Issue #14097: Backport 796d1371605d and subsequent changes.

The 2.7 tutorial introduction now matches the 3.x introduction as rewritten
by Ezio Melotti, with appropriate changes for 2.x.

Doc/tutorial/introduction.rst

@@ -5,7 +5,7 @@ An Informal Introduction to Python
 **********************************
 
 In the following examples, input and output are distinguished by the presence or
-absence of prompts (``>>>`` and ``...``): to repeat the example, you must type
+absence of prompts (:term:`>>>` and :term:`...`): to repeat the example, you must type
 everything after the prompt, when the prompt appears; lines that do not begin
 with a prompt are output from the interpreter. Note that a secondary prompt on a
 line by itself in an example means you must type a blank line; this is used to
@@ -22,9 +22,9 @@ be omitted when typing in examples.
 Some examples::
 
    # this is the first comment
-   SPAM = 1                 # and this is the second comment
-                            # ... and now a third!
-   STRING = "# This is not a comment."
+   spam = 1  # and this is the second comment
+             # ... and now a third!
+   text = "# This is not a comment because it's inside quotes."
 
 
 .. _tut-calculator:
@@ -44,43 +44,57 @@ Numbers
 The interpreter acts as a simple calculator: you can type an expression at it
 and it will write the value.  Expression syntax is straightforward: the
 operators ``+``, ``-``, ``*`` and ``/`` work just like in most other languages
-(for example, Pascal or C); parentheses can be used for grouping.  For example::
+(for example, Pascal or C); parentheses (``()``) can be used for grouping.
+For example::
 
-   >>> 2+2
+   >>> 2 + 2
    4
-   >>> # This is a comment
-   ... 2+2
-   4
-   >>> 2+2  # and a comment on the same line as code
-   4
-   >>> (50-5*6)/4
+   >>> 50 - 5*6
+   20
+   >>> (50 - 5.0*6) / 4
+   5.0
+   >>> 8 / 5.0
+   1.6
+
+The integer numbers (e.g. ``2``, ``4``, ``20``) have type :class:`int`,
+the ones with a fractional part (e.g. ``5.0``, ``1.6``) have type
+:class:`float`.  We will see more about numeric types later in the tutorial.
+
+The return type of a division (``/``) operation depends on its operands.  If
+both operands are of type :class:`int`, :term:`floor division` is performed
+and an :class:`int` is returned.  If either operand is a :class:`float`,
+classic division is performed and a :class:`float` is returned.  The ``//``
+operator is also provided for doing floor division no matter what the
+operands are.  The remainder can be calculated with the ``%`` operator::
+
+   >>> 17 / 3  # int / int -> int
    5
-   >>> # Integer division returns the floor:
-   ... 7/3
+   >>> 17 / 3.0  # int / float -> float
+   5.666666666666667
+   >>> 17 // 3.0  # explicit floor division discards the fractional part
+   5.0
+   >>> 17 % 3  # the % operator returns the remainder of the division
    2
-   >>> 7/-3
-   -3
+   >>> 5 * 3 + 2  # result * divisor + remainder
+   17
+
+With Python, it is possible to use the ``**`` operator to calculate powers [#]_::
+
+   >>> 5 ** 2  # 5 squared
+   25
+   >>> 2 ** 7  # 2 to the power of 7
+   128
 
-The equal sign (``'='``) is used to assign a value to a variable. Afterwards, no
+The equal sign (``=``) is used to assign a value to a variable. Afterwards, no
 result is displayed before the next interactive prompt::
 
    >>> width = 20
-   >>> height = 5*9
+   >>> height = 5 * 9
    >>> width * height
    900
 
-A value can be assigned to several variables simultaneously::
-
-   >>> x = y = z = 0  # Zero x, y and z
-   >>> x
-   0
-   >>> y
-   0
-   >>> z
-   0
-
-Variables must be "defined" (assigned a value) before they can be used, or an
-error will occur::
+If a variable is not "defined" (assigned a value), trying to use it will
+give you an error::
 
    >>> n  # try to access an undefined variable
    Traceback (most recent call last):
@@ -95,49 +109,6 @@ convert the integer operand to floating point::
    >>> 7.0 / 2
    3.5
 
