inspection of the source text for the code block: in the
+absence of \verb\global\ statements, a name that is bound anywhere in
+the code block is local in the entire code block; all other names are
+considered global.  The \verb\global\ statement forces global
+interpretation of selected names throughout the code block.  The
+following constructs bind names: formal parameters, \verb\import\
+statements, class and function definitions (these bind the class or
+function name), and targets that are identifiers if occurring in an
+assignment, \verb\for\ loop header, or \verb\except\ clause header.
+(A target occurring in a \verb\del\ statement does not bind a name.)
+
+When a global name is not found in the global name space, it is
+searched in the list of ``built-in'' names (which is actually the
+global name space of the module \verb\builtin\).  When a name is not
+found at all, the \verb\NameError\ exception is raised.
+
+The following table lists the meaning of the local and global name
+space for various types of code blocks.  The name space for a
+particular module is automatically created when the module is first
+referenced.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}
+\hline
+Code block type & Global name space & Local name space & Notes \\
+\hline
+Module & n.s. for this module & same as global & \\
+Script & n.s. for \verb\__main__\ & same as global & \\
+Interactive command & n.s. for \verb\__main__\ & same as global & \\
+Class definition & global n.s. of containing block & new n.s. & \\
+Function body & global n.s. of containing block & new n.s. & \\
+String passed to \verb\exec\ or \verb\eval\
+	& global n.s. of caller & local n.s. of caller & (1) \\
+File read by \verb\execfile\
+	& global n.s. of caller & local n.s. of caller & (1) \\
+Expression read by \verb\input\
+	& global n.s. of caller & local n.s. of caller & \\
+\hline
+\end{tabular}
+\end{center}
+
+Notes:
+
+\begin{description}
+
+\item[n.s.] means {\em name space}
+
+\item[(1)] The global and local name space for these functions can be
+overridden with optional extra arguments.
+
+\end{description}
+
+\section{Exceptions}
+
+Exceptions are a means of breaking out of the normal flow of control
+of a code block in order to handle errors or other exceptional
+conditions.  An exception is {\em raised} at the point where the error
+is detected; it may be {\em handled} by the surrounding code block or
+by any code block that directly or indirectly invoked the code block
+where the error occurred.
+\index{exception}
+\index{raise an exception}
+\index{handle an exception}
+\index{exception handler}
+\index{errors}
+\index{error handling}
+
+The Python interpreter raises an exception when it detects an run-time
+error (such as division by zero).  A Python program can also
+explicitly raise an exception with the \verb\raise\ statement.
+Exception handlers are specified with the \verb\try...except\
+statement.
+
+Python uses the ``termination'' model of error handling: an exception
+handler can find out what happened and continue execution at an outer
+level, but it cannot repair the cause of the error and retry the
+failing operation (except by re-entering the the offending piece of
+code from the top).
+
+When an exception is not handled at all, the interpreter terminates
+execution of the program, or returns to its interactive main loop.
+
+Exceptions are identified by string objects.  Two different string
+objects with the same value identify different exceptions.
+
+When an exception is raised, an object (maybe \verb\None\) is passed
+as the exception's ``parameter''; this object does not affect the
+selection of an exception handler, but is passed to the selected
+exception handler as additional information.
+
+See also the description of the \verb\try\ and \verb\raise\
+statements.

Doc/ref/ref5.tex

+\chapter{Expressions and conditions}
+\index{expression}
+\index{condition}
+
+{\bf Note:} In this and the following chapters, extended BNF notation
+will be used to describe syntax, not lexical analysis.
+\index{BNF}
+
+This chapter explains the meaning of the elements of expressions and
+conditions.  Conditions are a superset of expressions, and a condition
+may be used wherever an expression is required by enclosing it in
+parentheses.  The only places where expressions are used in the syntax
+instead of conditions is in expression statements and on the
+right-hand side of assignment statements; this catches some nasty bugs
+like accidentally writing \verb\x == 1\ instead of \verb\x = 1\.
+\indexii{assignment}{statement}
+
+The comma plays several roles in Python's syntax.  It is usually an
+operator with a lower precedence than all others, but occasionally
+serves other purposes as well; e.g. it separates function arguments,
+is used in list and dictionary constructors, and has special semantics
+in \verb\print\ statements.
+\index{comma}
+
+When (one alternative of) a syntax rule has the form
+
+\begin{verbatim}
+name:           othername
+\end{verbatim}
+
+and no semantics are given, the semantics of this form of \verb\name\
+are the same as for \verb\othername\.
+\index{syntax}
+
+\section{Arithmetic conversions}
+\indexii{arithmetic}{conversion}
+
+When a description of an arithmetic operator below uses the phrase
+``the numeric arguments are converted to a common type'',
+this both means that if either argument is not a number, a
+\verb\TypeError\ exception is raised, and that otherwise
+the following conversions are applied:
+\exindex{TypeError}
+\indexii{floating point}{number}
+\indexii{long}{integer}
+\indexii{plain}{integer}
+
+\begin{itemize}
+\item	first, if either argument is a floating point number,
+	the other is converted to floating point;
+\item	else, if either argument is a long integer,
+	the other is converted to long integer;
+\item	otherwise, both must be plain integers and no conversion
+	is necessary.
+\end{itemize}
+
+\section{Atoms}
+\index{atom}
+
+Atoms are the most basic elements of expressions.  Forms enclosed in
+reverse quotes or in parentheses, brackets or braces are also
+categorized syntactically as atoms.  The syntax for atoms is:
+
+\begin{verbatim}
+atom:           identifier | literal | enclosure
+enclosure:      parenth_form | list_display | dict_display | string_conversion
+\end{verbatim}
+
+\subsection{Identifiers (Names)}
+\index{name}
+\index{identifier}
+
+An identifier occurring as an atom is a reference to a local, global
+or built-in name binding.  If a name can be assigned to anywhere in a
+code block, and is not mentioned in a \verb\global\ statement in that
+code block, it refers to a local name throughout that code block.
+Otherwise, it refers to a global name if one exists, else to a
+built-in name.
+\indexii{name}{binding}
+\index{code block}
+\stindex{global}
+\indexii{built-in}{name}
+\indexii{global}{name}
+
+When the name is bound to an object, evaluation of the atom yields
+that object.  When a name is not bound, an attempt to evaluate it
+raises a \verb\NameError\ exception.
+\exindex{NameError}
+
+\subsection{Literals}
+\index{literal}
+
+Python knows string and numeric literals:
+
+\begin{verbatim}
+literal:        stringliteral | integer | longinteger | floatnumber
+\end{verbatim}
+
+Evaluation of a literal yields an object of the given type (string,
+integer, long integer, floating point number) with the given value.
+The value may be approximated in the case of floating point literals.
+See section \ref{literals} for details.
+
+All literals correspond to immutable data types, and hence the
+object's identity is less important than its value.  Multiple
+evaluations of literals with the same value (either the same
+occurrence in the program text or a different occurrence) may obtain
+the same object or a different object with the same value.
+\indexiii{immutable}{data}{type}
+
+(In the original implementation, all literals in the same code block
+with the same type and value yield the same object.)
+
+\subsection{Parenthesized forms}
+\index{parenthesized form}
+
+A parenthesized form is an optional condition list enclosed in
+parentheses:
+
+\begin{verbatim}
+parenth_form:      "(" [condition_list] ")"
+\end{verbatim}
+
+A parenthesized condition list yields whatever that condition list
+yields.
+
+An empty pair of parentheses yields an empty tuple object.  Since
+tuples are immutable, the rules for literals apply here.
+\indexii{empty}{tuple}
+
+(Note that tuples are not formed by the parentheses, but rather by use
+of the comma operator.  The exception is the empty tuple, for which
+parentheses {\em are} required --- allowing unparenthesized ``nothing''
+in expressions would causes ambiguities and allow common typos to
+pass uncaught.)
+\index{comma}
+\indexii{tuple}{display}
+
+\subsection{List displays}
+\indexii{list}{display}
+
+A list display is a possibly empty series of conditions enclosed in
+square brackets:
+
+\begin{verbatim}
+list_display:   "[" [condition_list] "]"
+\end{verbatim}
+
+A list display yields a new list object.
+\obindex{list}
+
+If it has no condition list, the list object has no items.  Otherwise,
+the elements of the condition list are evaluated from left to right
+and inserted in the list object in that order.
+\indexii{empty}{list}
+
+\subsection{Dictionary displays} \label{dict}
+\indexii{dictionary}{display}
+
+A dictionary display is a possibly empty series of key/datum pairs
+enclosed in curly braces:
+\index{key}
+\index{datum}
+\index{key/datum pair}
+
+\begin{verbatim}
+dict_display:   "{" [key_datum_list] "}"
+key_datum_list: key_datum ("," key_datum)* [","]
+key_datum:      condition ":" condition
+\end{verbatim}
+
+A dictionary display yields a new dictionary object.
+\obindex{dictionary}
+
+The key/datum pairs are evaluated from left to right to define the
+entries of the dictionary: each key object is used as a key into the
+dictionary to store the corresponding datum.
+
+Keys must be strings, otherwise a \verb\TypeError\ exception is
+raised.  Clashes between duplicate keys are not detected; the last
+datum (textually rightmost in the display) stored for a given key
+value prevails.
+\exindex{TypeError}
+
+\subsection{String conversions}
+\indexii{string}{conversion}
+
+A string conversion is a condition list enclosed in reverse (or
+backward) quotes:
+
+\begin{verbatim}
+string_conversion: "`" condition_list "`"
+\end{verbatim}
+
+A string conversion evaluates the contained condition list and
+converts the resulting object into a string according to rules
+specific to its type.
+
+If the object is a string, a number, \verb\None\, or a tuple, list or
+dictionary containing only objects whose type is one of these, the
+resulting string is a valid Python expression which can be passed to
+the built-in function \verb\eval()\ to yield an expression with the
+same value (or an approximation, if floating point numbers are
+involved).
+
+(In particular, converting a string adds quotes around it and converts
+``funny'' characters to escape sequences that are safe to print.)
+
+It is illegal to attempt to convert recursive objects (e.g. lists or
+dictionaries that contain a reference to themselves, directly or
+indirectly.)
+\obindex{recursive}
+
+\section{Primaries} \label{primaries}
+\index{primary}
+
+Primaries represent the most tightly bound operations of the language.
+Their syntax is:
+
+\begin{verbatim}
+primary:        atom | attributeref | subscription | slicing | call
+\end{verbatim}
+
+\subsection{Attribute references}
+\indexii{attribute}{reference}
+
+An attribute reference is a primary followed by a period and a name:
+
+\begin{verbatim}
+attributeref:   primary "." identifier
+\end{verbatim}
+
+The primary must evaluate to an object of a type that supports
+attribute references, e.g. a module or a list.  This object is then
+asked to produce the attribute whose name is the identifier.  If this
+attribute is not available, the exception \verb\AttributeError\ is
+raised.  Otherwise, the type and value of the object produced is
+determined by the object.  Multiple evaluations of the same attribute
+reference may yield different objects.
+\obindex{module}
+\obindex{list}
+
+\subsection{Subscriptions}
+\index{subscription}
+
+A subscription selects an item of a sequence (string, tuple or list)
+or mapping (dictionary) object:
+\obindex{sequence}
+\obindex{mapping}
+\obindex{string}
+\obindex{tuple}
+\obindex{list}
+\obindex{dictionary}
+\indexii{sequence}{item}
+
+\begin{verbatim}
+subscription:   primary "[" condition "]"
+\end{verbatim}
+
+The primary must evaluate to an object of a sequence or mapping type.
+
+If it is a mapping, the condition must evaluate to an object whose
+value is one of the keys of the mapping, and the subscription selects
+the value in the mapping that corresponds to that key.
+
+If it is a sequence, the condition must evaluate to a plain integer.
+If this value is negative, the length of the sequence is added to it
+(so that, e.g. \verb\x[-1]\ selects the last item of \verb\x\.)
+The resulting value must be a nonnegative integer smaller than the
+number of items in the sequence, and the subscription selects the item
+whose index is that value (counting from zero).
+
+A string's items are characters.  A character is not a separate data
+type but a string of exactly one character.
