.. XXX: reference/datamodel and this have quite a few overlaps!


.. _bltin-types:

**************
Built-in Types
**************

The following sections describe the standard types that are built into the
interpreter.

.. note::

   Historically (until release 2.2), Python's built-in types have differed from
   user-defined types because it was not possible to use the built-in types as the
   basis for object-oriented inheritance. This limitation no longer
   exists.

.. index:: pair: built-in; types

The principal built-in types are numerics, sequences, mappings, files, classes,
instances and exceptions.

.. index:: statement: print

Some operations are supported by several object types; in particular,
practically all objects can be compared, tested for truth value, and converted
to a string (with the :func:`repr` function or the slightly different
:func:`str` function).  The latter function is implicitly used when an object is
written by the :func:`print` function.


.. _truth:

Truth Value Testing
===================

.. index::
   statement: if
   statement: while
   pair: truth; value
   pair: Boolean; operations
   single: false

Any object can be tested for truth value, for use in an :keyword:`if` or
:keyword:`while` condition or as operand of the Boolean operations below. The
following values are considered false:

  .. index:: single: None (Built-in object)

* ``None``

  .. index:: single: False (Built-in object)

* ``False``

* zero of any numeric type, for example, ``0``, ``0L``, ``0.0``, ``0j``.

* any empty sequence, for example, ``''``, ``()``, ``[]``.

* any empty mapping, for example, ``{}``.

* instances of user-defined classes, if the class defines a :meth:`__nonzero__`
  or :meth:`__len__` method, when that method returns the integer zero or
  :class:`bool` value ``False``. [#]_

.. index:: single: true

All other values are considered true --- so objects of many types are always
true.

.. index::
   operator: or
   operator: and
   single: False
   single: True

Operations and built-in functions that have a Boolean result always return ``0``
or ``False`` for false and ``1`` or ``True`` for true, unless otherwise stated.
(Important exception: the Boolean operations ``or`` and ``and`` always return
one of their operands.)


.. _boolean:

Boolean Operations --- :keyword:`and`, :keyword:`or`, :keyword:`not`
====================================================================

.. index:: pair: Boolean; operations

These are the Boolean operations, ordered by ascending priority:

+-------------+---------------------------------+-------+
| Operation   | Result                          | Notes |
+=============+=================================+=======+
| ``x or y``  | if *x* is false, then *y*, else | \(1)  |
|             | *x*                             |       |
+-------------+---------------------------------+-------+
| ``x and y`` | if *x* is false, then *x*, else | \(2)  |
|             | *y*                             |       |
+-------------+---------------------------------+-------+
| ``not x``   | if *x* is false, then ``True``, | \(3)  |
|             | else ``False``                  |       |
+-------------+---------------------------------+-------+

.. index::
   operator: and
   operator: or
   operator: not

Notes:

(1)
   This is a short-circuit operator, so it only evaluates the second
   argument if the first one is :const:`False`.

(2)
   This is a short-circuit operator, so it only evaluates the second
   argument if the first one is :const:`True`.

(3)
   ``not`` has a lower priority than non-Boolean operators, so ``not a == b`` is
   interpreted as ``not (a == b)``, and ``a == not b`` is a syntax error.


.. _stdcomparisons:

Comparisons
===========

.. index:: pair: chaining; comparisons

Comparison operations are supported by all objects.  They all have the same
priority (which is higher than that of the Boolean operations). Comparisons can
be chained arbitrarily; for example, ``x < y <= z`` is equivalent to ``x < y 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).

This table summarizes the comparison operations:

+------------+-------------------------+-------+
| Operation  | Meaning                 | Notes |
+============+=========================+=======+
| ``<``      | strictly less than      |       |
+------------+-------------------------+-------+
| ``<=``     | less than or equal      |       |
+------------+-------------------------+-------+
| ``>``      | strictly greater than   |       |
+------------+-------------------------+-------+
| ``>=``     | greater than or equal   |       |
+------------+-------------------------+-------+
| ``==``     | equal                   |       |
+------------+-------------------------+-------+
| ``!=``     | not equal               | \(1)  |
+------------+-------------------------+-------+
| ``is``     | object identity         |       |
+------------+-------------------------+-------+
| ``is not`` | negated object identity |       |
+------------+-------------------------+-------+

.. index::
   pair: operator; comparison
   operator: ==
   operator: <
   operator: <=
   operator: >
   operator: >=
   operator: !=
   operator: is
   operator: is not

Notes:

(1)
    ``!=`` can also be written ``<>``, but this is an obsolete usage
    kept for backwards compatibility only. New code should always use
    ``!=``.

.. index::
   pair: object; numeric
   pair: objects; comparing

Objects of different types, except different numeric types and different string
types, never compare equal; such objects are ordered consistently but
arbitrarily (so that sorting a heterogeneous array yields a consistent result).
Furthermore, some types (for example, file objects) support only a degenerate
notion of comparison where any two objects of that type are unequal.  Again,
such objects are ordered arbitrarily but consistently. The ``<``, ``<=``, ``>``
and ``>=`` operators will raise a :exc:`TypeError` exception when any operand is
a complex number.

.. index:: single: __cmp__() (instance method)

Instances of a class normally compare as non-equal unless the class defines the
:meth:`__cmp__` method.  Refer to :ref:`customization`) for information on the
use of this method to effect object comparisons.