-Complex numbers are also supported; imaginary numbers are written with a suffix
-of ``j`` or ``J``.  Complex numbers with a nonzero real component are written as
-``(real+imagj)``, or can be created with the ``complex(real, imag)`` function.
-::
-
-   >>> 1j * 1J
-   (-1+0j)
-   >>> 1j * complex(0,1)
-   (-1+0j)
-   >>> 3+1j*3
-   (3+3j)
-   >>> (3+1j)*3
-   (9+3j)
-   >>> (1+2j)/(1+1j)
-   (1.5+0.5j)
-
-Complex numbers are always represented as two floating point numbers, the real
-and imaginary part.  To extract these parts from a complex number *z*, use
-``z.real`` and ``z.imag``.   ::
-
-   >>> a=1.5+0.5j
-   >>> a.real
-   1.5
-   >>> a.imag
-   0.5
-
-The conversion functions to floating point and integer (:func:`float`,
-:func:`int` and :func:`long`) don't work for complex numbers --- there is no one
-correct way to convert a complex number to a real number.  Use ``abs(z)`` to get
-its magnitude (as a float) or ``z.real`` to get its real part. ::
-
-   >>> a=3.0+4.0j
-   >>> float(a)
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in ?
-   TypeError: can't convert complex to float; use abs(z)
-   >>> a.real
-   3.0
-   >>> a.imag
-   4.0
-   >>> abs(a)  # sqrt(a.real**2 + a.imag**2)
-   5.0
-
 In interactive mode, the last printed expression is assigned to the variable
 ``_``.  This means that when you are using Python as a desk calculator, it is
 somewhat easier to continue calculations, for example::
@@ -155,20 +126,28 @@ This variable should be treated as read-only by the user.  Don't explicitly
 assign a value to it --- you would create an independent local variable with the
 same name masking the built-in variable with its magic behavior.
 
+In addition to :class:`int` and :class:`float`, Python supports other types of
+numbers, such as :class:`~decimal.Decimal` and :class:`~fractions.Fraction`.
+Python also has built-in support for :ref:`complex numbers <typesnumeric>`,
+and uses the ``j`` or ``J`` suffix to indicate the imaginary part
+(e.g. ``3+5j``).
+
 
 .. _tut-strings:
 
 Strings
 -------
 
-Besides numbers, Python can also manipulate strings, which can be expressed in
-several ways.  They can be enclosed in single quotes or double quotes::
+Besides numbers, Python can also manipulate strings, which can be expressed
+in several ways.  They can be enclosed in single quotes (``'...'``) or
+double quotes (``"..."``) with the same result [#]_.  ``\`` can be used
+to escape quotes::
 
-   >>> 'spam eggs'
+   >>> 'spam eggs'  # single quotes
    'spam eggs'
-   >>> 'doesn\'t'
+   >>> 'doesn\'t'  # use \' to escape the single quote...
    "doesn't"
-   >>> "doesn't"
+   >>> "doesn't"  # ...or use double quotes instead
    "doesn't"
    >>> '"Yes," he said.'
    '"Yes," he said.'
@@ -177,45 +156,48 @@ several ways.  They can be enclosed in single quotes or double quotes::
    >>> '"Isn\'t," she said.'
    '"Isn\'t," she said.'
 
-The interpreter prints the result of string operations in the same way as they
-are typed for input: inside quotes, and with quotes and other funny characters
-escaped by backslashes, to show the precise value.  The string is enclosed in
-double quotes if the string contains a single quote and no double quotes, else
-it's enclosed in single quotes.  The :keyword:`print` statement produces a more
-readable output for such input strings.
+In the interactive interpreter, the output string is enclosed in quotes and
+special characters are escaped with backslashes.  While this might sometimes
+look different from the input (the enclosing quotes could change), the two
+strings are equivalent.  The string is enclosed in double quotes if
+the string contains a single quote and no double quotes, otherwise it is
+enclosed in single quotes.  The :keyword:`print` statement produces a more
+readable output, by omitting the enclosing quotes and by printing escaped
+and special characters::
 