+\index{character}
+\indexii{string}{item}
+
+\subsection{Slicings}
+\index{slicing}
+\index{slice}
+
+A slicing (or slice) selects a range of items in a sequence (string,
+tuple or list) object:
+\obindex{sequence}
+\obindex{string}
+\obindex{tuple}
+\obindex{list}
+
+\begin{verbatim}
+slicing:        primary "[" [condition] ":" [condition] "]"
+\end{verbatim}
+
+The primary must evaluate to a sequence object.  The lower and upper
+bound expressions, if present, must evaluate to plain integers;
+defaults are zero and the sequence's length, respectively.  If either
+bound is negative, the sequence's length is added to it.  The slicing
+now selects all items with index $k$ such that $i <= k < j$ where $i$
+and $j$ are the specified lower and upper bounds.  This may be an
+empty sequence.  It is not an error if $i$ or $j$ lie outside the
+range of valid indexes (such items don't exist so they aren't
+selected).
+
+\subsection{Calls} \label{calls}
+\index{call}
+
+A call calls a callable object (e.g. a function) with a possibly empty
+series of arguments:
+\obindex{callable}
+
+\begin{verbatim}
+call:           primary "(" [condition_list] ")"
+\end{verbatim}
+
+The primary must evaluate to a callable object (user-defined
+functions, built-in functions, methods of built-in objects, class
+objects, and methods of class instances are callable).  If it is a
+class, the argument list must be empty; otherwise, the arguments are
+evaluated.
+
+A call always returns some value, possibly \verb\None\, unless it
+raises an exception.  How this value is computed depends on the type
+of the callable object.  If it is:
+
+\begin{description}
+
+\item[a user-defined function:] the code block for the function is
+executed, passing it the argument list.  The first thing the code
+block will do is bind the formal parameters to the arguments; this is
+described in section \ref{function}.  When the code block executes a
+\verb\return\ statement, this specifies the return value of the
+function call.
+\indexii{function}{call}
+\indexiii{user-defined}{function}{call}
+\obindex{user-defined function}
+\obindex{function}
+
+\item[a built-in function or method:] the result is up to the
+interpreter; see the library reference manual for the descriptions of
+built-in functions and methods.
+\indexii{function}{call}
+\indexii{built-in function}{call}
+\indexii{method}{call}
+\indexii{built-in method}{call}
+\obindex{built-in method}
+\obindex{built-in function}
+\obindex{method}
+\obindex{function}
+
+\item[a class object:] a new instance of that class is returned.
+\obindex{class}
+\indexii{class object}{call}
+
+\item[a class instance method:] the corresponding user-defined
+function is called, with an argument list that is one longer than the
+argument list of the call: the instance becomes the first argument.
+\obindex{class instance}
+\obindex{instance}
+\indexii{instance}{call}
+\indexii{class instance}{call}
+
+\end{description}
+
+\section{Unary arithmetic operations}
+\indexiii{unary}{arithmetic}{operation}
+\indexiii{unary}{bit-wise}{operation}
+
+All unary arithmetic (and bit-wise) operations have the same priority:
+
+\begin{verbatim}
+u_expr:         primary | "-" u_expr | "+" u_expr | "~" u_expr
+\end{verbatim}
+
+The unary \verb\"-"\ (minus) operator yields the negation of its
+numeric argument.
+\index{negation}
+\index{minus}
+
+The unary \verb\"+"\ (plus) operator yields its numeric argument
+unchanged.
+\index{plus}
+
+The unary \verb\"~"\ (invert) operator yields the bit-wise inversion
+of its plain or long integer argument.  The bit-wise inversion of
+\verb\x\ is defined as \verb\-(x+1)\.
+\index{inversion}
+
+In all three cases, if the argument does not have the proper type,
+a \verb\TypeError\ exception is raised.
+\exindex{TypeError}
+
+\section{Binary arithmetic operations}
+\indexiii{binary}{arithmetic}{operation}
+
+The binary arithmetic operations have the conventional priority
+levels.  Note that some of these operations also apply to certain
+non-numeric types.  There is no ``power'' operator, so there are only
+two levels, one for multiplicative operators and one for additive
+operators:
+
+\begin{verbatim}
+m_expr:         u_expr | m_expr "*" u_expr
+              | m_expr "/" u_expr | m_expr "%" u_expr
+a_expr:         m_expr | aexpr "+" m_expr | aexpr "-" m_expr
+\end{verbatim}
+
+The \verb\"*"\ (multiplication) operator yields the product of its
+arguments.  The arguments must either both be numbers, or one argument
+must be a plain integer and the other must be a sequence.  In the
+former case, the numbers are converted to a common type and then
+multiplied together.  In the latter case, sequence repetition is
+performed; a negative repetition factor yields an empty sequence.
+\index{multiplication}
+
+The \verb\"/"\ (division) operator yields the quotient of its
+arguments.  The numeric arguments are first converted to a common
+type.  Plain or long integer division yields an integer of the same
+type; the result is that of mathematical division with the `floor'
+function applied to the result.  Division by zero raises the
+\verb\ZeroDivisionError\ exception.
+\exindex{ZeroDivisionError}
+\index{division}
+
+The \verb\"%"\ (modulo) operator yields the remainder from the
+division of the first argument by the second.  The numeric arguments
+are first converted to a common type.  A zero right argument raises
+the \verb\ZeroDivisionError\ exception.  The arguments may be floating
+point numbers, e.g. \verb\3.14 % 0.7\ equals \verb\0.34\.  The modulo
+operator always yields a result with the same sign as its second
+operand (or zero); the absolute value of the result is strictly
+smaller than the second operand.
+\index{modulo}
+
+The integer division and modulo operators are connected by the
+following identity: \verb\x == (x/y)*y + (x%y)\.  Integer division and
+modulo are also connected with the built-in function \verb\divmod()\:
+\verb\divmod(x, y) == (x/y, x%y)\.  These identities don't hold for
+floating point numbers; there a similar identity holds where
+\verb\x/y\ is replaced by \verb\floor(x/y)\).
+
+The \verb\"+"\ (addition) operator yields the sum of its arguments.
+The arguments must either both be numbers, or both sequences of the
+same type.  In the former case, the numbers are converted to a common
+type and then added together.  In the latter case, the sequences are
+concatenated.
+\index{addition}
+
+The \verb\"-"\ (subtraction) operator yields the difference of its
+arguments.  The numeric arguments are first converted to a common
+type.
+\index{subtraction}
+
+\section{Shifting operations}
+\indexii{shifting}{operation}
+
+The shifting operations have lower priority than the arithmetic
+operations:
+
+\begin{verbatim}
+shift_expr:     a_expr | shift_expr ( "<<" | ">>" ) a_expr
+\end{verbatim}
+
+These operators accept plain or long integers as arguments.  The
+arguments are converted to a common type.  They shift the first
+argument to the left or right by the number of bits given by the
+second argument.
+
+A right shift by $n$ bits is defined as division by $2^n$.  A left
+shift by $n$ bits is defined as multiplication with $2^n$; for plain
+integers there is no overflow check so this drops bits and flip the
+sign if the result is not less than $2^{31}$ in absolute value.
+
+Negative shift counts raise a \verb\ValueError\ exception.
+\exindex{ValueError}
+
+\section{Binary bit-wise operations}
+\indexiii{binary}{bit-wise}{operation}
+
+Each of the three bitwise operations has a different priority level:
+
+\begin{verbatim}
+and_expr:       shift_expr | and_expr "&" shift_expr
+xor_expr:       and_expr | xor_expr "^" and_expr
+or_expr:       xor_expr | or_expr "|" xor_expr
+\end{verbatim}
+
+The \verb\"&"\ operator yields the bitwise AND of its arguments, which
+must be plain or long integers.  The arguments are converted to a
+common type.
+\indexii{bit-wise}{and}
+
+The \verb\"^"\ operator yields the bitwise XOR (exclusive OR) of its
+arguments, which must be plain or long integers.  The arguments are
+converted to a common type.
+\indexii{bit-wise}{xor}
+\indexii{exclusive}{or}
+
+The \verb\"|"\ operator yields the bitwise (inclusive) OR of its
+arguments, which must be plain or long integers.  The arguments are
+converted to a common type.
+\indexii{bit-wise}{or}
+\indexii{inclusive}{or}
+
+\section{Comparisons}
+\index{comparison}
+
+Contrary to C, all comparison operations in Python have the same
+priority, which is lower than that of any arithmetic, shifting or
+bitwise operation.  Also contrary to C, expressions like
+\verb\a < b < c\ have the interpretation that is conventional in
+mathematics:
+\index{C}
+
+\begin{verbatim}
+comparison:     or_expr (comp_operator or_expr)*
+comp_operator:  "<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"
+\end{verbatim}
+
+Comparisons yield integer values: 1 for true, 0 for false.
+
+Comparisons can be chained arbitrarily, e.g. $x < y <= z$ is
+equivalent to $x < y$ \verb\and\ $y <= z$, except that $y$ is
+evaluated only once (but in both cases $z$ is not evaluated at all
+when $x < y$ is found to be false).
+\indexii{chaining}{comparisons}
+
+Formally, $e_0 op_1 e_1 op_2 e_2 ...e_{n-1} op_n e_n$ is equivalent to
+$e_0 op_1 e_1$ \verb\and\ $e_1 op_2 e_2$ \verb\and\ ... \verb\and\
+$e_{n-1} op_n e_n$, except that each expression is evaluated at most once.
+
+Note that $e_0 op_1 e_1 op_2 e_2$ does not imply any kind of comparison
+between $e_0$ and $e_2$, e.g. $x < y > z$ is perfectly legal.
+
+The forms \verb\<>\ and \verb\!=\ are equivalent; for consistency with
+C, \verb\!=\ is preferred; where \verb\!=\ is mentioned below
+\verb\<>\ is also implied.
+
+The operators {\tt "<", ">", "==", ">=", "<="}, and {\tt "!="} compare
+the values of two objects.  The objects needn't have the same type.
+If both are numbers, they are coverted to a common type.  Otherwise,
+objects of different types {\em always} compare unequal, and are
+ordered consistently but arbitrarily.
+
+(This unusual definition of comparison is done to simplify the
+definition of operations like sorting and the \verb\in\ and \verb\not
+in\ operators.)
+
+Comparison of objects of the same type depends on the type:
+
+\begin{itemize}
+
+\item
+Numbers are compared arithmetically.
+
+\item
+Strings are compared lexicographically using the numeric equivalents
+(the result of the built-in function \verb\ord\) of their characters.
+
+\item
+Tuples and lists are compared lexicographically using comparison of
+corresponding items.
+
+\item
+Mappings (dictionaries) are compared through lexicographic
+comparison of their sorted (key, value) lists.%
+\footnote{This is expensive since it requires sorting the keys first,
+but about the only sensible definition.  It was tried to compare
+dictionaries by identity only, but this caused surprises because
+people expected to be able to test a dictionary for emptiness by
+comparing it to {\tt \{\}}.}
+
+\item
+Most other types compare unequal unless they are the same object;
+the choice whether one object is considered smaller or larger than
+another one is made arbitrarily but consistently within one
+execution of a program.
+
+\end{itemize}
+
+The operators \verb\in\ and \verb\not in\ test for sequence
+membership: if $y$ is a sequence, $x ~\verb\in\~ y$ is true if and
+only if there exists an index $i$ such that $x = y[i]$.
+$x ~\verb\not in\~ y$ yields the inverse truth value.  The exception
+\verb\TypeError\ is raised when $y$ is not a sequence, or when $y$ is
+a string and $x$ is not a string of length one.%
+\footnote{The latter restriction is sometimes a nuisance.}
+\opindex{in}
+\opindex{not in}
+\indexii{membership}{test}
+\obindex{sequence}
+
+The operators \verb\is\ and \verb\is not\ test for object identity:
+$x ~\verb\is\~ y$ is true if and only if $x$ and $y$ are the same
+object.  $x ~\verb\is not\~ y$ yields the inverse truth value.