**Implementation note:** Objects of different types except numbers are ordered
by their type names; objects of the same types that don't support proper
comparison are ordered by their address.

.. index::
   operator: in
   operator: not in

Two more operations with the same syntactic priority, ``in`` and ``not in``, are
supported only by sequence types (below).


.. _typesnumeric:

Numeric Types --- :class:`int`, :class:`float`, :class:`long`, :class:`complex`
===============================================================================

.. index::
   object: numeric
   object: Boolean
   object: integer
   object: long integer
   object: floating point
   object: complex number
   pair: C; language

There are four distinct numeric types: :dfn:`plain integers`, :dfn:`long
integers`,  :dfn:`floating point numbers`, and :dfn:`complex numbers`. In
addition, Booleans are a subtype of plain integers. Plain integers (also just
called :dfn:`integers`) are implemented using :ctype:`long` in C, which gives
them at least 32 bits of precision (``sys.maxint`` is always set to the maximum
plain integer value for the current platform, the minimum value is
``-sys.maxint - 1``).  Long integers have unlimited precision. Floating point
numbers are implemented using :ctype:`double` in C. All bets on their precision
are off unless you happen to know the machine you are working with.

Complex numbers have a real and imaginary part, which are each implemented using
:ctype:`double` in C.  To extract these parts from a complex number *z*, use
``z.real`` and ``z.imag``.

.. index::
   pair: numeric; literals
   pair: integer; literals
   triple: long; integer; literals
   pair: floating point; literals
   pair: complex number; literals
   pair: hexadecimal; literals
   pair: octal; literals

Numbers are created by numeric literals or as the result of built-in functions
and operators.  Unadorned integer literals (including binary, hex, and octal
numbers) yield plain integers unless the value they denote is too large to be
represented as a plain integer, in which case they yield a long integer.
Integer literals with an ``'L'`` or ``'l'`` suffix yield long integers (``'L'``
is preferred because ``1l`` looks too much like eleven!).  Numeric literals
containing a decimal point or an exponent sign yield floating point numbers.
Appending ``'j'`` or ``'J'`` to a numeric literal yields a complex number with a
zero real part. A complex numeric literal is the sum of a real and an imaginary
part.

.. index::
   single: arithmetic
   builtin: int
   builtin: long
   builtin: float
   builtin: complex

Python fully supports mixed arithmetic: when a binary arithmetic operator has
operands of different numeric types, the operand with the "narrower" type is
widened to that of the other, where plain integer is narrower than long integer
is narrower than floating point is narrower than complex. Comparisons between
numbers of mixed type use the same rule. [#]_ The constructors :func:`int`,
:func:`long`, :func:`float`, and :func:`complex` can be used to produce numbers
of a specific type.

All builtin numeric types support the following operations. See
:ref:`power` and later sections for the operators' priorities.

+--------------------+---------------------------------+--------+
| Operation          | Result                          | Notes  |
+====================+=================================+========+
| ``x + y``          | sum of *x* and *y*              |        |
+--------------------+---------------------------------+--------+
| ``x - y``          | difference of *x* and *y*       |        |
+--------------------+---------------------------------+--------+
| ``x * y``          | product of *x* and *y*          |        |
+--------------------+---------------------------------+--------+
| ``x / y``          | quotient of *x* and *y*         | \(1)   |
+--------------------+---------------------------------+--------+
| ``x // y``         | (floored) quotient of *x* and   | (4)(5) |
|                    | *y*                             |        |
+--------------------+---------------------------------+--------+
| ``x % y``          | remainder of ``x / y``          | \(4)   |
+--------------------+---------------------------------+--------+
| ``-x``             | *x* negated                     |        |
+--------------------+---------------------------------+--------+
| ``+x``             | *x* unchanged                   |        |
+--------------------+---------------------------------+--------+
| ``abs(x)``         | absolute value or magnitude of  | \(3)   |
|                    | *x*                             |        |
+--------------------+---------------------------------+--------+
| ``int(x)``         | *x* converted to integer        | \(2)   |
+--------------------+---------------------------------+--------+
| ``long(x)``        | *x* converted to long integer   | \(2)   |
+--------------------+---------------------------------+--------+
| ``float(x)``       | *x* converted to floating point | \(6)   |
+--------------------+---------------------------------+--------+
| ``complex(re,im)`` | a complex number with real part |        |
|                    | *re*, imaginary part *im*.      |        |
|                    | *im* defaults to zero.          |        |
+--------------------+---------------------------------+--------+
| ``c.conjugate()``  | conjugate of the complex number |        |
|                    | *c*. (Identity on real numbers) |        |
+--------------------+---------------------------------+--------+
| ``divmod(x, y)``   | the pair ``(x // y, x % y)``    | (3)(4) |
+--------------------+---------------------------------+--------+
| ``pow(x, y)``      | *x* to the power *y*            | (3)(7) |
+--------------------+---------------------------------+--------+
| ``x ** y``         | *x* to the power *y*            | \(7)   |
+--------------------+---------------------------------+--------+

.. index::
   triple: operations on; numeric; types
   single: conjugate() (complex number method)

Notes:

(1)
   .. index::
      pair: integer; division
      triple: long; integer; division

   For (plain or long) integer division, the result is an integer. The result is
   always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and
   (-1)/(-2) is 0.  Note that the result is a long integer if either operand is a
   long integer, regardless of the numeric value.