-String literals can span multiple lines in several ways.  Continuation lines can
-be used, with a backslash as the last character on the line indicating that the
-next line is a logical continuation of the line::
-
-   hello = "This is a rather long string containing\n\
-   several lines of text just as you would do in C.\n\
-       Note that whitespace at the beginning of the line is\
-    significant."
-
-   print hello
-
-Note that newlines still need to be embedded in the string using ``\n`` -- the
-newline following the trailing backslash is discarded.  This example would print
-the following:
-
-.. code-block:: text
-
-   This is a rather long string containing
-   several lines of text just as you would do in C.
-       Note that whitespace at the beginning of the line is significant.
-
-Or, strings can be surrounded in a pair of matching triple-quotes: ``"""`` or
-``'''``.  End of lines do not need to be escaped when using triple-quotes, but
-they will be included in the string. ::
-
-   print """
+   >>> '"Isn\'t," she said.'
+   '"Isn\'t," she said.'
+   >>> print '"Isn\'t," she said.'
+   "Isn't," she said.
+   >>> s = 'First line.\nSecond line.'  # \n means newline
+   >>> s  # without print(), \n is included in the output
+   'First line.\nSecond line.'
+   >>> print s  # with print, \n produces a new line
+   First line.
+   Second line.
+
+If you don't want characters prefaced by ``\`` to be interpreted as
+special characters, you can use *raw strings* by adding an ``r`` before
+the first quote::
+
+   >>> print 'C:\some\name'  # here \n means newline!
+   C:\some
+   ame
+   >>> print r'C:\some\name'  # note the r before the quote
+   C:\some\name
+
+String literals can span multiple lines.  One way is using triple-quotes:
+``"""..."""`` or ``'''...'''``.  End of lines are automatically
+included in the string, but it's possible to prevent this by adding a ``\`` at
+the end of the line.  The following example::
+
+   print """\
    Usage: thingy [OPTIONS]
         -h                        Display this usage message
         -H hostname               Hostname to connect to
    """
 
-produces the following output:
+produces the following output (note that the initial newline is not included):
 
 .. code-block:: text
 
@@ -223,142 +205,100 @@ produces the following output:
         -h                        Display this usage message
         -H hostname               Hostname to connect to
 
-If we make the string literal a "raw" string, ``\n`` sequences are not converted
-to newlines, but the backslash at the end of the line, and the newline character
-in the source, are both included in the string as data.  Thus, the example::
-
-   hello = r"This is a rather long string containing\n\
-   several lines of text much as you would do in C."
-
-   print hello
-
-would print:
-
-.. code-block:: text
-
-   This is a rather long string containing\n\
-   several lines of text much as you would do in C.
-
 Strings can be concatenated (glued together) with the ``+`` operator, and
 repeated with ``*``::
 
-   >>> word = 'Help' + 'A'
-   >>> word
-   'HelpA'
-   >>> '<' + word*5 + '>'
-   '<HelpAHelpAHelpAHelpAHelpA>'
-
-Two string literals next to each other are automatically concatenated; the first
-line above could also have been written ``word = 'Help' 'A'``; this only works
-with two literals, not with arbitrary string expressions::
-
-   >>> 'str' 'ing'                   #  <-  This is ok
-   'string'
-   >>> 'str'.strip() + 'ing'   #  <-  This is ok
-   'string'
-   >>> 'str'.strip() 'ing'     #  <-  This is invalid
-     File "<stdin>", line 1, in ?
-       'str'.strip() 'ing'
-                         ^
-   SyntaxError: invalid syntax
+   >>> # 3 times 'un', followed by 'ium'
+   >>> 3 * 'un' + 'ium'
+   'unununium'
 
-Strings can be subscripted (indexed); like in C, the first character of a string
-has subscript (index) 0.  There is no separate character type; a character is
-simply a string of size one.  Like in Icon, substrings can be specified with the
-*slice notation*: two indices separated by a colon. ::
+Two or more *string literals* (i.e. the ones enclosed between quotes) next
+to each other are automatically concatenated. ::
 