+\opindex{is}
+\opindex{is not}
+\indexii{identity}{test}
+
+\section{Boolean operations} \label{Booleans}
+\indexii{Boolean}{operation}
+
+Boolean operations have the lowest priority of all Python operations:
+
+\begin{verbatim}
+condition:      or_test
+or_test:        and_test | or_test "or" and_test
+and_test:       not_test | and_test "and" not_test
+not_test:       comparison | "not" not_test
+\end{verbatim}
+
+In the context of Boolean operations, and also when conditions are
+used by control flow statements, the following values are interpreted
+as false: \verb\None\, numeric zero of all types, empty sequences
+(strings, tuples and lists), and empty mappings (dictionaries).  All
+other values are interpreted as true.
+
+The operator \verb\not\ yields 1 if its argument is false, 0 otherwise.
+\opindex{not}
+
+The condition $x ~\verb\and\~ y$ first evaluates $x$; if $x$ is false,
+its value is returned; otherwise, $y$ is evaluated and the resulting
+value is returned.
+\opindex{and}
+
+The condition $x ~\verb\or\~ y$ first evaluates $x$; if $x$ is true,
+its value is returned; otherwise, $y$ is evaluated and the resulting
+value is returned.
+\opindex{or}
+
+(Note that \verb\and\ and \verb\or\ do not restrict the value and type
+they return to 0 and 1, but rather return the last evaluated argument.
+This is sometimes useful, e.g. if \verb\s\ is a string that should be
+replaced by a default value if it is empty, the expression
+\verb\s or 'foo'\ yields the desired value.  Because \verb\not\ has to
+invent a value anyway, it does not bother to return a value of the
+same type as its argument, so e.g. \verb\not 'foo'\ yields \verb\0\,
+not \verb\''\.)
+
+\section{Expression lists and condition lists}
+\indexii{expression}{list}
+\indexii{condition}{list}
+
+\begin{verbatim}
+expr_list:      or_expr ("," or_expr)* [","]
+cond_list:      condition ("," condition)* [","]
+\end{verbatim}
+
+The only difference between expression lists and condition lists is
+the lowest priority of operators that can be used in them without
+being enclosed in parentheses; condition lists allow all operators,
+while expression lists don't allow comparisons and Boolean operators
+(they do allow bitwise and shift operators though).
+
+Expression lists are used in expression statements and assignments;
+condition lists are used everywhere else where a list of
+comma-separated values is required.
+
+An expression (condition) list containing at least one comma yields a
+tuple.  The length of the tuple is the number of expressions
+(conditions) in the list.  The expressions (conditions) are evaluated
+from left to right.  (Conditions lists are used syntactically is a few
+places where no tuple is constructed but a list of values is needed
+nevertheless.)
+\obindex{tuple}
+
+The trailing comma is required only to create a single tuple (a.k.a. a
+{\em singleton}); it is optional in all other cases.  A single
+expression (condition) without a trailing comma doesn't create a
+tuple, but rather yields the value of that expression (condition).
+\indexii{trailing}{comma}
+
+(To create an empty tuple, use an empty pair of parentheses:
+\verb\()\.)

Doc/ref1.tex

+\chapter{Introduction}
+
+This reference manual describes the Python programming language.
+It is not intended as a tutorial.
+
+While I am trying to be as precise as possible, I chose to use English
+rather than formal specifications for everything except syntax and
+lexical analysis.  This should make the document better understandable
+to the average reader, but will leave room for ambiguities.
+Consequently, if you were coming from Mars and tried to re-implement
+Python from this document alone, you might have to guess things and in
+fact you would probably end up implementing quite a different language.
+On the other hand, if you are using
+Python and wonder what the precise rules about a particular area of
+the language are, you should definitely be able to find them here.
+
+It is dangerous to add too many implementation details to a language
+reference document --- the implementation may change, and other
+implementations of the same language may work differently.  On the
+other hand, there is currently only one Python implementation, and
+its particular quirks are sometimes worth being mentioned, especially
+where the implementation imposes additional limitations.  Therefore,
+you'll find short ``implementation notes'' sprinkled throughout the
+text.
+
+Every Python implementation comes with a number of built-in and
+standard modules.  These are not documented here, but in the separate
+{\em Python Library Reference} document.  A few built-in modules are
+mentioned when they interact in a significant way with the language
+definition.
+
+\section{Notation}
+
+The descriptions of lexical analysis and syntax use a modified BNF
+grammar notation.  This uses the following style of definition:
+\index{BNF}
+\index{grammar}
+\index{syntax}
+\index{notation}
+
+\begin{verbatim}
+name:           lc_letter (lc_letter | "_")*
+lc_letter:      "a"..."z"
+\end{verbatim}
+
+The first line says that a \verb\name\ is an \verb\lc_letter\ followed by
+a sequence of zero or more \verb\lc_letter\s and underscores.  An
+\verb\lc_letter\ in turn is any of the single characters `a' through `z'.
+(This rule is actually adhered to for the names defined in lexical and
+grammar rules in this document.)
+
+Each rule begins with a name (which is the name defined by the rule)
+and a colon.  A vertical bar (\verb\|\) is used to separate
+alternatives; it is the least binding operator in this notation.  A
+star (\verb\*\) means zero or more repetitions of the preceding item;
+likewise, a plus (\verb\+\) means one or more repetitions, and a
+phrase enclosed in square brackets (\verb\[ ]\) means zero or one
+occurrences (in other words, the enclosed phrase is optional).  The
+\verb\*\ and \verb\+\ operators bind as tightly as possible;
+parentheses are used for grouping.  Literal strings are enclosed in
+double quotes.  White space is only meaningful to separate tokens.
+Rules are normally contained on a single line; rules with many
+alternatives may be formatted alternatively with each line after the
+first beginning with a vertical bar.
+
+In lexical definitions (as the example above), two more conventions
+are used: Two literal characters separated by three dots mean a choice
+of any single character in the given (inclusive) range of ASCII
+characters.  A phrase between angular brackets (\verb\<...>\) gives an
+informal description of the symbol defined; e.g. this could be used
+to describe the notion of `control character' if needed.
+\index{lexical definitions}
+\index{ASCII}
+
+Even though the notation used is almost the same, there is a big
+difference between the meaning of lexical and syntactic definitions:
+a lexical definition operates on the individual characters of the
+input source, while a syntax definition operates on the stream of
+tokens generated by the lexical analysis.  All uses of BNF in the next
+chapter (``Lexical Analysis'') are lexical definitions; uses in
+subsequent chapters are syntactic definitions.

Doc/ref2.tex

+\chapter{Lexical analysis}
+
+A Python program is read by a {\em parser}.  Input to the parser is a
+stream of {\em tokens}, generated by the {\em lexical analyzer}.  This
+chapter describes how the lexical analyzer breaks a file into tokens.
+\index{lexical analysis}
+\index{parser}
+\index{token}
+
+\section{Line structure}
+
+A Python program is divided in a number of logical lines.  The end of
+a logical line is represented by the token NEWLINE.  Statements cannot
+cross logical line boundaries except where NEWLINE is allowed by the
+syntax (e.g. between statements in compound statements).
+\index{line structure}
+\index{logical line}
+\index{NEWLINE token}
+
+\subsection{Comments}
+
+A comment starts with a hash character (\verb\#\) that is not part of
+a string literal, and ends at the end of the physical line.  A comment
+always signifies the end of the logical line.  Comments are ignored by
+the syntax.
+\index{comment}
+\index{logical line}
+\index{physical line}
+\index{hash character}
+
+\subsection{Line joining}
+
+Two or more physical lines may be joined into logical lines using
+backslash characters (\verb/\/), as follows: when a physical line ends
+in a backslash that is not part of a string literal or comment, it is
+joined with the following forming a single logical line, deleting the
+backslash and the following end-of-line character.  For example:
+\index{physical line}
+\index{line joining}
+\index{backslash character}
+%
+\begin{verbatim}
+month_names = ['Januari', 'Februari', 'Maart',     \
+               'April',   'Mei',      'Juni',      \
+               'Juli',    'Augustus', 'September', \
+               'Oktober', 'November', 'December']
+\end{verbatim}
+
+\subsection{Blank lines}
+
+A logical line that contains only spaces, tabs, and possibly a
+comment, is ignored (i.e., no NEWLINE token is generated), except that
+during interactive input of statements, an entirely blank logical line
+terminates a multi-line statement.
+\index{blank line}
+
+\subsection{Indentation}
+
+Leading whitespace (spaces and tabs) at the beginning of a logical
+line is used to compute the indentation level of the line, which in
+turn is used to determine the grouping of statements.
+\index{indentation}
+\index{whitespace}
+\index{leading whitespace}
+\index{space}
+\index{tab}
+\index{grouping}
+\index{statement grouping}
+
+First, tabs are replaced (from left to right) by one to eight spaces
+such that the total number of characters up to there is a multiple of
+eight (this is intended to be the same rule as used by {\UNIX}).  The
+total number of spaces preceding the first non-blank character then
+determines the line's indentation.  Indentation cannot be split over
+multiple physical lines using backslashes.
+
+The indentation levels of consecutive lines are used to generate
+INDENT and DEDENT tokens, using a stack, as follows.
+\index{INDENT token}
+\index{DEDENT token}
+
+Before the first line of the file is read, a single zero is pushed on
+the stack; this will never be popped off again.  The numbers pushed on
+the stack will always be strictly increasing from bottom to top.  At
+the beginning of each logical line, the line's indentation level is
+compared to the top of the stack.  If it is equal, nothing happens.
+If it is larger, it is pushed on the stack, and one INDENT token is
+generated.  If it is smaller, it {\em must} be one of the numbers
+occurring on the stack; all numbers on the stack that are larger are
+popped off, and for each number popped off a DEDENT token is
+generated.  At the end of the file, a DEDENT token is generated for
+each number remaining on the stack that is larger than zero.
+
+Here is an example of a correctly (though confusingly) indented piece
+of Python code:
+
+\begin{verbatim}
+def perm(l):
+        # Compute the list of all permutations of l
+
+    if len(l) <= 1:
+                  return [l]
+    r = []
+    for i in range(len(l)):
+             s = l[:i] + l[i+1:]
+             p = perm(s)
+             for x in p:
+              r.append(l[i:i+1] + x)
+    return r
+\end{verbatim}
+
+The following example shows various indentation errors:
+
+\begin{verbatim}
+    def perm(l):                        # error: first line indented
+    for i in range(len(l)):             # error: not indented
+        s = l[:i] + l[i+1:]
+            p = perm(l[:i] + l[i+1:])   # error: unexpected indent
+            for x in p:
+                    r.append(l[i:i+1] + x)
+                return r                # error: inconsistent dedent
+\end{verbatim}
+
+(Actually, the first three errors are detected by the parser; only the
+last error is found by the lexical analyzer --- the indentation of
+\verb\return r\ does not match a level popped off the stack.)
+
+\section{Other tokens}
+
+Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
+exist: identifiers, keywords, literals, operators, and delimiters.
+Spaces and tabs are not tokens, but serve to delimit tokens.  Where
+ambiguity exists, a token comprises the longest possible string that
+forms a legal token, when read from left to right.
+
+\section{Identifiers}
+
+Identifiers (also referred to as names) are described by the following
+lexical definitions:
+\index{identifier}
+\index{name}
+
+\begin{verbatim}
+identifier:     (letter|"_") (letter|digit|"_")*
+letter:         lowercase | uppercase
+lowercase:      "a"..."z"
+uppercase:      "A"..."Z"
+digit:          "0"..."9"
+\end{verbatim}
+
+Identifiers are unlimited in length.  Case is significant.
+
+\subsection{Keywords}
+
+The following identifiers are used as reserved words, or {\em
+keywords} of the language, and cannot be used as ordinary
+identifiers.  They must be spelled exactly as written here:
+\index{keyword}
+\index{reserved word}
+
+\begin{verbatim}
+and        del        for        in         print
+break      elif       from       is         raise
+class      else       global     not        return
+continue   except     if         or         try
+def        finally    import     pass       while
+\end{verbatim}
+
+%	# This Python program sorts and formats the above table
+%	import string
+%	l = []
+%	try:
+%		while 1:
+%			l = l + string.split(raw_input())
+%	except EOFError:
+%		pass
+%	l.sort()
+%	for i in range((len(l)+4)/5):
+%		for j in range(i, len(l), 5):
+%			print string.ljust(l[j], 10),
+%		print
+
+\section{Literals} \label{literals}
+
+Literals are notations for constant values of some built-in types.