(2)
   .. index::
      module: math
      single: floor() (in module math)
      single: ceil() (in module math)
      single: trunc() (in module math)
      pair: numeric; conversions

   Conversion from floats using :func:`int` or :func:`long` truncates toward
   zero like the related function, :func:`math.trunc`.  Use the function
   :func:`math.floor` to round downward and :func:`math.ceil` to round
   upward.

(3)
   See :ref:`built-in-funcs` for a full description.

(4)
   Complex floor division operator, modulo operator, and :func:`divmod`.

   .. deprecated:: 2.3
      Instead convert to float using :func:`abs` if appropriate.

(5)
   Also referred to as integer division.  The resultant value is a whole integer,
   though the result's type is not necessarily int.

(6)
   float also accepts the strings "nan" and "inf" with an optional prefix "+"
   or "-" for Not a Number (NaN) and positive or negative infinity.

   .. versionadded:: 2.6

(7)
   Python defines ``pow(0, 0)`` and ``0 ** 0`` to be ``1``, as is common for
   programming languages.

All :class:`numbers.Real` types (:class:`int`, :class:`long`, and
:class:`float`) also include the following operations:

+--------------------+------------------------------------+--------+
| Operation          | Result                             | Notes  |
+====================+====================================+========+
| ``math.trunc(x)``  | *x* truncated to Integral          |        |
+--------------------+------------------------------------+--------+
| ``round(x[, n])``  | *x* rounded to n digits,           |        |
|                    | rounding half to even. If n is     |        |
|                    | omitted, it defaults to 0.         |        |
+--------------------+------------------------------------+--------+
| ``math.floor(x)``  | the greatest integral float <= *x* |        |
+--------------------+------------------------------------+--------+
| ``math.ceil(x)``   | the least integral float >= *x*    |        |
+--------------------+------------------------------------+--------+

.. XXXJH exceptions: overflow (when? what operations?) zerodivision


.. _bitstring-ops:

Bit-string Operations on Integer Types
--------------------------------------

.. _bit-string-operations:

Plain and long integer types support additional operations that make sense only
for bit-strings.  Negative numbers are treated as their 2's complement value
(for long integers, this assumes a sufficiently large number of bits that no
overflow occurs during the operation).

The priorities of the binary bitwise operations are all lower than the numeric
operations and higher than the comparisons; the unary operation ``~`` has the
same priority as the other unary numeric operations (``+`` and ``-``).

This table lists the bit-string operations sorted in ascending priority:

+------------+--------------------------------+----------+
| Operation  | Result                         | Notes    |
+============+================================+==========+
| ``x | y``  | bitwise :dfn:`or` of *x* and   |          |
|            | *y*                            |          |
+------------+--------------------------------+----------+
| ``x ^ y``  | bitwise :dfn:`exclusive or` of |          |
|            | *x* and *y*                    |          |
+------------+--------------------------------+----------+
| ``x & y``  | bitwise :dfn:`and` of *x* and  |          |
|            | *y*                            |          |
+------------+--------------------------------+----------+
| ``x << n`` | *x* shifted left by *n* bits   | (1)(2)   |
+------------+--------------------------------+----------+
| ``x >> n`` | *x* shifted right by *n* bits  | (1)(3)   |
+------------+--------------------------------+----------+
| ``~x``     | the bits of *x* inverted       |          |
+------------+--------------------------------+----------+

.. index::
   triple: operations on; integer; types
   pair: bit-string; operations
   pair: shifting; operations
   pair: masking; operations

Notes:

(1)
   Negative shift counts are illegal and cause a :exc:`ValueError` to be raised.

(2)
   A left shift by *n* bits is equivalent to multiplication by ``pow(2, n)``.  A
   long integer is returned if the result exceeds the range of plain integers.

(3)
   A right shift by *n* bits is equivalent to division by ``pow(2, n)``.


Additional Methods on Float
---------------------------

The float type has some additional methods.

.. method:: float.as_integer_ratio()

    Return a pair of integers whose ratio is exactly equal to the
    original float and with a positive denominator.  Raises
    :exc:`OverflowError` on infinities and a :exc:`ValueError` on
    NaNs.

    .. versionadded:: 2.6

Two methods support conversion to
and from hexadecimal strings.  Since Python's floats are stored
internally as binary numbers, converting a float to or from a
*decimal* string usually involves a small rounding error.  In
contrast, hexadecimal strings allow exact representation and
specification of floating-point numbers.  This can be useful when
debugging, and in numerical work.


.. method:: float.hex()

   Return a representation of a floating-point number as a hexadecimal
   string.  For finite floating-point numbers, this representation
   will always include a leading ``0x`` and a trailing ``p`` and
   exponent.

   .. versionadded:: 2.6


.. method:: float.fromhex(s)

   Class method to return the float represented by a hexadecimal
   string *s*.  The string *s* may have leading and trailing
   whitespace.

   .. versionadded:: 2.6


Note that :meth:`float.hex` is an instance method, while
:meth:`float.fromhex` is a class method.