-   >>> word[4]
-   'A'
-   >>> word[0:2]
-   'He'
-   >>> word[2:4]
-   'lp'
+   >>> 'Py' 'thon'
+   'Python'
 
-Slice indices have useful defaults; an omitted first index defaults to zero, an
-omitted second index defaults to the size of the string being sliced. ::
+This only works with two literals though, not with variables or expressions::
 
-   >>> word[:2]    # The first two characters
-   'He'
-   >>> word[2:]    # Everything except the first two characters
-   'lpA'
+   >>> prefix = 'Py'
+   >>> prefix 'thon'  # can't concatenate a variable and a string literal
+     ...
+   SyntaxError: invalid syntax
+   >>> ('un' * 3) 'ium'
+     ...
+   SyntaxError: invalid syntax
 
-Unlike a C string, Python strings cannot be changed.  Assigning to an indexed
-position in the string results in an error::
+If you want to concatenate variables or a variable and a literal, use ``+``::
 
-   >>> word[0] = 'x'
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in ?
-   TypeError: object does not support item assignment
-   >>> word[:1] = 'Splat'
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in ?
-   TypeError: object does not support slice assignment
+   >>> prefix + 'thon'
+   'Python'
 
-However, creating a new string with the combined content is easy and efficient::
+This feature is particularly useful when you want to break long strings::
 
-   >>> 'x' + word[1:]
-   'xelpA'
-   >>> 'Splat' + word[4]
-   'SplatA'
+   >>> text = ('Put several strings within parentheses '
+               'to have them joined together.')
+   >>> text
+   'Put several strings within parentheses to have them joined together.'
 
-Here's a useful invariant of slice operations: ``s[:i] + s[i:]`` equals ``s``.
-::
+Strings can be *indexed* (subscripted), with the first character having index 0.
+There is no separate character type; a character is simply a string of size
+one::
 
-   >>> word[:2] + word[2:]
-   'HelpA'
-   >>> word[:3] + word[3:]
-   'HelpA'
+   >>> word = 'Python'
+   >>> word[0]  # character in position 0
+   'P'
+   >>> word[5]  # character in position 5
+   'n'
 
-Degenerate slice indices are handled gracefully: an index that is too large is
-replaced by the string size, an upper bound smaller than the lower bound returns
-an empty string. ::
+Indices may also be negative numbers, to start counting from the right::
 
-   >>> word[1:100]
-   'elpA'
-   >>> word[10:]
-   ''
-   >>> word[2:1]
-   ''
+   >>> word[-1]  # last character
+   'n'
+   >>> word[-2]  # second-last character
+   'o'
+   >>> word[-6]
+   'P'
 
-Indices may be negative numbers, to start counting from the right. For example::
+Note that since -0 is the same as 0, negative indices start from -1.
 
-   >>> word[-1]     # The last character
-   'A'
-   >>> word[-2]     # The last-but-one character
-   'p'
-   >>> word[-2:]    # The last two characters
-   'pA'
-   >>> word[:-2]    # Everything except the last two characters
-   'Hel'
+In addition to indexing, *slicing* is also supported.  While indexing is used
+to obtain individual characters, *slicing* allows you to obtain a substring::
 
-But note that -0 is really the same as 0, so it does not count from the right!
-::
+   >>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
+   'Py'
+   >>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
+   'tho'
 
-   >>> word[-0]     # (since -0 equals 0)
-   'H'
+Note how the start is always included, and the end always excluded.  This
+makes sure that ``s[:i] + s[i:]`` is always equal to ``s``::
 
-Out-of-range negative slice indices are truncated, but don't try this for
-single-element (non-slice) indices::
+   >>> word[:2] + word[2:]
+   'Python'
+   >>> word[:4] + word[4:]
+   'Python'
 
-   >>> word[-100:]
-   'HelpA'
-   >>> word[-10]    # error
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in ?
-   IndexError: string index out of range
+Slice indices have useful defaults; an omitted first index defaults to zero, an
+omitted second index defaults to the size of the string being sliced. ::
+
+   >>> word[:2]  # character from the beginning to position 2 (excluded)
+   'Py'
+   >>> word[4:]  # characters from position 4 (included) to the end
+   'on'
+   >>> word[-2:] # characters from the second-last (included) to the end
+   'on'
 