+\index{literal}
+\index{constant}
+
+\subsection{String literals}
+
+String literals are described by the following lexical definitions:
+\index{string literal}
+
+\begin{verbatim}
+stringliteral:  "'" stringitem* "'"
+stringitem:     stringchar | escapeseq
+stringchar:     <any ASCII character except newline or "\" or "'">
+escapeseq:      "'" <any ASCII character except newline>
+\end{verbatim}
+\index{ASCII}
+
+String literals cannot span physical line boundaries.  Escape
+sequences in strings are actually interpreted according to rules
+similar to those used by Standard C.  The recognized escape sequences
+are:
+\index{physical line}
+\index{escape sequence}
+\index{Standard C}
+\index{C}
+
+\begin{center}
+\begin{tabular}{|l|l|}
+\hline
+\verb/\\/	& Backslash (\verb/\/) \\
+\verb/\'/	& Single quote (\verb/'/) \\
+\verb/\a/	& ASCII Bell (BEL) \\
+\verb/\b/	& ASCII Backspace (BS) \\
+%\verb/\E/	& ASCII Escape (ESC) \\
+\verb/\f/	& ASCII Formfeed (FF) \\
+\verb/\n/	& ASCII Linefeed (LF) \\
+\verb/\r/	& ASCII Carriage Return (CR) \\
+\verb/\t/	& ASCII Horizontal Tab (TAB) \\
+\verb/\v/	& ASCII Vertical Tab (VT) \\
+\verb/\/{\em ooo}	& ASCII character with octal value {\em ooo} \\
+\verb/\x/{\em xx...}	& ASCII character with hex value {\em xx...} \\
+\hline
+\end{tabular}
+\end{center}
+\index{ASCII}
+
+In strict compatibility with Standard C, up to three octal digits are
+accepted, but an unlimited number of hex digits is taken to be part of
+the hex escape (and then the lower 8 bits of the resulting hex number
+are used in all current implementations...).
+
+All unrecognized escape sequences are left in the string unchanged,
+i.e., {\em the backslash is left in the string.}  (This behavior is
+useful when debugging: if an escape sequence is mistyped, the
+resulting output is more easily recognized as broken.  It also helps a
+great deal for string literals used as regular expressions or
+otherwise passed to other modules that do their own escape handling.)
+\index{unrecognized escape sequence}
+
+\subsection{Numeric literals}
+
+There are three types of numeric literals: plain integers, long
+integers, and floating point numbers.
+\index{number}
+\index{numeric literal}
+\index{integer literal}
+\index{plain integer literal}
+\index{long integer literal}
+\index{floating point literal}
+\index{hexadecimal literal}
+\index{octal literal}
+\index{decimal literal}
+
+Integer and long integer literals are described by the following
+lexical definitions:
+
+\begin{verbatim}
+longinteger:    integer ("l"|"L")
+integer:        decimalinteger | octinteger | hexinteger
+decimalinteger: nonzerodigit digit* | "0"
+octinteger:     "0" octdigit+
+hexinteger:     "0" ("x"|"X") hexdigit+
+
+nonzerodigit:   "1"..."9"
+octdigit:       "0"..."7"
+hexdigit:        digit|"a"..."f"|"A"..."F"
+\end{verbatim}
+
+Although both lower case `l' and upper case `L' are allowed as suffix
+for long integers, it is strongly recommended to always use `L', since
+the letter `l' looks too much like the digit `1'.
+
+Plain integer decimal literals must be at most $2^{31} - 1$ (i.e., the
+largest positive integer, assuming 32-bit arithmetic).  Plain octal and
+hexadecimal literals may be as large as $2^{32} - 1$, but values
+larger than $2^{31} - 1$ are converted to a negative value by
+subtracting $2^{32}$.  There is no limit for long integer literals.
+
+Some examples of plain and long integer literals:
+
+\begin{verbatim}
+7     2147483647                        0177    0x80000000
+3L    79228162514264337593543950336L    0377L   0x100000000L
+\end{verbatim}
+
+Floating point literals are described by the following lexical
+definitions:
+
+\begin{verbatim}
+floatnumber:    pointfloat | exponentfloat
+pointfloat:     [intpart] fraction | intpart "."
+exponentfloat:  (intpart | pointfloat) exponent
+intpart:        digit+
+fraction:       "." digit+
+exponent:       ("e"|"E") ["+"|"-"] digit+
+\end{verbatim}
+
+The allowed range of floating point literals is
+implementation-dependent.
+
+Some examples of floating point literals:
+
+\begin{verbatim}
+3.14    10.    .001    1e100    3.14e-10
+\end{verbatim}
+
+Note that numeric literals do not include a sign; a phrase like
+\verb\-1\ is actually an expression composed of the operator
+\verb\-\ and the literal \verb\1\.
+
+\section{Operators}
+
+The following tokens are operators:
+\index{operators}
+
+\begin{verbatim}
++       -       *       /       %
+<<      >>      &       |       ^       ~
+<       ==      >       <=      <>      !=      >=
+\end{verbatim}
+
+The comparison operators \verb\<>\ and \verb\!=\ are alternate
+spellings of the same operator.
+
+\section{Delimiters}
+
+The following tokens serve as delimiters or otherwise have a special
+meaning:
+\index{delimiters}
+
+\begin{verbatim}
+(       )       [       ]       {       }
+;       ,       :       .       `       =
+\end{verbatim}
+
+The following printing ASCII characters are not used in Python.  Their
+occurrence outside string literals and comments is an unconditional
+error:
+\index{ASCII}
+
+\begin{verbatim}
+@       $       "       ?
+\end{verbatim}
+
+They may be used by future versions of the language though!

Doc/ref3.tex

+\chapter{Data model}
+
+\section{Objects, values and types}
+
+{\em Objects} are Python's abstraction for data.  All data in a Python
+program is represented by objects or by relations between objects.
+(In a sense, and in conformance to Von Neumann's model of a
+``stored program computer'', code is also represented by objects.)
+\index{object}
+\index{data}
+
+Every object has an identity, a type and a value.  An object's {\em
+identity} never changes once it has been created; you may think of it
+as the object's address in memory.  An object's {\em type} is also
+unchangeable.  It determines the operations that an object supports
+(e.g. ``does it have a length?'') and also defines the possible
+values for objects of that type.  The {\em value} of some objects can
+change.  Objects whose value can change are said to be {\em mutable};
+objects whose value is unchangeable once they are created are called
+{\em immutable}.  The type determines an object's (im)mutability.
+\index{identity of an object}
+\index{value of an object}
+\index{type of an object}
+\index{mutable object}
+\index{immutable object}
+
+Objects are never explicitly destroyed; however, when they become
+unreachable they may be garbage-collected.  An implementation is
+allowed to delay garbage collection or omit it altogether --- it is a
+matter of implementation quality how garbage collection is
+implemented, as long as no objects are collected that are still
+reachable.  (Implementation note: the current implementation uses a
+reference-counting scheme which collects most objects as soon as they
+become unreachable, but never collects garbage containing circular
+references.)
+\index{garbage collection}
+\index{reference counting}
+\index{unreachable object}
+
+Note that the use of the implementation's tracing or debugging
+facilities may keep objects alive that would normally be collectable.
+
+Some objects contain references to ``external'' resources such as open
+files or windows.  It is understood that these resources are freed
+when the object is garbage-collected, but since garbage collection is
+not guaranteed to happen, such objects also provide an explicit way to
+release the external resource, usually a \verb\close\ method.
+Programs are strongly recommended to always explicitly close such
+objects.
+
+Some objects contain references to other objects; these are called
+{\em containers}.  Examples of containers are tuples, lists and
+dictionaries.  The references are part of a container's value.  In
+most cases, when we talk about the value of a container, we imply the
+values, not the identities of the contained objects; however, when we
+talk about the (im)mutability of a container, only the identities of
+the immediately contained objects are implied.  (So, if an immutable
+container contains a reference to a mutable object, its value changes
+if that mutable object is changed.)
+\index{container}
+
+Types affect almost all aspects of objects' lives.  Even the meaning
+of object identity is affected in some sense: for immutable types,
+operations that compute new values may actually return a reference to
+any existing object with the same type and value, while for mutable
+objects this is not allowed.  E.g. after
+
+\begin{verbatim}
+a = 1; b = 1; c = []; d = []
+\end{verbatim}
+
+\verb\a\ and \verb\b\ may or may not refer to the same object with the
+value one, depending on the implementation, but \verb\c\ and \verb\d\
+are guaranteed to refer to two different, unique, newly created empty
+lists.
+
+\section{The standard type hierarchy} \label{types}
+
+Below is a list of the types that are built into Python.  Extension
+modules written in C can define additional types.  Future versions of
+Python may add types to the type hierarchy (e.g. rational or complex
+numbers, efficiently stored arrays of integers, etc.).
+\index{type}
+\indexii{data}{type}
+\indexii{type}{hierarchy}
+\indexii{extension}{module}
+\index{C}
+
+Some of the type descriptions below contain a paragraph listing
+`special attributes'.  These are attributes that provide access to the
+implementation and are not intended for general use.  Their definition
+may change in the future.  There are also some `generic' special
+attributes, not listed with the individual objects: \verb\__methods__\
+is a list of the method names of a built-in object, if it has any;
+\verb\__members__\ is a list of the data attribute names of a built-in
+object, if it has any.
+\index{attribute}
+\indexii{special}{attribute}
+\indexiii{generic}{special}{attribute}
+\ttindex{__methods__}
+\ttindex{__members__}
+
+\begin{description}
+
+\item[None]
+This type has a single value.  There is a single object with this value.
+This object is accessed through the built-in name \verb\None\.
+It is returned from functions that don't explicitly return an object.
+\ttindex{None}
+\obindex{None@{\tt None}}
+
+\item[Numbers]
+These are created by numeric literals and returned as results by
+arithmetic operators and arithmetic built-in functions.  Numeric
+objects are immutable; once created their value never changes.  Python
+numbers are of course strongly related to mathematical numbers, but
+subject to the limitations of numerical representation in computers.
+\obindex{number}
+\obindex{numeric}
+
+Python distinguishes between integers and floating point numbers:
+
+\begin{description}
+\item[Integers]
+These represent elements from the mathematical set of whole numbers.
+\obindex{integer}
+
+There are two types of integers:
+
+\begin{description}
+
+\item[Plain integers]
+These represent numbers in the range $-2^{31}$ through $2^{31}-1$.
+(The range may be larger on machines with a larger natural word
+size, but not smaller.)
+When the result of an operation falls outside this range, the
+exception \verb\OverflowError\ is raised.
+For the purpose of shift and mask operations, integers are assumed to
+have a binary, 2's complement notation using 32 or more bits, and
+hiding no bits from the user (i.e., all $2^{32}$ different bit
+patterns correspond to different values).
+\obindex{plain integer}
+
+\item[Long integers]
+These represent numbers in an unlimited range, subject to available
+(virtual) memory only.  For the purpose of shift and mask operations,
+a binary representation is assumed, and negative numbers are
+represented in a variant of 2's complement which gives the illusion of
+an infinite string of sign bits extending to the left.
+\obindex{long integer}
+
+\end{description} % Integers
+
+The rules for integer representation are intended to give the most
+meaningful interpretation of shift and mask operations involving
+negative integers and the least surprises when switching between the
+plain and long integer domains.  For any operation except left shift,
+if it yields a result in the plain integer domain without causing
+overflow, it will yield the same result in the long integer domain or
+when using mixed operands.
+\indexii{integer}{representation}
+
+\item[Floating point numbers]
+These represent machine-level double precision floating point numbers.  
+You are at the mercy of the underlying machine architecture and
+C implementation for the accepted range and handling of overflow.
+\obindex{floating point}
+\indexii{floating point}{number}
+\index{C}
+
+\end{description} % Numbers
+
+\item[Sequences]
+These represent finite ordered sets indexed by natural numbers.
+The built-in function \verb\len()\ returns the number of elements
+of a sequence.  When this number is $n$, the index set contains
+the numbers $0, 1, \ldots, n-1$.  Element \verb\i\ of sequence
+\verb\a\ is selected by \verb\a[i]\.
