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.. 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.
.. index:: pair: built-in; types
The principal built-in types are numerics, sequences, mappings, files, classes,
instances and exceptions.
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``, ``0.0``, ``0j``.
* any empty sequence, for example, ``''``, ``()``, ``[]``.
* any empty mapping, for example, ``{}``.
* instances of user-defined classes, if the class defines a :meth:`__bool__` 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
There are eight comparison operations in Python. 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).
.. index::
pair: operator; comparison
operator: ==
operator: <
operator: >
operator: <=
operator: >=
operator: !=
operator: is
operator: is not
This table summarizes the comparison operations:
+------------+-------------------------+
| Operation | Meaning |
+============+=========================+
| ``<`` | strictly less than |
+------------+-------------------------+
| ``<=`` | less than or equal |
+------------+-------------------------+
| ``>`` | strictly greater than |
+------------+-------------------------+
| ``>=`` | greater than or equal |
+------------+-------------------------+
| ``==`` | equal |
+------------+-------------------------+
| ``!=`` | not equal |
+------------+-------------------------+
| ``is`` | object identity |
+------------+-------------------------+
| ``is not`` | negated object identity |
+------------+-------------------------+
.. index::
pair: object; numeric
pair: objects; comparing
Objects of different types, except different numeric types, never compare equal.
Furthermore, some types (for example, file objects) support only a degenerate
notion of comparison where any two objects of that type are unequal. The ``<``,
``<=``, ``>`` and ``>=`` operators will raise a :exc:`TypeError` exception when
any operand is a complex number, the objects are of different types that cannot
be compared, or other cases where there is no defined ordering.
.. index::
single: __eq__() (instance method)
single: __ne__() (instance method)
single: __lt__() (instance method)
single: __le__() (instance method)
single: __gt__() (instance method)
single: __ge__() (instance method)
Non-identical instances of a class normally compare as non-equal unless the
class defines the :meth:`__eq__` method.
Instances of a class cannot be ordered with respect to other instances of the
same class, or other types of object, unless the class defines enough of the
methods :meth:`__lt__`, :meth:`__le__`, :meth:`__gt__`, and :meth:`__ge__` (in
general, :meth:`__lt__` and :meth:`__eq__` are sufficient, if you want the
conventional meanings of the comparison operators).
The behavior of the :keyword:`is` and :keyword:`is not` operators cannot be
customized; also they can be applied to any two objects and never raise an
exception.
.. 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:`complex`
================================================================
.. index::
object: numeric
object: Boolean
object: integer
object: floating point
object: complex number
pair: C; language
There are three distinct numeric types: :dfn:`integers`, :dfn:`floating
point numbers`, and :dfn:`complex numbers`. In addition, Booleans are a
subtype of integers. 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``. (The standard library
includes additional numeric types, :mod:`fractions` that hold rationals,
and :mod:`decimal` that hold floating-point numbers with user-definable
precision.)
.. index::
pair: numeric; literals
pair: integer; literals
pair: floating point; literals
pair: complex number; literals
pair: hexadecimal; literals
pair: octal; literals
pair: binary; literals
Numbers are created by numeric literals or as the result of built-in functions
and operators. Unadorned integer literals (including hex, octal and binary
numbers) yield integers. Numeric literals containing a decimal point or an
exponent sign yield floating point numbers. Appending ``'j'`` or ``'J'`` to a
numeric literal yields an imaginary number (a complex number with a zero real
part) which you can add to an integer or float to get a complex number with real
and imaginary parts.
.. index::
single: arithmetic
builtin: int
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 integer is narrower than floating point,
which is narrower than complex. Comparisons between numbers of mixed type use
the same rule. [#]_ The constructors :func:`int`, :func:`float`, and
:func:`complex` can be used to produce numbers of a specific type.