A hexadecimal string takes the form::

   [sign] ['0x'] integer ['.' fraction] ['p' exponent]

where the optional ``sign`` may by either ``+`` or ``-``, ``integer``
and ``fraction`` are strings of hexadecimal digits, and ``exponent``
is a decimal integer with an optional leading sign.  Case is not
significant, and there must be at least one hexadecimal digit in
either the integer or the fraction.  This syntax is similar to the
syntax specified in section 6.4.4.2 of the C99 standard, and also to
the syntax used in Java 1.5 onwards.  In particular, the output of
:meth:`float.hex` is usable as a hexadecimal floating-point literal in
C or Java code, and hexadecimal strings produced by C's ``%a`` format
character or Java's ``Double.toHexString`` are accepted by
:meth:`float.fromhex`.


Note that the exponent is written in decimal rather than hexadecimal,
and that it gives the power of 2 by which to multiply the coefficient.
For example, the hexadecimal string ``0x3.a7p10`` represents the
floating-point number ``(3 + 10./16 + 7./16**2) * 2.0**10``, or
``3740.0``::

   >>> float.fromhex('0x3.a7p10')
   3740.0


Applying the reverse conversion to ``3740.0`` gives a different
hexadecimal string representing the same number::

   >>> float.hex(3740.0)
   '0x1.d380000000000p+11'


.. _typeiter:

Iterator Types
==============

.. versionadded:: 2.2

.. index::
   single: iterator protocol
   single: protocol; iterator
   single: sequence; iteration
   single: container; iteration over

Python supports a concept of iteration over containers.  This is implemented
using two distinct methods; these are used to allow user-defined classes to
support iteration.  Sequences, described below in more detail, always support
the iteration methods.

One method needs to be defined for container objects to provide iteration
support:

.. XXX duplicated in reference/datamodel!

.. method:: container.__iter__()

   Return an iterator object.  The object is required to support the iterator
   protocol described below.  If a container supports different types of
   iteration, additional methods can be provided to specifically request
   iterators for those iteration types.  (An example of an object supporting
   multiple forms of iteration would be a tree structure which supports both
   breadth-first and depth-first traversal.)  This method corresponds to the
   :attr:`tp_iter` slot of the type structure for Python objects in the Python/C
   API.

The iterator objects themselves are required to support the following two
methods, which together form the :dfn:`iterator protocol`:


.. method:: iterator.__iter__()

   Return the iterator object itself.  This is required to allow both containers
   and iterators to be used with the :keyword:`for` and :keyword:`in` statements.
   This method corresponds to the :attr:`tp_iter` slot of the type structure for
   Python objects in the Python/C API.


.. method:: iterator.next()

   Return the next item from the container.  If there are no further items, raise
   the :exc:`StopIteration` exception.  This method corresponds to the
   :attr:`tp_iternext` slot of the type structure for Python objects in the
   Python/C API.

Python defines several iterator objects to support iteration over general and
specific sequence types, dictionaries, and other more specialized forms.  The
specific types are not important beyond their implementation of the iterator
protocol.

The intention of the protocol is that once an iterator's :meth:`next` method
raises :exc:`StopIteration`, it will continue to do so on subsequent calls.
Implementations that do not obey this property are deemed broken.  (This
constraint was added in Python 2.3; in Python 2.2, various iterators are broken
according to this rule.)

Python's :term:`generator`\s provide a convenient way to implement the iterator
protocol.  If a container object's :meth:`__iter__` method is implemented as a
generator, it will automatically return an iterator object (technically, a
generator object) supplying the :meth:`__iter__` and :meth:`next` methods.


.. _typesseq:

Sequence Types --- :class:`str`, :class:`unicode`, :class:`list`, :class:`tuple`, :class:`buffer`, :class:`xrange`
==================================================================================================================

There are six sequence types: strings, Unicode strings, lists, tuples, buffers,
and xrange objects.

For other containers see the built in :class:`dict` and :class:`set` classes,
and the :mod:`collections` module.


.. index::
   object: sequence
   object: string
   object: Unicode
   object: tuple
   object: list
   object: buffer
   object: xrange

String literals are written in single or double quotes: ``'xyzzy'``,
``"frobozz"``.  See :ref:`strings` for more about string literals.
Unicode strings are much like strings, but are specified in the syntax
using a preceding ``'u'`` character: ``u'abc'``, ``u"def"``. In addition
to the functionality described here, there are also string-specific
methods described in the :ref:`string-methods` section. Lists are
constructed with square brackets, separating items with commas: ``[a, b, c]``.
Tuples are constructed by the comma operator (not within square
brackets), with or without enclosing parentheses, but an empty tuple
must have the enclosing parentheses, such as ``a, b, c`` or ``()``.  A
single item tuple must have a trailing comma, such as ``(d,)``.

Buffer objects are not directly supported by Python syntax, but can be created
by calling the builtin function :func:`buffer`.  They don't support
concatenation or repetition.

Objects of type xrange are similar to buffers in that there is no specific syntax to
create them, but they are created using the :func:`xrange` function.  They don't
support slicing, concatenation or repetition, and using ``in``, ``not in``,
:func:`min` or :func:`max` on them is inefficient.

Most sequence types support the following operations.  The ``in`` and ``not in``
operations have the same priorities as the comparison operations.  The ``+`` and
``*`` operations have the same priority as the corresponding numeric operations.
[#]_ Additional methods are provided for :ref:`typesseq-mutable`.