+\obindex{seqence}
+\bifuncindex{len}
+\index{index operation}
+\index{item selection}
+\index{subscription}
+
+Sequences also support slicing: \verb\a[i:j]\ selects all elements
+with index $k$ such that $i <= k < j$.  When used as an expression,
+a slice is a sequence of the same type --- this implies that the
+index set is renumbered so that it starts at 0 again.
+\index{slicing}
+
+Sequences are distinguished according to their mutability:
+
+\begin{description}
+%
+\item[Immutable sequences]
+An object of an immutable sequence type cannot change once it is
+created.  (If the object contains references to other objects,
+these other objects may be mutable and may be changed; however
+the collection of objects directly referenced by an immutable object
+cannot change.)
+\obindex{immutable sequence}
+\obindex{immutable}
+
+The following types are immutable sequences:
+
+\begin{description}
+
+\item[Strings]
+The elements of a string are characters.  There is no separate
+character type; a character is represented by a string of one element.
+Characters represent (at least) 8-bit bytes.  The built-in
+functions \verb\chr()\ and \verb\ord()\ convert between characters
+and nonnegative integers representing the byte values.
+Bytes with the values 0-127 represent the corresponding ASCII values.
+The string data type is also used to represent arrays of bytes, e.g.
+to hold data read from a file.
+\obindex{string}
+\index{character}
+\index{byte}
+\index{ASCII}
+\bifuncindex{chr}
+\bifuncindex{ord}
+
+(On systems whose native character set is not ASCII, strings may use
+EBCDIC in their internal representation, provided the functions
+\verb\chr()\ and \verb\ord()\ implement a mapping between ASCII and
+EBCDIC, and string comparison preserves the ASCII order.
+Or perhaps someone can propose a better rule?)
+\index{ASCII}
+\index{EBCDIC}
+\index{character set}
+\indexii{string}{comparison}
+\bifuncindex{chr}
+\bifuncindex{ord}
+
+\item[Tuples]
+The elements of a tuple are arbitrary Python objects.
+Tuples of two or more elements are formed by comma-separated lists
+of expressions.  A tuple of one element (a `singleton') can be formed
+by affixing a comma to an expression (an expression by itself does
+not create a tuple, since parentheses must be usable for grouping of
+expressions).  An empty tuple can be formed by enclosing `nothing' in
+parentheses.
+\obindex{tuple}
+\indexii{singleton}{tuple}
+\indexii{empty}{tuple}
+
+\end{description} % Immutable sequences
+
+\item[Mutable sequences]
+Mutable sequences can be changed after they are created.  The
+subscription and slicing notations can be used as the target of
+assignment and \verb\del\ (delete) statements.
+\obindex{mutable sequece}
+\obindex{mutable}
+\indexii{assignment}{statement}
+\index{delete}
+\stindex{del}
+\index{subscription}
+\index{slicing}
+
+There is currently a single mutable sequence type:
+
+\begin{description}
+
+\item[Lists]
+The elements of a list are arbitrary Python objects.  Lists are formed
+by placing a comma-separated list of expressions in square brackets.
+(Note that there are no special cases needed to form lists of length 0
+or 1.)
+\obindex{list}
+
+\end{description} % Mutable sequences
+
+\end{description} % Sequences
+
+\item[Mapping types]
+These represent finite sets of objects indexed by arbitrary index sets.
+The subscript notation \verb\a[k]\ selects the element indexed
+by \verb\k\ from the mapping \verb\a\; this can be used in
+expressions and as the target of assignments or \verb\del\ statements.
+The built-in function \verb\len()\ returns the number of elements
+in a mapping.
+\bifuncindex{len}
+\index{subscription}
+\obindex{mapping}
+
+There is currently a single mapping type:
+
+\begin{description}
+
+\item[Dictionaries]
+These represent finite sets of objects indexed by strings.
+Dictionaries are mutable; they are created by the \verb\{...}\
+notation (see section \ref{dict}).  (Implementation note: the strings
+used for indexing must not contain null bytes.)
+\obindex{dictionary}
+\obindex{mutable}
+
+\end{description} % Mapping types
+
+\item[Callable types]
+These are the types to which the function call (invocation) operation,
+written as \verb\function(argument, argument, ...)\, can be applied:
+\indexii{function}{call}
+\index{invocation}
+\indexii{function}{argument}
+\obindex{callable}
+
+\begin{description}
+
+\item[User-defined functions]
+A user-defined function object is created by a function definition
+(see section \ref{function}).  It should be called with an argument
+list containing the same number of items as the function's formal
+parameter list.
+\indexii{user-defined}{function}
+\obindex{function}
+\obindex{user-defined function}
+
+Special read-only attributes: \verb\func_code\ is the code object
+representing the compiled function body, and \verb\func_globals\ is (a
+reference to) the dictionary that holds the function's global
+variables --- it implements the global name space of the module in
+which the function was defined.
+\ttindex{func_code}
+\ttindex{func_globals}
+\indexii{global}{name space}
+
+\item[User-defined methods]
+A user-defined method (a.k.a. {\em object closure}) is a pair of a
+class instance object and a user-defined function.  It should be
+called with an argument list containing one item less than the number
+of items in the function's formal parameter list.  When called, the
+class instance becomes the first argument, and the call arguments are
+shifted one to the right.
+\obindex{method}
+\obindex{user-defined method}
+\indexii{user-defined}{method}
+\index{object closure}
+
+Special read-only attributes: \verb\im_self\ is the class instance
+object, \verb\im_func\ is the function object.
+\ttindex{im_func}
+\ttindex{im_self}
+
+\item[Built-in functions]
+A built-in function object is a wrapper around a C function.  Examples
+of built-in functions are \verb\len\ and \verb\math.sin\.  There
+are no special attributes.  The number and type of the arguments are
+determined by the C function.
+\obindex{built-in function}
+\obindex{function}
+\index{C}
+
+\item[Built-in methods]
+This is really a different disguise of a built-in function, this time
+containing an object passed to the C function as an implicit extra
+argument.  An example of a built-in method is \verb\list.append\ if
+\verb\list\ is a list object.
+\obindex{built-in method}
+\obindex{method}
+\indexii{built-in}{method}
+
+\item[Classes]
+Class objects are described below.  When a class object is called as a
+parameterless function, a new class instance (also described below) is
+created and returned.  The class's initialization function is not
+called --- this is the responsibility of the caller.  It is illegal to
+call a class object with one or more arguments.
+\obindex{class}
+\obindex{class instance}
+\obindex{instance}
+\indexii{class object}{call}
+
+\end{description}
+
+\item[Modules]
+Modules are imported by the \verb\import\ statement (see section
+\ref{import}).  A module object is a container for a module's name
+space, which is a dictionary (the same dictionary as referenced by the
+\verb\func_globals\ attribute of functions defined in the module).
+Module attribute references are translated to lookups in this
+dictionary.  A module object does not contain the code object used to
+initialize the module (since it isn't needed once the initialization
+is done).
+\stindex{import}
+\obindex{module}
+
+Attribute assignment update the module's name space dictionary.
+
+Special read-only attributes: \verb\__dict__\ yields the module's name
+space as a dictionary object; \verb\__name__\ yields the module's name
+as a string object.
+\ttindex{__dict__}
+\ttindex{__name__}
+\indexii{module}{name space}
+
+\item[Classes]
+Class objects are created by class definitions (see section
+\ref{class}).  A class is a container for a dictionary containing the
+class's name space.  Class attribute references are translated to
+lookups in this dictionary.  When an attribute name is not found
+there, the attribute search continues in the base classes.  The search
+is depth-first, left-to-right in the order of their occurrence in the
+base class list.
+\obindex{class}
+\obindex{class instance}
+\obindex{instance}
+\indexii{class object}{call}
+\index{container}
+\index{dictionary}
+\indexii{class}{attribute}
+
+Class attribute assignments update the class's dictionary, never the
+dictionary of a base class.
+\indexiii{class}{attribute}{assignment}
+
+A class can be called as a parameterless function to yield a class
+instance (see above).
+\indexii{class object}{call}
+
+Special read-only attributes: \verb\__dict__\ yields the dictionary
+containing the class's name space; \verb\__bases__\ yields a tuple
+(possibly empty or a singleton) containing the base classes, in the
+order of their occurrence in the base class list.
+\ttindex{__dict__}
+\ttindex{__bases__}
+
+\item[Class instances]
+A class instance is created by calling a class object as a
+parameterless function.  A class instance has a dictionary in which
+attribute references are searched.  When an attribute is not found
+there, and the instance's class has an attribute by that name, and
+that class attribute is a user-defined function (and in no other
+cases), the instance attribute reference yields a user-defined method
+object (see above) constructed from the instance and the function.
+\obindex{class instance}
+\obindex{instance}
+\indexii{class}{instance}
+\indexii{class instance}{attribute}
+
+Attribute assignments update the instance's dictionary.
+\indexiii{class instance}{attribute}{assignment}
+
+Class instances can pretend to be numbers, sequences, or mappings if
+they have methods with certain special names.  These are described in
+section \ref{specialnames}.
+\obindex{number}
+\obindex{sequence}
+\obindex{mapping}
+
+Special read-only attributes: \verb\__dict__\ yields the attribute
+dictionary; \verb\__class__\ yields the instance's class.
+\ttindex{__dict__}
+\ttindex{__class__}
+
+\item[Files]
+A file object represents an open file.  (It is a wrapper around a C
+{\tt stdio} file pointer.)  File objects are created by the
+\verb\open()\ built-in function, and also by \verb\posix.popen()\ and
+the \verb\makefile\ method of socket objects.  \verb\sys.stdin\,
+\verb\sys.stdout\ and \verb\sys.stderr\ are file objects corresponding
+the the interpreter's standard input, output and error streams.
+See the Python Library Reference for methods of file objects and other
+details.
+\obindex{file}
+\index{C}
+\index{stdio}
+\bifuncindex{open}
+\bifuncindex{popen}
+\bifuncindex{makefile}
+\ttindex{stdin}
+\ttindex{stdout}
+\ttindex{stderr}
+\ttindex{sys.stdin}
+\ttindex{sys.stdout}
+\ttindex{sys.stderr}
+
+\item[Internal types]
+A few types used internally by the interpreter are exposed to the user.
+Their definition may change with future versions of the interpreter,
+but they are mentioned here for completeness.
+\index{internal type}
+
+\begin{description}
+
+\item[Code objects]
+Code objects represent executable code.  The difference between a code
+object and a function object is that the function object contains an
+explicit reference to the function's context (the module in which it
+was defined) which a code object contains no context.  There is no way
+to execute a bare code object.
+\obindex{code}
+
+Special read-only attributes: \verb\co_code\ is a string representing
+the sequence of instructions; \verb\co_consts\ is a list of literals
+used by the code; \verb\co_names\ is a list of names (strings) used by
+the code; \verb\co_filename\ is the filename from which the code was
+compiled.  (To find out the line numbers, you would have to decode the
+instructions; the standard library module \verb\dis\ contains an
+example of how to do this.)
+\ttindex{co_code}
+\ttindex{co_consts}
+\ttindex{co_names}
+\ttindex{co_filename}
+
+\item[Frame objects]
+Frame objects represent execution frames.  They may occur in traceback
+objects (see below).
+\obindex{frame}
+
+Special read-only attributes: \verb\f_back\ is to the previous
+stack frame (towards the caller), or \verb\None\ if this is the bottom
+stack frame; \verb\f_code\ is the code object being executed in this
+frame; \verb\f_globals\ is the dictionary used to look up global
+variables; \verb\f_locals\ is used for local variables;
+\verb\f_lineno\ gives the line number and \verb\f_lasti\ gives the
+precise instruction (this is an index into the instruction string of
+the code object).
+\ttindex{f_back}
+\ttindex{f_code}
+\ttindex{f_globals}
+\ttindex{f_locals}
+\ttindex{f_lineno}
+\ttindex{f_lasti}
+
+\item[Traceback objects]
+Traceback objects represent a stack trace of an exception.  A
+traceback object is created when an exception occurs.  When the search
+for an exception handler unwinds the execution stack, at each unwound
+level a traceback object is inserted in front of the current
+traceback.  When an exception handler is entered, the stack trace is
+made available to the program as \verb\sys.exc_traceback\.  When the
+program contains no suitable handler, the stack trace is written
+(nicely formatted) to the standard error stream; if the interpreter is
+interactive, it is also made available to the user as
+\verb\sys.last_traceback\.