 One way to remember how slices work is to think of the indices as pointing
 *between* characters, with the left edge of the first character numbered 0.
 Then the right edge of the last character of a string of *n* characters has
 index *n*, for example::
 
-    +---+---+---+---+---+
-    | H | e | l | p | A |
-    +---+---+---+---+---+
-    0   1   2   3   4   5
-   -5  -4  -3  -2  -1
+    +---+---+---+---+---+---+
+    | P | y | t | h | o | n |
+    +---+---+---+---+---+---+
+    0   1   2   3   4   5   6
+   -6  -5  -4  -3  -2  -1
 
-The first row of numbers gives the position of the indices 0...5 in the string;
+The first row of numbers gives the position of the indices 0...6 in the string;
 the second row gives the corresponding negative indices. The slice from *i* to
 *j* consists of all characters between the edges labeled *i* and *j*,
 respectively.
@@ -367,6 +307,38 @@ For non-negative indices, the length of a slice is the difference of the
 indices, if both are within bounds.  For example, the length of ``word[1:3]`` is
 2.
 
+Attempting to use a index that is too large will result in an error::
+
+   >>> word[42]  # the word only has 7 characters
+   Traceback (most recent call last):
+     File "<stdin>", line 1, in <module>
+   IndexError: string index out of range
+
+However, out of range slice indexes are handled gracefully when used for
+slicing::
+
+   >>> word[4:42]
+   'on'
+   >>> word[42:]
+   ''
+
+Python strings cannot be changed --- they are :term:`immutable`.
+Therefore, assigning to an indexed position in the string results in an error::
+
+   >>> word[0] = 'J'
+     ...
+   TypeError: 'str' object does not support item assignment
+   >>> word[2:] = 'py'
+     ...
+   TypeError: 'str' object does not support item assignment
+
+If you need a different string, you should create a new one::
+
+   >>> 'J' + word[1:]
+   'Jython'
+   >>> word[:2] + 'py'
+   'Pypy'
+
 The built-in function :func:`len` returns the length of a string::
 
    >>> s = 'supercalifragilisticexpialidocious'
@@ -499,94 +471,89 @@ Lists
 
 Python knows a number of *compound* data types, used to group together other
 values.  The most versatile is the *list*, which can be written as a list of
-comma-separated values (items) between square brackets.  List items need not all
-have the same type. ::
-
-   >>> a = ['spam', 'eggs', 100, 1234]
-   >>> a
-   ['spam', 'eggs', 100, 1234]
-
-Like string indices, list indices start at 0, and lists can be sliced,
-concatenated and so on::
-
-   >>> a[0]
-   'spam'
-   >>> a[3]
-   1234
-   >>> a[-2]
-   100
-   >>> a[1:-1]
-   ['eggs', 100]
-   >>> a[:2] + ['bacon', 2*2]
-   ['spam', 'eggs', 'bacon', 4]
-   >>> 3*a[:3] + ['Boo!']
-   ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']
+comma-separated values (items) between square brackets.  Lists might contain
+items of different types, but usually the items all have the same type. ::
+
+   >>> squares = [1, 4, 9, 16, 25]
+   >>> squares
+   [1, 4, 9, 16, 25]
+
+Like strings (and all other built-in :term:`sequence` type), lists can be
+indexed and sliced::
+
+   >>> squares[0]  # indexing returns the item
+   1
+   >>> squares[-1]
+   25
+   >>> squares[-3:]  # slicing returns a new list
+   [9, 16, 25]
 
 All slice operations return a new list containing the requested elements.  This
-means that the following slice returns a shallow copy of the list *a*::
+means that the following slice returns a new (shallow) copy of the list::
+
+   >>> squares[:]
+   [1, 4, 9, 16, 25]
+
+Lists also supports operations like concatenation::
 
-   >>> a[:]
-   ['spam', 'eggs', 100, 1234]
+   >>> squares + [36, 49, 64, 81, 100]
+   [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
 
-Unlike strings, which are *immutable*, it is possible to change individual
-elements of a list::
+Unlike strings, which are :term:`immutable`, lists are a :term:`mutable`
+type, i.e. it is possible to change their content::
 