This table lists the sequence operations sorted in ascending priority
(operations in the same box have the same priority).  In the table, *s* and *t*
are sequences of the same type; *n*, *i* and *j* are integers:

+------------------+--------------------------------+----------+
| Operation        | Result                         | Notes    |
+==================+================================+==========+
| ``x in s``       | ``True`` if an item of *s* is  | \(1)     |
|                  | equal to *x*, else ``False``   |          |
+------------------+--------------------------------+----------+
| ``x not in s``   | ``False`` if an item of *s* is | \(1)     |
|                  | equal to *x*, else ``True``    |          |
+------------------+--------------------------------+----------+
| ``s + t``        | the concatenation of *s* and   | \(6)     |
|                  | *t*                            |          |
+------------------+--------------------------------+----------+
| ``s * n, n * s`` | *n* shallow copies of *s*      | \(2)     |
|                  | concatenated                   |          |
+------------------+--------------------------------+----------+
| ``s[i]``         | *i*'th item of *s*, origin 0   | \(3)     |
+------------------+--------------------------------+----------+
| ``s[i:j]``       | slice of *s* from *i* to *j*   | (3)(4)   |
+------------------+--------------------------------+----------+
| ``s[i:j:k]``     | slice of *s* from *i* to *j*   | (3)(5)   |
|                  | with step *k*                  |          |
+------------------+--------------------------------+----------+
| ``len(s)``       | length of *s*                  |          |
+------------------+--------------------------------+----------+
| ``min(s)``       | smallest item of *s*           |          |
+------------------+--------------------------------+----------+
| ``max(s)``       | largest item of *s*            |          |
+------------------+--------------------------------+----------+

Sequence types also support comparisons. In particular, tuples and lists
are compared lexicographically by comparing corresponding
elements. This means that to compare equal, every element must compare
equal and the two sequences must be of the same type and have the same
length. (For full details see :ref:`comparisons` in the language
reference.)

.. index::
   triple: operations on; sequence; types
   builtin: len
   builtin: min
   builtin: max
   pair: concatenation; operation
   pair: repetition; operation
   pair: subscript; operation
   pair: slice; operation
   pair: extended slice; operation
   operator: in
   operator: not in

Notes:

(1)
   When *s* is a string or Unicode string object the ``in`` and ``not in``
   operations act like a substring test.  In Python versions before 2.3, *x* had to
   be a string of length 1. In Python 2.3 and beyond, *x* may be a string of any
   length.

(2)
   Values of *n* less than ``0`` are treated as ``0`` (which yields an empty
   sequence of the same type as *s*).  Note also that the copies are shallow;
   nested structures are not copied.  This often haunts new Python programmers;
   consider:

      >>> lists = [[]] * 3
      >>> lists
      [[], [], []]
      >>> lists[0].append(3)
      >>> lists
      [[3], [3], [3]]

   What has happened is that ``[[]]`` is a one-element list containing an empty
   list, so all three elements of ``[[]] * 3`` are (pointers to) this single empty
   list.  Modifying any of the elements of ``lists`` modifies this single list.
   You can create a list of different lists this way:

      >>> lists = [[] for i in range(3)]
      >>> lists[0].append(3)
      >>> lists[1].append(5)
      >>> lists[2].append(7)
      >>> lists
      [[3], [5], [7]]

(3)
   If *i* or *j* is negative, the index is relative to the end of the string:
   ``len(s) + i`` or ``len(s) + j`` is substituted.  But note that ``-0`` is still
   ``0``.

(4)
   The slice of *s* from *i* to *j* is defined as the sequence of items with index
   *k* such that ``i <= k < j``.  If *i* or *j* is greater than ``len(s)``, use
   ``len(s)``.  If *i* is omitted or ``None``, use ``0``.  If *j* is omitted or
   ``None``, use ``len(s)``.  If *i* is greater than or equal to *j*, the slice is
   empty.

(5)
   The slice of *s* from *i* to *j* with step *k* is defined as the sequence of
   items with index  ``x = i + n*k`` such that ``0 <= n < (j-i)/k``.  In other words,
   the indices are ``i``, ``i+k``, ``i+2*k``, ``i+3*k`` and so on, stopping when
   *j* is reached (but never including *j*).  If *i* or *j* is greater than
   ``len(s)``, use ``len(s)``.  If *i* or *j* are omitted or ``None``, they become
   "end" values (which end depends on the sign of *k*).  Note, *k* cannot be zero.
   If *k* is ``None``, it is treated like ``1``.

(6)
   If *s* and *t* are both strings, some Python implementations such as CPython can
   usually perform an in-place optimization for assignments of the form ``s=s+t``
   or ``s+=t``.  When applicable, this optimization makes quadratic run-time much
   less likely.  This optimization is both version and implementation dependent.
   For performance sensitive code, it is preferable to use the :meth:`str.join`
   method which assures consistent linear concatenation performance across versions
   and implementations.

   .. versionchanged:: 2.4
      Formerly, string concatenation never occurred in-place.


.. _string-methods:

String Methods
--------------

.. index:: pair: string; methods

Below are listed the string methods which both 8-bit strings and Unicode objects
support. Note that none of these methods take keyword arguments.

In addition, Python's strings support the sequence type methods
described in the :ref:`typesseq` section. To output formatted strings
use template strings or the ``%`` operator described in the
:ref:`string-formatting` section. Also, see the :mod:`re` module for
string functions based on regular expressions.

.. method:: str.capitalize()

   Return a copy of the string with only its first character capitalized.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.center(width[, fillchar])

   Return centered in a string of length *width*. Padding is done using the
   specified *fillchar* (default is a space).