+\obindex{traceback}
+\indexii{stack}{trace}
+\indexii{exception}{handler}
+\indexii{execution}{stack}
+\ttindex{exc_traceback}
+\ttindex{last_traceback}
+\ttindex{sys.exc_traceback}
+\ttindex{sys.last_traceback}
+
+Special read-only attributes: \verb\tb_next\ is the next level in the
+stack trace (towards the frame where the exception occurred), or
+\verb\None\ if there is no next level; \verb\tb_frame\ points to the
+execution frame of the current level; \verb\tb_lineno\ gives the line
+number where the exception occurred; \verb\tb_lasti\ indicates the
+precise instruction.  The line number and last instruction in the
+traceback may differ from the line number of its frame object if the
+exception occurred in a \verb\try\ statement with no matching
+\verb\except\ clause or with a \verb\finally\ clause.
+\ttindex{tb_next}
+\ttindex{tb_frame}
+\ttindex{tb_lineno}
+\ttindex{tb_lasti}
+\stindex{try}
+
+\end{description} % Internal types
+
+\end{description} % Types
+
+
+\section{Special method names} \label{specialnames}
+
+A class can implement certain operations that are invoked by special
+syntax (such as subscription or arithmetic operations) by defining
+methods with special names.  For instance, if a class defines a
+method named \verb\__getitem__\, and \verb\x\ is an instance of this
+class, then \verb\x[i]\ is equivalent to \verb\x.__getitem__(i)\.
+(The reverse is not true --- if \verb\x\ is a list object,
+\verb\x.__getitem__(i)\ is not equivalent to \verb\x[i]\.)
+
+Except for \verb\__repr__\ and \verb\__cmp__\, attempts to execute an
+operation raise an exception when no appropriate method is defined.
+For \verb\__repr__\ and \verb\__cmp__\, the traditional
+interpretations are used in this case.
+
+
+\subsection{Special methods for any type}
+
+\begin{description}
+
+\item[\tt __repr__(self)]
+Called by the \verb\print\ statement and conversions (reverse quotes) to
+compute the string representation of an object.
+
+\item[\tt _cmp__(self, other)]
+Called by all comparison operations.  Should return -1 if
+\verb\self < other\,  0 if \verb\self == other\, +1 if
+\verb\self > other\.  (Implementation note: due to limitations in the
+interpreter, exceptions raised by comparisons are ignored, and the
+objects will be considered equal in this case.)
+
+\end{description}
+
+
+\subsection{Special methods for sequence and mapping types}
+
+\begin{description}
+
+\item[\tt __len__(self)]
+Called to implement the built-in function \verb\len()\.  Should return
+the length of the object, an integer \verb\>=\ 0.  Also, an object
+whose \verb\__len__()\ method returns 0 is considered to be false in a
+Boolean context.
+
+\item[\tt __getitem__(self, key)]
+Called to implement evaluation of \verb\self[key]\.  Note that the
+special interpretation of negative keys (if the class wishes to
+emulate a sequence type) is up to the \verb\__getitem__\ method.
+
+\item[\tt __setitem__(self, key, value)]
+Called to implement assignment to \verb\self[key]\.  Same note as for
+\verb\__getitem__\.
+
+\item[\tt __delitem__(self, key)]
+Called to implement deletion of \verb\self[key]\.  Same note as for
+\verb\__getitem__\.
+
+\end{description}
+
+
+\subsection{Special methods for sequence types}
+
+\begin{description}
+
+\item[\tt __getslice__(self, i, j)]
+Called to implement evaluation of \verb\self[i:j]\.  Note that missing
+\verb\i\ or \verb\j\ are replaced by 0 or \verb\len(self)\,
+respectively, and \verb\len(self)\ has been added (once) to originally
+negative \verb\i\ or \verb\j\ by the time this function is called
+(unlike for \verb\__getitem__\).
+
+\item[\tt __setslice__(self, i, j, sequence)]
+Called to implement assignment to \verb\self[i:j]\.  Same notes as for
+\verb\__getslice__\.
+
+\item[\tt __delslice__(self, i, j)]
+Called to implement deletion of \verb\self[i:j]\.  Same notes as for
+\verb\__getslice__\.
+
+\end{description}
+
+
+\subsection{Special methods for numeric types}
+
+\begin{description}
+
+\item[\tt __add__(self, other)]\itemjoin
+\item[\tt __sub__(self, other)]\itemjoin
+\item[\tt __mul__(self, other)]\itemjoin
+\item[\tt __div__(self, other)]\itemjoin
+\item[\tt __mod__(self, other)]\itemjoin
+\item[\tt __divmod__(self, other)]\itemjoin
+\item[\tt __pow__(self, other)]\itemjoin
+\item[\tt __lshift__(self, other)]\itemjoin
+\item[\tt __rshift__(self, other)]\itemjoin
+\item[\tt __and__(self, other)]\itemjoin
+\item[\tt __xor__(self, other)]\itemjoin
+\item[\tt __or__(self, other)]\itembreak
+Called to implement the binary arithmetic operations (\verb\+\,
+\verb\-\, \verb\*\, \verb\/\, \verb\%\, \verb\divmod()\, \verb\pow()\,
+\verb\<<\, \verb\>>\, \verb\&\, \verb\^\, \verb\|\).
+
+\item[\tt __neg__(self)]\itemjoin
+\item[\tt __pos__(self)]\itemjoin
+\item[\tt __abs__(self)]\itemjoin
+\item[\tt __invert__(self)]\itembreak
+Called to implement the unary arithmetic operations (\verb\-\, \verb\+\,
+\verb\abs()\ and \verb\~\).
+
+\item[\tt __nonzero__(self)]
+Called to implement boolean testing; should return 0 or 1.  An
+alternative name for this method is \verb\__len__\.
+
+\item[\tt __coerce__(self, other)]
+Called to implement ``mixed-mode'' numeric arithmetic.  Should either
+return a tuple containing self and other converted to a common numeric
+type, or None if no way of conversion is known.  When the common type
+would be the type of other, it is sufficient to return None, since the
+interpreter will also ask the other object to attempt a coercion (but
+sometimes, if the implementation of the other type cannot be changed,
+it is useful to do the conversion to the other type here).
+
+Note that this method is not called to coerce the arguments to \verb\+\
+and \verb\*\, because these are also used to implement sequence
+concatenation and repetition, respectively.  Also note that, for the
+same reason, in \verb\n*x\, where \verb\n\ is a built-in number and
+\verb\x\ is an instance, a call to \verb\x.__mul__(n)\ is made.%
+\footnote{The interpreter should really distinguish between
+user-defined classes implementing sequences, mappings or numbers, but
+currently it doesn't --- hence this strange exception.}
+
+\item[\tt __int__(self)]\itemjoin
+\item[\tt __long__(self)]\itemjoin
+\item[\tt __float__(self)]\itembreak
+Called to implement the built-in functions \verb\int()\, \verb\long()\
+and \verb\float()\.  Should return a value of the appropriate type.
+
+\end{description}

Doc/ref4.tex

+\chapter{Execution model}
+\index{execution model}
+
+\section{Code blocks, execution frames, and name spaces} \label{execframes}
+\index{code block}
+\indexii{execution}{frame}
+\index{name space}
+
+A {\em code block} is a piece of Python program text that can be
+executed as a unit, such as a module, a class definition or a function
+body.  Some code blocks (like modules) are executed only once, others
+(like function bodies) may be executed many times.  Code block may
+textually contain other code blocks.  Code blocks may invoke other
+code blocks (that may or may not be textually contained in them) as
+part of their execution, e.g. by invoking (calling) a function.
+\index{code block}
+\indexii{code}{block}
+
+The following are code blocks:  A module is a code block.  A function
+body is a code block.  A class definition is a code block.  Each
+command typed interactively is a separate code block; a script file is
+a code block.  The string argument passed to the built-in functions
+\verb\eval\ and \verb\exec\ are code blocks.  And finally, the
+expression read and evaluated by the built-in function \verb\input\ is
+a code block.
+
+A code block is executed in an execution frame.  An {\em execution
+frame} contains some administrative information (used for debugging),
+determines where and how execution continues after the code block's
+execution has completed, and (perhaps most importantly) defines two
+name spaces, the local and the global name space, that affect
+execution of the code block.
+\indexii{execution}{frame}
+
+A {\em name space} is a mapping from names (identifiers) to objects.
+A particular name space may be referenced by more than one execution
+frame, and from other places as well.  Adding a name to a name space
+is called {\em binding} a name (to an object); changing the mapping of
+a name is called {\em rebinding}; removing a name is {\em unbinding}.
+Name spaces are functionally equivalent to dictionaries.
+\index{name space}
+\indexii{binding}{name}
+\indexii{rebinding}{name}
+\indexii{unbinding}{name}
+
+The {\em local name space} of an execution frame determines the default
+place where names are defined and searched.  The {\em global name
+space} determines the place where names listed in \verb\global\
+statements are defined and searched, and where names that are not
+explicitly bound in the current code block are searched.
+\indexii{local}{name space}
+\indexii{global}{name space}
+\stindex{global}
+
+Whether a name is local or global in a code block is determined by
+static inspection of the source text for the code block: in the
+absence of \verb\global\ statements, a name that is bound anywhere in
+the code block is local in the entire code block; all other names are
+considered global.  The \verb\global\ statement forces global
+interpretation of selected names throughout the code block.  The
+following constructs bind names: formal parameters, \verb\import\
+statements, class and function definitions (these bind the class or
+function name), and targets that are identifiers if occurring in an
+assignment, \verb\for\ loop header, or \verb\except\ clause header.
+(A target occurring in a \verb\del\ statement does not bind a name.)
+
+When a global name is not found in the global name space, it is
+searched in the list of ``built-in'' names (which is actually the
+global name space of the module \verb\builtin\).  When a name is not
+found at all, the \verb\NameError\ exception is raised.
+
+The following table lists the meaning of the local and global name
+space for various types of code blocks.  The name space for a
+particular module is automatically created when the module is first
+referenced.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}
+\hline
+Code block type & Global name space & Local name space & Notes \\
+\hline
+Module & n.s. for this module & same as global & \\
+Script & n.s. for \verb\__main__\ & same as global & \\
+Interactive command & n.s. for \verb\__main__\ & same as global & \\
+Class definition & global n.s. of containing block & new n.s. & \\
+Function body & global n.s. of containing block & new n.s. & \\
+String passed to \verb\exec\ or \verb\eval\
+	& global n.s. of caller & local n.s. of caller & (1) \\
+File read by \verb\execfile\
+	& global n.s. of caller & local n.s. of caller & (1) \\
+Expression read by \verb\input\
+	& global n.s. of caller & local n.s. of caller & \\
+\hline
+\end{tabular}
+\end{center}
+
+Notes:
+
+\begin{description}
+
+\item[n.s.] means {\em name space}
+
+\item[(1)] The global and local name space for these functions can be
+overridden with optional extra arguments.
+
+\end{description}
+
+\section{Exceptions}
+
+Exceptions are a means of breaking out of the normal flow of control
+of a code block in order to handle errors or other exceptional
+conditions.  An exception is {\em raised} at the point where the error
+is detected; it may be {\em handled} by the surrounding code block or
+by any code block that directly or indirectly invoked the code block
+where the error occurred.
+\index{exception}
+\index{raise an exception}
+\index{handle an exception}
+\index{exception handler}
+\index{errors}
+\index{error handling}
+
+The Python interpreter raises an exception when it detects an run-time
+error (such as division by zero).  A Python program can also
+explicitly raise an exception with the \verb\raise\ statement.
+Exception handlers are specified with the \verb\try...except\
+statement.
+
+Python uses the ``termination'' model of error handling: an exception
+handler can find out what happened and continue execution at an outer
+level, but it cannot repair the cause of the error and retry the
+failing operation (except by re-entering the the offending piece of
+code from the top).