-   >>> a
-   ['spam', 'eggs', 100, 1234]
-   >>> a[2] = a[2] + 23
-   >>> a
-   ['spam', 'eggs', 123, 1234]
+    >>> cubes = [1, 8, 27, 65, 125]  # something's wrong here
+    >>> 4 ** 3  # the cube of 4 is 64, not 65!
+    64
+    >>> cubes[3] = 64  # replace the wrong value
+    >>> cubes
+    [1, 8, 27, 64, 125]
+
+You can also add new items at the end of the list, by using
+the :meth:`~list.append` *method* (we will see more about methods later)::
+
+   >>> cubes.append(216)  # add the cube of 6
+   >>> cubes.append(7 ** 3)  # and the cube of 7
+   >>> cubes
+   [1, 8, 27, 64, 125, 216, 343]
 
 Assignment to slices is also possible, and this can even change the size of the
 list or clear it entirely::
 
-   >>> # Replace some items:
-   ... a[0:2] = [1, 12]
-   >>> a
-   [1, 12, 123, 1234]
-   >>> # Remove some:
-   ... a[0:2] = []
-   >>> a
-   [123, 1234]
-   >>> # Insert some:
-   ... a[1:1] = ['bletch', 'xyzzy']
-   >>> a
-   [123, 'bletch', 'xyzzy', 1234]
-   >>> # Insert (a copy of) itself at the beginning
-   >>> a[:0] = a
-   >>> a
-   [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]
-   >>> # Clear the list: replace all items with an empty list
-   >>> a[:] = []
-   >>> a
+   >>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
+   >>> letters
+   ['a', 'b', 'c', 'd', 'e', 'f', 'g']
+   >>> # replace some values
+   >>> letters[2:5] = ['C', 'D', 'E']
+   >>> letters
+   ['a', 'b', 'C', 'D', 'E', 'f', 'g']
+   >>> # now remove them
+   >>> letters[2:5] = []
+   >>> letters
+   ['a', 'b', 'f', 'g']
+   >>> # clear the list by replacing all the elements with an empty list
+   >>> letters[:] = []
+   >>> letters
    []
 
 The built-in function :func:`len` also applies to lists::
 
-   >>> a = ['a', 'b', 'c', 'd']
-   >>> len(a)
+   >>> letters = ['a', 'b', 'c', 'd']
+   >>> len(letters)
    4
 
 It is possible to nest lists (create lists containing other lists), for
 example::
 
-   >>> q = [2, 3]
-   >>> p = [1, q, 4]
-   >>> len(p)
-   3
-   >>> p[1]
-   [2, 3]
-   >>> p[1][0]
-   2
-   >>> p[1].append('xtra')     # See section 5.1
-   >>> p
-   [1, [2, 3, 'xtra'], 4]
-   >>> q
-   [2, 3, 'xtra']
-
-Note that in the last example, ``p[1]`` and ``q`` really refer to the same
-object!  We'll come back to *object semantics* later.
-
+   >>> a = ['a', 'b', 'c']
+   >>> n = [1, 2, 3]
+   >>> x = [a, n]
+   >>> x
+   [['a', 'b', 'c'], [1, 2, 3]]
+   >>> x[0]
+   ['a', 'b', 'c']
+   >>> x[0][1]
+   'b'
 
 .. _tut-firststeps:
 
@@ -658,3 +625,14 @@ This example introduces several new features.
 
   Note that the interpreter inserts a newline before it prints the next prompt if
   the last line was not completed.
+
+.. rubric:: Footnotes
+
+.. [#] Since ``**`` has higher precedence than ``-``, ``-3**2`` will be
+   interpreted as ``-(3**2)`` and thus result in ``-9``.  To avoid this
+   and get ``9``, you can use ``(-3)**2``.
+
+.. [#] Unlike other languages, special characters such as ``\n`` have the
+   same meaning with both single (``'...'``) and double (``"..."``) quotes.
+   The only difference between the two is that within single quotes you don't
+   need to escape ``"`` (but you have to escape ``\'``) and vice versa.