   .. versionchanged:: 2.4
      Support for the *fillchar* argument.


.. method:: str.count(sub[, start[, end]])

   Return the number of non-overlapping occurrences of substring *sub* in the
   range [*start*, *end*].  Optional arguments *start* and *end* are
   interpreted as in slice notation.


.. method:: str.decode([encoding[, errors]])

   Decodes the string using the codec registered for *encoding*. *encoding*
   defaults to the default string encoding.  *errors* may be given to set a
   different error handling scheme.  The default is ``'strict'``, meaning that
   encoding errors raise :exc:`UnicodeError`.  Other possible values are
   ``'ignore'``, ``'replace'`` and any other name registered via
   :func:`codecs.register_error`, see section :ref:`codec-base-classes`.

   .. versionadded:: 2.2

   .. versionchanged:: 2.3
      Support for other error handling schemes added.


.. method:: str.encode([encoding[,errors]])

   Return an encoded version of the string.  Default encoding is the current
   default string encoding.  *errors* may be given to set a different error
   handling scheme.  The default for *errors* is ``'strict'``, meaning that
   encoding errors raise a :exc:`UnicodeError`.  Other possible values are
   ``'ignore'``, ``'replace'``, ``'xmlcharrefreplace'``, ``'backslashreplace'`` and
   any other name registered via :func:`codecs.register_error`, see section
   :ref:`codec-base-classes`. For a list of possible encodings, see section
   :ref:`standard-encodings`.

   .. versionadded:: 2.0

   .. versionchanged:: 2.3
      Support for ``'xmlcharrefreplace'`` and ``'backslashreplace'`` and other error
      handling schemes added.


.. method:: str.endswith(suffix[, start[, end]])

   Return ``True`` if the string ends with the specified *suffix*, otherwise return
   ``False``.  *suffix* can also be a tuple of suffixes to look for.  With optional
   *start*, test beginning at that position.  With optional *end*, stop comparing
   at that position.

   .. versionchanged:: 2.5
      Accept tuples as *suffix*.


.. method:: str.expandtabs([tabsize])

   Return a copy of the string where all tab characters are replaced by one or
   more spaces, depending on the current column and the given tab size.  The
   column number is reset to zero after each newline occurring in the string.
   If *tabsize* is not given, a tab size of ``8`` characters is assumed.  This
   doesn't understand other non-printing characters or escape sequences.


.. method:: str.find(sub[, start[, end]])

   Return the lowest index in the string where substring *sub* is found, such that
   *sub* is contained in the range [*start*, *end*].  Optional arguments *start*
   and *end* are interpreted as in slice notation.  Return ``-1`` if *sub* is not
   found.


.. method:: str.format(format_string, *args, **kwargs)

   Perform a string formatting operation.  The *format_string* argument can
   contain literal text or replacement fields delimited by braces ``{}``.  Each
   replacement field contains either the numeric index of a positional argument,
   or the name of a keyword argument.  Returns a copy of *format_string* where
   each replacement field is replaced with the string value of the corresponding
   argument.

      >>> "The sum of 1 + 2 is {0}".format(1+2)
      'The sum of 1 + 2 is 3'

   See :ref:`formatstrings` for a description of the various formatting options
   that can be specified in format strings.

   This method of string formatting is the new standard in Python 3.0, and
   should be preferred to the ``%`` formatting described in
   :ref:`string-formatting` in new code.

   .. versionadded:: 2.6


.. method:: str.index(sub[, start[, end]])

   Like :meth:`find`, but raise :exc:`ValueError` when the substring is not found.


.. method:: str.isalnum()

   Return true if all characters in the string are alphanumeric and there is at
   least one character, false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.isalpha()

   Return true if all characters in the string are alphabetic and there is at least
   one character, false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.isdigit()

   Return true if all characters in the string are digits and there is at least one
   character, false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.islower()

   Return true if all cased characters in the string are lowercase and there is at
   least one cased character, false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.isspace()

   Return true if there are only whitespace characters in the string and there is
   at least one character, false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.istitle()

   Return true if the string is a titlecased string and there is at least one
   character, for example uppercase characters may only follow uncased characters
   and lowercase characters only cased ones.  Return false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.isupper()

   Return true if all cased characters in the string are uppercase and there is at
   least one cased character, false otherwise.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.join(seq)

   Return a string which is the concatenation of the strings in the sequence *seq*.
   The separator between elements is the string providing this method.


.. method:: str.ljust(width[, fillchar])

   Return the string left justified in a string of length *width*. Padding is done
   using the specified *fillchar* (default is a space).  The original string is
   returned if *width* is less than ``len(s)``.

   .. versionchanged:: 2.4
      Support for the *fillchar* argument.


.. method:: str.lower()

   Return a copy of the string converted to lowercase.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.lstrip([chars])

   Return a copy of the string with leading characters removed.  The *chars*
   argument is a string specifying the set of characters to be removed.  If omitted
   or ``None``, the *chars* argument defaults to removing whitespace.  The *chars*
   argument is not a prefix; rather, all combinations of its values are stripped:

      >>> '   spacious   '.lstrip()
      'spacious   '
      >>> 'www.example.com'.lstrip('cmowz.')
      'example.com'

   .. versionchanged:: 2.2.2
      Support for the *chars* argument.