+
+When an exception is not handled at all, the interpreter terminates
+execution of the program, or returns to its interactive main loop.
+
+Exceptions are identified by string objects.  Two different string
+objects with the same value identify different exceptions.
+
+When an exception is raised, an object (maybe \verb\None\) is passed
+as the exception's ``parameter''; this object does not affect the
+selection of an exception handler, but is passed to the selected
+exception handler as additional information.
+
+See also the description of the \verb\try\ and \verb\raise\
+statements.

Doc/ref5.tex

+\chapter{Expressions and conditions}
+\index{expression}
+\index{condition}
+
+{\bf Note:} In this and the following chapters, extended BNF notation
+will be used to describe syntax, not lexical analysis.
+\index{BNF}
+
+This chapter explains the meaning of the elements of expressions and
+conditions.  Conditions are a superset of expressions, and a condition
+may be used wherever an expression is required by enclosing it in
+parentheses.  The only places where expressions are used in the syntax
+instead of conditions is in expression statements and on the
+right-hand side of assignment statements; this catches some nasty bugs
+like accidentally writing \verb\x == 1\ instead of \verb\x = 1\.
+\indexii{assignment}{statement}
+
+The comma plays several roles in Python's syntax.  It is usually an
+operator with a lower precedence than all others, but occasionally
+serves other purposes as well; e.g. it separates function arguments,
+is used in list and dictionary constructors, and has special semantics
+in \verb\print\ statements.
+\index{comma}
+
+When (one alternative of) a syntax rule has the form
+
+\begin{verbatim}
+name:           othername
+\end{verbatim}
+
+and no semantics are given, the semantics of this form of \verb\name\
+are the same as for \verb\othername\.
+\index{syntax}
+
+\section{Arithmetic conversions}
+\indexii{arithmetic}{conversion}
+
+When a description of an arithmetic operator below uses the phrase
+``the numeric arguments are converted to a common type'',
+this both means that if either argument is not a number, a
+\verb\TypeError\ exception is raised, and that otherwise
+the following conversions are applied:
+\exindex{TypeError}
+\indexii{floating point}{number}
+\indexii{long}{integer}
+\indexii{plain}{integer}
+
+\begin{itemize}
+\item	first, if either argument is a floating point number,
+	the other is converted to floating point;
+\item	else, if either argument is a long integer,
+	the other is converted to long integer;
+\item	otherwise, both must be plain integers and no conversion
+	is necessary.
+\end{itemize}
+
+\section{Atoms}
+\index{atom}
+
+Atoms are the most basic elements of expressions.  Forms enclosed in
+reverse quotes or in parentheses, brackets or braces are also
+categorized syntactically as atoms.  The syntax for atoms is:
+
+\begin{verbatim}
+atom:           identifier | literal | enclosure
+enclosure:      parenth_form | list_display | dict_display | string_conversion
+\end{verbatim}
+
+\subsection{Identifiers (Names)}
+\index{name}
+\index{identifier}
+
+An identifier occurring as an atom is a reference to a local, global
+or built-in name binding.  If a name can be assigned to anywhere in a
+code block, and is not mentioned in a \verb\global\ statement in that
+code block, it refers to a local name throughout that code block.
+Otherwise, it refers to a global name if one exists, else to a
+built-in name.
+\indexii{name}{binding}
+\index{code block}
+\stindex{global}
+\indexii{built-in}{name}
+\indexii{global}{name}
+
+When the name is bound to an object, evaluation of the atom yields
+that object.  When a name is not bound, an attempt to evaluate it
+raises a \verb\NameError\ exception.
+\exindex{NameError}
+
+\subsection{Literals}
+\index{literal}
+
+Python knows string and numeric literals:
+
+\begin{verbatim}
+literal:        stringliteral | integer | longinteger | floatnumber
+\end{verbatim}
+
+Evaluation of a literal yields an object of the given type (string,
+integer, long integer, floating point number) with the given value.
+The value may be approximated in the case of floating point literals.
+See section \ref{literals} for details.
+
+All literals correspond to immutable data types, and hence the
+object's identity is less important than its value.  Multiple
+evaluations of literals with the same value (either the same
+occurrence in the program text or a different occurrence) may obtain
+the same object or a different object with the same value.
+\indexiii{immutable}{data}{type}
+
+(In the original implementation, all literals in the same code block
+with the same type and value yield the same object.)
+
+\subsection{Parenthesized forms}
+\index{parenthesized form}
+
+A parenthesized form is an optional condition list enclosed in
+parentheses:
+
+\begin{verbatim}
+parenth_form:      "(" [condition_list] ")"
+\end{verbatim}
+
+A parenthesized condition list yields whatever that condition list
+yields.
+
+An empty pair of parentheses yields an empty tuple object.  Since
+tuples are immutable, the rules for literals apply here.
+\indexii{empty}{tuple}
+
+(Note that tuples are not formed by the parentheses, but rather by use
+of the comma operator.  The exception is the empty tuple, for which
+parentheses {\em are} required --- allowing unparenthesized ``nothing''
+in expressions would causes ambiguities and allow common typos to
+pass uncaught.)
+\index{comma}
+\indexii{tuple}{display}
+
+\subsection{List displays}
+\indexii{list}{display}
+
+A list display is a possibly empty series of conditions enclosed in
+square brackets:
+
+\begin{verbatim}
+list_display:   "[" [condition_list] "]"
+\end{verbatim}
+
+A list display yields a new list object.
+\obindex{list}
+
+If it has no condition list, the list object has no items.  Otherwise,
+the elements of the condition list are evaluated from left to right
+and inserted in the list object in that order.
+\indexii{empty}{list}
+
+\subsection{Dictionary displays} \label{dict}
+\indexii{dictionary}{display}
+
+A dictionary display is a possibly empty series of key/datum pairs
+enclosed in curly braces:
+\index{key}
+\index{datum}
+\index{key/datum pair}
+
+\begin{verbatim}
+dict_display:   "{" [key_datum_list] "}"
+key_datum_list: key_datum ("," key_datum)* [","]
+key_datum:      condition ":" condition
+\end{verbatim}
+
+A dictionary display yields a new dictionary object.
+\obindex{dictionary}
+
+The key/datum pairs are evaluated from left to right to define the
+entries of the dictionary: each key object is used as a key into the
+dictionary to store the corresponding datum.
+
+Keys must be strings, otherwise a \verb\TypeError\ exception is
+raised.  Clashes between duplicate keys are not detected; the last
+datum (textually rightmost in the display) stored for a given key
+value prevails.
+\exindex{TypeError}
+
+\subsection{String conversions}
+\indexii{string}{conversion}
+
+A string conversion is a condition list enclosed in reverse (or
+backward) quotes:
+
+\begin{verbatim}
+string_conversion: "`" condition_list "`"
+\end{verbatim}
+
+A string conversion evaluates the contained condition list and
+converts the resulting object into a string according to rules
+specific to its type.
+
+If the object is a string, a number, \verb\None\, or a tuple, list or
+dictionary containing only objects whose type is one of these, the
+resulting string is a valid Python expression which can be passed to
+the built-in function \verb\eval()\ to yield an expression with the
+same value (or an approximation, if floating point numbers are
+involved).
+
+(In particular, converting a string adds quotes around it and converts
+``funny'' characters to escape sequences that are safe to print.)
+
+It is illegal to attempt to convert recursive objects (e.g. lists or
+dictionaries that contain a reference to themselves, directly or
+indirectly.)
+\obindex{recursive}
+
+\section{Primaries} \label{primaries}
+\index{primary}
+
+Primaries represent the most tightly bound operations of the language.
+Their syntax is:
+
+\begin{verbatim}
+primary:        atom | attributeref | subscription | slicing | call
+\end{verbatim}
+
+\subsection{Attribute references}
+\indexii{attribute}{reference}
+
+An attribute reference is a primary followed by a period and a name:
+
+\begin{verbatim}
+attributeref:   primary "." identifier
+\end{verbatim}
+
+The primary must evaluate to an object of a type that supports
+attribute references, e.g. a module or a list.  This object is then
+asked to produce the attribute whose name is the identifier.  If this
+attribute is not available, the exception \verb\AttributeError\ is
+raised.  Otherwise, the type and value of the object produced is
+determined by the object.  Multiple evaluations of the same attribute
+reference may yield different objects.
+\obindex{module}
+\obindex{list}
+
+\subsection{Subscriptions}
+\index{subscription}
+
+A subscription selects an item of a sequence (string, tuple or list)
+or mapping (dictionary) object:
+\obindex{sequence}
+\obindex{mapping}
+\obindex{string}
+\obindex{tuple}
+\obindex{list}
+\obindex{dictionary}
+\indexii{sequence}{item}
+
+\begin{verbatim}
+subscription:   primary "[" condition "]"
+\end{verbatim}
+
+The primary must evaluate to an object of a sequence or mapping type.
+
+If it is a mapping, the condition must evaluate to an object whose
+value is one of the keys of the mapping, and the subscription selects
+the value in the mapping that corresponds to that key.
+
+If it is a sequence, the condition must evaluate to a plain integer.
+If this value is negative, the length of the sequence is added to it
+(so that, e.g. \verb\x[-1]\ selects the last item of \verb\x\.)
+The resulting value must be a nonnegative integer smaller than the
+number of items in the sequence, and the subscription selects the item
+whose index is that value (counting from zero).
+
+A string's items are characters.  A character is not a separate data
+type but a string of exactly one character.
+\index{character}
+\indexii{string}{item}
+
+\subsection{Slicings}
+\index{slicing}
+\index{slice}
+
+A slicing (or slice) selects a range of items in a sequence (string,
+tuple or list) object:
+\obindex{sequence}
+\obindex{string}
+\obindex{tuple}
+\obindex{list}
+
+\begin{verbatim}
+slicing:        primary "[" [condition] ":" [condition] "]"
+\end{verbatim}
+
+The primary must evaluate to a sequence object.  The lower and upper
+bound expressions, if present, must evaluate to plain integers;
+defaults are zero and the sequence's length, respectively.  If either
+bound is negative, the sequence's length is added to it.  The slicing
+now selects all items with index $k$ such that $i <= k < j$ where $i$
+and $j$ are the specified lower and upper bounds.  This may be an
+empty sequence.  It is not an error if $i$ or $j$ lie outside the
+range of valid indexes (such items don't exist so they aren't
+selected).
+
+\subsection{Calls} \label{calls}
+\index{call}
+
+A call calls a callable object (e.g. a function) with a possibly empty
+series of arguments:
+\obindex{callable}
+
+\begin{verbatim}
+call:           primary "(" [condition_list] ")"
+\end{verbatim}
+
+The primary must evaluate to a callable object (user-defined
+functions, built-in functions, methods of built-in objects, class
+objects, and methods of class instances are callable).  If it is a
+class, the argument list must be empty; otherwise, the arguments are
+evaluated.
+
+A call always returns some value, possibly \verb\None\, unless it
+raises an exception.  How this value is computed depends on the type
+of the callable object.  If it is:
+
+\begin{description}
+
+\item[a user-defined function:] the code block for the function is
+executed, passing it the argument list.  The first thing the code
+block will do is bind the formal parameters to the arguments; this is
+described in section \ref{function}.  When the code block executes a
+\verb\return\ statement, this specifies the return value of the
+function call.
+\indexii{function}{call}
+\indexiii{user-defined}{function}{call}
+\obindex{user-defined function}
+\obindex{function}
+
+\item[a built-in function or method:] the result is up to the
+interpreter; see the library reference manual for the descriptions of
+built-in functions and methods.
+\indexii{function}{call}
+\indexii{built-in function}{call}
+\indexii{method}{call}
+\indexii{built-in method}{call}
+\obindex{built-in method}
+\obindex{built-in function}
+\obindex{method}
+\obindex{function}
+
+\item[a class object:] a new instance of that class is returned.
+\obindex{class}
+\indexii{class object}{call}
+
+\item[a class instance method:] the corresponding user-defined
+function is called, with an argument list that is one longer than the
+argument list of the call: the instance becomes the first argument.