.. method:: str.partition(sep)

   Split the string at the first occurrence of *sep*, and return a 3-tuple
   containing the part before the separator, the separator itself, and the part
   after the separator.  If the separator is not found, return a 3-tuple containing
   the string itself, followed by two empty strings.

   .. versionadded:: 2.5


.. method:: str.replace(old, new[, count])

   Return a copy of the string with all occurrences of substring *old* replaced by
   *new*.  If the optional argument *count* is given, only the first *count*
   occurrences are replaced.


.. method:: str.rfind(sub [,start [,end]])

   Return the highest index in the string where substring *sub* is found, such that
   *sub* is contained within s[start,end].  Optional arguments *start* and *end*
   are interpreted as in slice notation.  Return ``-1`` on failure.


.. method:: str.rindex(sub[, start[, end]])

   Like :meth:`rfind` but raises :exc:`ValueError` when the substring *sub* is not
   found.


.. method:: str.rjust(width[, fillchar])

   Return the string right justified in a string of length *width*. Padding is done
   using the specified *fillchar* (default is a space). The original string is
   returned if *width* is less than ``len(s)``.

   .. versionchanged:: 2.4
      Support for the *fillchar* argument.


.. method:: str.rpartition(sep)

   Split the string at the last occurrence of *sep*, and return a 3-tuple
   containing the part before the separator, the separator itself, and the part
   after the separator.  If the separator is not found, return a 3-tuple containing
   two empty strings, followed by the string itself.

   .. versionadded:: 2.5


.. method:: str.rsplit([sep [,maxsplit]])

   Return a list of the words in the string, using *sep* as the delimiter string.
   If *maxsplit* is given, at most *maxsplit* splits are done, the *rightmost*
   ones.  If *sep* is not specified or ``None``, any whitespace string is a
   separator.  Except for splitting from the right, :meth:`rsplit` behaves like
   :meth:`split` which is described in detail below.

   .. versionadded:: 2.4


.. method:: str.rstrip([chars])

   Return a copy of the string with trailing characters removed.  The *chars*
   argument is a string specifying the set of characters to be removed.  If omitted
   or ``None``, the *chars* argument defaults to removing whitespace.  The *chars*
   argument is not a suffix; rather, all combinations of its values are stripped:

      >>> '   spacious   '.rstrip()
      '   spacious'
      >>> 'mississippi'.rstrip('ipz')
      'mississ'

   .. versionchanged:: 2.2.2
      Support for the *chars* argument.


.. method:: str.split([sep[, maxsplit]])

   Return a list of the words in the string, using *sep* as the delimiter
   string.  If *maxsplit* is given, at most *maxsplit* splits are done (thus,
   the list will have at most ``maxsplit+1`` elements).  If *maxsplit* is not
   specified, then there is no limit on the number of splits (all possible
   splits are made).

   If *sep* is given, consecutive delimiters are not grouped together and are
   deemed to delimit empty strings (for example, ``'1,,2'.split(',')`` returns
   ``['1', '', '2']``).  The *sep* argument may consist of multiple characters
   (for example, ``'1<>2<>3'.split('<>')`` returns ``['1', '2', '3']``).
   Splitting an empty string with a specified separator returns ``['']``.

   If *sep* is not specified or is ``None``, a different splitting algorithm is
   applied: runs of consecutive whitespace are regarded as a single separator,
   and the result will contain no empty strings at the start or end if the
   string has leading or trailing whitespace.  Consequently, splitting an empty
   string or a string consisting of just whitespace with a ``None`` separator
   returns ``[]``.

   For example, ``' 1  2   3  '.split()`` returns ``['1', '2', '3']``, and
   ``'  1  2   3  '.split(None, 1)`` returns ``['1', '2   3  ']``.


.. method:: str.splitlines([keepends])

   Return a list of the lines in the string, breaking at line boundaries.  Line
   breaks are not included in the resulting list unless *keepends* is given and
   true.


.. method:: str.startswith(prefix[, start[, end]])

   Return ``True`` if string starts with the *prefix*, otherwise return ``False``.
   *prefix* can also be a tuple of prefixes to look for.  With optional *start*,
   test string beginning at that position.  With optional *end*, stop comparing
   string at that position.

   .. versionchanged:: 2.5
      Accept tuples as *prefix*.


.. method:: str.strip([chars])

   Return a copy of the string with the leading and trailing characters removed.
   The *chars* argument is a string specifying the set of characters to be removed.
   If omitted or ``None``, the *chars* argument defaults to removing whitespace.
   The *chars* argument is not a prefix or suffix; rather, all combinations of its
   values are stripped:

      >>> '   spacious   '.strip()
      'spacious'
      >>> 'www.example.com'.strip('cmowz.')
      'example'

   .. versionchanged:: 2.2.2
      Support for the *chars* argument.


.. method:: str.swapcase()

   Return a copy of the string with uppercase characters converted to lowercase and
   vice versa.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.title()

   Return a titlecased version of the string where words start with an uppercase
   character and the remaining characters are lowercase.