+\obindex{class instance}
+\obindex{instance}
+\indexii{instance}{call}
+\indexii{class instance}{call}
+
+\end{description}
+
+\section{Unary arithmetic operations}
+\indexiii{unary}{arithmetic}{operation}
+\indexiii{unary}{bit-wise}{operation}
+
+All unary arithmetic (and bit-wise) operations have the same priority:
+
+\begin{verbatim}
+u_expr:         primary | "-" u_expr | "+" u_expr | "~" u_expr
+\end{verbatim}
+
+The unary \verb\"-"\ (minus) operator yields the negation of its
+numeric argument.
+\index{negation}
+\index{minus}
+
+The unary \verb\"+"\ (plus) operator yields its numeric argument
+unchanged.
+\index{plus}
+
+The unary \verb\"~"\ (invert) operator yields the bit-wise inversion
+of its plain or long integer argument.  The bit-wise inversion of
+\verb\x\ is defined as \verb\-(x+1)\.
+\index{inversion}
+
+In all three cases, if the argument does not have the proper type,
+a \verb\TypeError\ exception is raised.
+\exindex{TypeError}
+
+\section{Binary arithmetic operations}
+\indexiii{binary}{arithmetic}{operation}
+
+The binary arithmetic operations have the conventional priority
+levels.  Note that some of these operations also apply to certain
+non-numeric types.  There is no ``power'' operator, so there are only
+two levels, one for multiplicative operators and one for additive
+operators:
+
+\begin{verbatim}
+m_expr:         u_expr | m_expr "*" u_expr
+              | m_expr "/" u_expr | m_expr "%" u_expr
+a_expr:         m_expr | aexpr "+" m_expr | aexpr "-" m_expr
+\end{verbatim}
+
+The \verb\"*"\ (multiplication) operator yields the product of its
+arguments.  The arguments must either both be numbers, or one argument
+must be a plain integer and the other must be a sequence.  In the
+former case, the numbers are converted to a common type and then
+multiplied together.  In the latter case, sequence repetition is
+performed; a negative repetition factor yields an empty sequence.
+\index{multiplication}
+
+The \verb\"/"\ (division) operator yields the quotient of its
+arguments.  The numeric arguments are first converted to a common
+type.  Plain or long integer division yields an integer of the same
+type; the result is that of mathematical division with the `floor'
+function applied to the result.  Division by zero raises the
+\verb\ZeroDivisionError\ exception.
+\exindex{ZeroDivisionError}
+\index{division}
+
+The \verb\"%"\ (modulo) operator yields the remainder from the
+division of the first argument by the second.  The numeric arguments
+are first converted to a common type.  A zero right argument raises
+the \verb\ZeroDivisionError\ exception.  The arguments may be floating
+point numbers, e.g. \verb\3.14 % 0.7\ equals \verb\0.34\.  The modulo
+operator always yields a result with the same sign as its second
+operand (or zero); the absolute value of the result is strictly
+smaller than the second operand.
+\index{modulo}
+
+The integer division and modulo operators are connected by the
+following identity: \verb\x == (x/y)*y + (x%y)\.  Integer division and
+modulo are also connected with the built-in function \verb\divmod()\:
+\verb\divmod(x, y) == (x/y, x%y)\.  These identities don't hold for
+floating point numbers; there a similar identity holds where
+\verb\x/y\ is replaced by \verb\floor(x/y)\).
+
+The \verb\"+"\ (addition) operator yields the sum of its arguments.
+The arguments must either both be numbers, or both sequences of the
+same type.  In the former case, the numbers are converted to a common
+type and then added together.  In the latter case, the sequences are
+concatenated.
+\index{addition}
+
+The \verb\"-"\ (subtraction) operator yields the difference of its
+arguments.  The numeric arguments are first converted to a common
+type.
+\index{subtraction}
+
+\section{Shifting operations}
+\indexii{shifting}{operation}
+
+The shifting operations have lower priority than the arithmetic
+operations:
+
+\begin{verbatim}
+shift_expr:     a_expr | shift_expr ( "<<" | ">>" ) a_expr
+\end{verbatim}
+
+These operators accept plain or long integers as arguments.  The
+arguments are converted to a common type.  They shift the first
+argument to the left or right by the number of bits given by the
+second argument.
+
+A right shift by $n$ bits is defined as division by $2^n$.  A left
+shift by $n$ bits is defined as multiplication with $2^n$; for plain
+integers there is no overflow check so this drops bits and flip the
+sign if the result is not less than $2^{31}$ in absolute value.
+
+Negative shift counts raise a \verb\ValueError\ exception.
+\exindex{ValueError}
+
+\section{Binary bit-wise operations}
+\indexiii{binary}{bit-wise}{operation}
+
+Each of the three bitwise operations has a different priority level:
+
+\begin{verbatim}
+and_expr:       shift_expr | and_expr "&" shift_expr
+xor_expr:       and_expr | xor_expr "^" and_expr
+or_expr:       xor_expr | or_expr "|" xor_expr
+\end{verbatim}
+
+The \verb\"&"\ operator yields the bitwise AND of its arguments, which
+must be plain or long integers.  The arguments are converted to a
+common type.
+\indexii{bit-wise}{and}
+
+The \verb\"^"\ operator yields the bitwise XOR (exclusive OR) of its
+arguments, which must be plain or long integers.  The arguments are
+converted to a common type.
+\indexii{bit-wise}{xor}
+\indexii{exclusive}{or}
+
+The \verb\"|"\ operator yields the bitwise (inclusive) OR of its
+arguments, which must be plain or long integers.  The arguments are
+converted to a common type.
+\indexii{bit-wise}{or}
+\indexii{inclusive}{or}
+
+\section{Comparisons}
+\index{comparison}
+
+Contrary to C, all comparison operations in Python have the same
+priority, which is lower than that of any arithmetic, shifting or
+bitwise operation.  Also contrary to C, expressions like
+\verb\a < b < c\ have the interpretation that is conventional in
+mathematics:
+\index{C}
+
+\begin{verbatim}
+comparison:     or_expr (comp_operator or_expr)*
+comp_operator:  "<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"
+\end{verbatim}
+
+Comparisons yield integer values: 1 for true, 0 for false.
+
+Comparisons can be chained arbitrarily, e.g. $x < y <= z$ is
+equivalent to $x < y$ \verb\and\ $y <= z$, except that $y$ is
+evaluated only once (but in both cases $z$ is not evaluated at all
+when $x < y$ is found to be false).
+\indexii{chaining}{comparisons}
+
+Formally, $e_0 op_1 e_1 op_2 e_2 ...e_{n-1} op_n e_n$ is equivalent to
+$e_0 op_1 e_1$ \verb\and\ $e_1 op_2 e_2$ \verb\and\ ... \verb\and\
+$e_{n-1} op_n e_n$, except that each expression is evaluated at most once.
+
+Note that $e_0 op_1 e_1 op_2 e_2$ does not imply any kind of comparison
+between $e_0$ and $e_2$, e.g. $x < y > z$ is perfectly legal.
+
+The forms \verb\<>\ and \verb\!=\ are equivalent; for consistency with
+C, \verb\!=\ is preferred; where \verb\!=\ is mentioned below
+\verb\<>\ is also implied.
+
+The operators {\tt "<", ">", "==", ">=", "<="}, and {\tt "!="} compare
+the values of two objects.  The objects needn't have the same type.
+If both are numbers, they are coverted to a common type.  Otherwise,
+objects of different types {\em always} compare unequal, and are
+ordered consistently but arbitrarily.
+
+(This unusual definition of comparison is done to simplify the
+definition of operations like sorting and the \verb\in\ and \verb\not
+in\ operators.)
+
+Comparison of objects of the same type depends on the type:
+
+\begin{itemize}
+
+\item
+Numbers are compared arithmetically.
+
+\item
+Strings are compared lexicographically using the numeric equivalents
+(the result of the built-in function \verb\ord\) of their characters.
+
+\item
+Tuples and lists are compared lexicographically using comparison of
+corresponding items.
+
+\item
+Mappings (dictionaries) are compared through lexicographic
+comparison of their sorted (key, value) lists.%
+\footnote{This is expensive since it requires sorting the keys first,
+but about the only sensible definition.  It was tried to compare
+dictionaries by identity only, but this caused surprises because
+people expected to be able to test a dictionary for emptiness by
+comparing it to {\tt \{\}}.}
+
+\item
+Most other types compare unequal unless they are the same object;
+the choice whether one object is considered smaller or larger than
+another one is made arbitrarily but consistently within one
+execution of a program.
+
+\end{itemize}
+
+The operators \verb\in\ and \verb\not in\ test for sequence
+membership: if $y$ is a sequence, $x ~\verb\in\~ y$ is true if and
+only if there exists an index $i$ such that $x = y[i]$.
+$x ~\verb\not in\~ y$ yields the inverse truth value.  The exception
+\verb\TypeError\ is raised when $y$ is not a sequence, or when $y$ is
+a string and $x$ is not a string of length one.%
+\footnote{The latter restriction is sometimes a nuisance.}
+\opindex{in}
+\opindex{not in}
+\indexii{membership}{test}
+\obindex{sequence}
+
+The operators \verb\is\ and \verb\is not\ test for object identity:
+$x ~\verb\is\~ y$ is true if and only if $x$ and $y$ are the same
+object.  $x ~\verb\is not\~ y$ yields the inverse truth value.
+\opindex{is}
+\opindex{is not}
+\indexii{identity}{test}
+
+\section{Boolean operations} \label{Booleans}
+\indexii{Boolean}{operation}
+
+Boolean operations have the lowest priority of all Python operations:
+
+\begin{verbatim}
+condition:      or_test
+or_test:        and_test | or_test "or" and_test
+and_test:       not_test | and_test "and" not_test
+not_test:       comparison | "not" not_test
+\end{verbatim}
+
+In the context of Boolean operations, and also when conditions are
+used by control flow statements, the following values are interpreted
+as false: \verb\None\, numeric zero of all types, empty sequences
+(strings, tuples and lists), and empty mappings (dictionaries).  All
+other values are interpreted as true.
+
+The operator \verb\not\ yields 1 if its argument is false, 0 otherwise.
+\opindex{not}
+
+The condition $x ~\verb\and\~ y$ first evaluates $x$; if $x$ is false,
+its value is returned; otherwise, $y$ is evaluated and the resulting
+value is returned.
+\opindex{and}
+
+The condition $x ~\verb\or\~ y$ first evaluates $x$; if $x$ is true,
+its value is returned; otherwise, $y$ is evaluated and the resulting
+value is returned.
+\opindex{or}
+
+(Note that \verb\and\ and \verb\or\ do not restrict the value and type
+they return to 0 and 1, but rather return the last evaluated argument.
+This is sometimes useful, e.g. if \verb\s\ is a string that should be
+replaced by a default value if it is empty, the expression
+\verb\s or 'foo'\ yields the desired value.  Because \verb\not\ has to
+invent a value anyway, it does not bother to return a value of the
+same type as its argument, so e.g. \verb\not 'foo'\ yields \verb\0\,
+not \verb\''\.)
+
+\section{Expression lists and condition lists}
+\indexii{expression}{list}
+\indexii{condition}{list}
+
+\begin{verbatim}
+expr_list:      or_expr ("," or_expr)* [","]
+cond_list:      condition ("," condition)* [","]
+\end{verbatim}
+
+The only difference between expression lists and condition lists is
+the lowest priority of operators that can be used in them without
+being enclosed in parentheses; condition lists allow all operators,
+while expression lists don't allow comparisons and Boolean operators
+(they do allow bitwise and shift operators though).
+
+Expression lists are used in expression statements and assignments;
+condition lists are used everywhere else where a list of
+comma-separated values is required.
+
+An expression (condition) list containing at least one comma yields a
+tuple.  The length of the tuple is the number of expressions
+(conditions) in the list.  The expressions (conditions) are evaluated
+from left to right.  (Conditions lists are used syntactically is a few
+places where no tuple is constructed but a list of values is needed
+nevertheless.)
+\obindex{tuple}
+
+The trailing comma is required only to create a single tuple (a.k.a. a
+{\em singleton}); it is optional in all other cases.  A single
+expression (condition) without a trailing comma doesn't create a
+tuple, but rather yields the value of that expression (condition).
+\indexii{trailing}{comma}
+
+(To create an empty tuple, use an empty pair of parentheses:
+\verb\()\.)