   The algorithm uses a simple language-independent definition of a word as
   groups of consecutive letters.  The definition works in many contexts but
   it means that apostrophes in contractions and possessives form word
   boundaries, which may not be the desired result::

        >>> "they're bill's friends from the UK".title()
        "They'Re Bill'S Friends From The Uk"

   A workaround for apostrophes can be constructed using regular expressions::

        >>> import re
        >>> def titlecase(s):
                return re.sub(r"[A-Za-z]+('[A-Za-z]+)?",
                              lambda mo: mo.group(0)[0].upper() +
                                         mo.group(0)[1:].lower(),
                              s)

        >>> titlecase("they're bill's friends.")
        "They're Bill's Friends."

   For 8-bit strings, this method is locale-dependent.


.. method:: str.translate(table[, deletechars])

   Return a copy of the string where all characters occurring in the optional
   argument *deletechars* are removed, and the remaining characters have been
   mapped through the given translation table, which must be a string of length
   256.

   You can use the :func:`maketrans` helper function in the :mod:`string` module to
   create a translation table. For string objects, set the *table* argument to
   ``None`` for translations that only delete characters:

      >>> 'read this short text'.translate(None, 'aeiou')
      'rd ths shrt txt'

   .. versionadded:: 2.6
      Support for a ``None`` *table* argument.

   For Unicode objects, the :meth:`translate` method does not accept the optional
   *deletechars* argument.  Instead, it returns a copy of the *s* where all
   characters have been mapped through the given translation table which must be a
   mapping of Unicode ordinals to Unicode ordinals, Unicode strings or ``None``.
   Unmapped characters are left untouched. Characters mapped to ``None`` are
   deleted.  Note, a more flexible approach is to create a custom character mapping
   codec using the :mod:`codecs` module (see :mod:`encodings.cp1251` for an
   example).


.. method:: str.upper()

   Return a copy of the string converted to uppercase.

   For 8-bit strings, this method is locale-dependent.


.. method:: str.zfill(width)

   Return the numeric string left filled with zeros in a string of length
   *width*.  A sign prefix is handled correctly.  The original string is
   returned if *width* is less than ``len(s)``.


   .. versionadded:: 2.2.2

The following methods are present only on unicode objects:

.. method:: unicode.isnumeric()

   Return ``True`` if there are only numeric characters in S, ``False``
   otherwise. Numeric characters include digit characters, and all characters
   that have the Unicode numeric value property, e.g. U+2155,
   VULGAR FRACTION ONE FIFTH.

.. method:: unicode.isdecimal()

   Return ``True`` if there are only decimal characters in S, ``False``
   otherwise. Decimal characters include digit characters, and all characters
   that that can be used to form decimal-radix numbers, e.g. U+0660,
   ARABIC-INDIC DIGIT ZERO.


.. _string-formatting:

String Formatting Operations
----------------------------

.. index::
   single: formatting, string (%)
   single: interpolation, string (%)
   single: string; formatting
   single: string; interpolation
   single: printf-style formatting
   single: sprintf-style formatting
   single: % formatting
   single: % interpolation

String and Unicode objects have one unique built-in operation: the ``%``
operator (modulo).  This is also known as the string *formatting* or
*interpolation* operator.  Given ``format % values`` (where *format* is a string
or Unicode object), ``%`` conversion specifications in *format* are replaced
with zero or more elements of *values*.  The effect is similar to the using
:cfunc:`sprintf` in the C language.  If *format* is a Unicode object, or if any
of the objects being converted using the ``%s`` conversion are Unicode objects,
the result will also be a Unicode object.

If *format* requires a single argument, *values* may be a single non-tuple
object. [#]_  Otherwise, *values* must be a tuple with exactly the number of
items specified by the format string, or a single mapping object (for example, a
dictionary).

A conversion specifier contains two or more characters and has the following
components, which must occur in this order:

#. The ``'%'`` character, which marks the start of the specifier.

#. Mapping key (optional), consisting of a parenthesised sequence of characters
   (for example, ``(somename)``).

#. Conversion flags (optional), which affect the result of some conversion
   types.

#. Minimum field width (optional).  If specified as an ``'*'`` (asterisk), the
   actual width is read from the next element of the tuple in *values*, and the
   object to convert comes after the minimum field width and optional precision.

#. Precision (optional), given as a ``'.'`` (dot) followed by the precision.  If
   specified as ``'*'`` (an asterisk), the actual width is read from the next
   element of the tuple in *values*, and the value to convert comes after the
   precision.

#. Length modifier (optional).

#. Conversion type.

When the right argument is a dictionary (or other mapping type), then the
formats in the string *must* include a parenthesised mapping key into that
dictionary inserted immediately after the ``'%'`` character. The mapping key
selects the value to be formatted from the mapping.  For example:

   >>> print '%(language)s has %(#)03d quote types.' % \
   ...       {'language': "Python", "#": 2}
   Python has 002 quote types.

In this case no ``*`` specifiers may occur in a format (since they require a
sequential parameter list).

The conversion flag characters are:

+---------+---------------------------------------------------------------------+
| Flag    | Meaning                                                             |
+=========+=====================================================================+
| ``'#'`` | The value conversion will use the "alternate form" (where defined   |
|         | below).                                                             |
+---------+---------------------------------------------------------------------+
| ``'0'`` | The conversion will be zero padded for numeric values.              |
+---------+---------------------------------------------------------------------+
| ``'-'`` | The converted value is left adjusted (overrides the ``'0'``         |
|         | conversion if both are given).                                      |
+---------+---------------------------------------------------------------------+
| ``' '`` | (a space) A blank should be left before a positiv…