All numeric types (except complex) support the following operations, sorted by
ascending priority (operations in the same box have the same priority; all
numeric operations have a higher priority than comparison operations):
+---------------------+---------------------------------+-------+--------------------+
| Operation | Result | Notes | Full documentation |
+=====================+=================================+=======+====================+
| ``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* | | |
+---------------------+---------------------------------+-------+--------------------+
| ``x // y`` | floored quotient of *x* and | \(1) | |
| | *y* | | |
+---------------------+---------------------------------+-------+--------------------+
| ``x % y`` | remainder of ``x / y`` | \(2) | |
+---------------------+---------------------------------+-------+--------------------+
| ``-x`` | *x* negated | | |
+---------------------+---------------------------------+-------+--------------------+
| ``+x`` | *x* unchanged | | |
+---------------------+---------------------------------+-------+--------------------+
| ``abs(x)`` | absolute value or magnitude of | | :func:`abs` |
| | *x* | | |
+---------------------+---------------------------------+-------+--------------------+
| ``int(x)`` | *x* converted to integer | \(3) | :func:`int` |
+---------------------+---------------------------------+-------+--------------------+
| ``float(x)`` | *x* converted to floating point | \(4) | :func:`float` |
+---------------------+---------------------------------+-------+--------------------+
| ``complex(re, im)`` | a complex number with real part | | :func:`complex` |
| | *re*, imaginary part *im*. | | |
| | *im* defaults to zero. | | |
+---------------------+---------------------------------+-------+--------------------+
| ``c.conjugate()`` | conjugate of the complex number | | |
| | *c* | | |
+---------------------+---------------------------------+-------+--------------------+
| ``divmod(x, y)`` | the pair ``(x // y, x % y)`` | \(2) | :func:`divmod` |
+---------------------+---------------------------------+-------+--------------------+
| ``pow(x, y)`` | *x* to the power *y* | \(5) | :func:`pow` |
+---------------------+---------------------------------+-------+--------------------+
| ``x ** y`` | *x* to the power *y* | \(5) | |
+---------------------+---------------------------------+-------+--------------------+
.. index::
triple: operations on; numeric; types
single: conjugate() (complex number method)
Notes:
(1)
Also referred to as integer division. The resultant value is a whole
integer, though the result's type is not necessarily int. 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``.
(2)
Not for complex numbers. Instead convert to floats using :func:`abs` if
appropriate.
(3)
.. index::
module: math
single: floor() (in module math)
single: ceil() (in module math)
single: trunc() (in module math)
pair: numeric; conversions
pair: C; language
Conversion from floating point to integer may round or truncate
as in C; see functions :func:`floor` and :func:`ceil` in the :mod:`math` module
for well-defined conversions.
(4)
float also accepts the strings "nan" and "inf" with an optional prefix "+"
or "-" for Not a Number (NaN) and positive or negative infinity.
(5)
Python defines ``pow(0, 0)`` and ``0 ** 0`` to be ``1``, as is common for
programming languages.
All :class:`numbers.Real` types (:class:`int` and
:class:`float`) also include the following operations:
+--------------------+--------------------------------+--------+
| Operation | Result | Notes |
+====================+================================+========+
| ``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 <= *x* | |
+--------------------+--------------------------------+--------+
| ``math.ceil(x)`` | the least Integral >= *x* | |
+--------------------+--------------------------------+--------+
For additional numeric operations see the :mod:`math` and :mod:`cmath`
modules.
.. XXXJH exceptions: overflow (when? what operations?) zerodivision
.. _bitstring-ops:
Bit-string Operations on Integer Types
--------------------------------------
.. _bit-string-operations:
Integers support additional operations that make sense only for bit-strings.
Negative numbers are treated as their 2's complement value (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
(operations in the same box have the same 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)``
without overflow check.
(3)
A right shift by *n* bits is equivalent to division by ``pow(2, n)`` without
overflow check.
Additional Methods on Integer Types
-----------------------------------
.. method:: int.bit_length()
Return the number of bits necessary to represent an integer in binary,
excluding the sign and leading zeros::
>>> n = -37
>>> bin(n)
'-0b100101'
>>> n.bit_length()
6
More precisely, if ``x`` is nonzero, then ``x.bit_length()`` is the
unique positive integer ``k`` such that ``2**(k-1) <= abs(x) < 2**k``.
Equivalently, when ``abs(x)`` is small enough to have a correctly
rounded logarithm, then ``k = 1 + int(log(abs(x), 2))``.
If ``x`` is zero, then ``x.bit_length()`` returns ``0``.
Equivalent to::
def bit_length(self):
s = bin(x) # binary representation: bin(-37) --> '-0b100101'
s = s.lstrip('-0b') # remove leading zeros and minus sign
return len(s) # len('100101') --> 6
.. versionadded:: 3.1
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.
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.
.. 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.
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
==============
.. 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.
Once an iterator's :meth:`__next__` method raises :exc:`StopIteration`, it must
continue to do so on subsequent calls. Implementations that do not obey this
property are deemed broken.
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:`bytes`, :class:`bytearray`, :class:`list`, :class:`tuple`, :class:`range`
==================================================================================================================
There are six sequence types: strings, byte sequences (:class:`bytes` objects),
byte arrays (:class:`bytearray` objects), lists, tuples, and range objects.
For other containers see the built-in :class:`dict`, :class:`list`,
:class:`set`, and :class:`tuple` classes, and the :mod:`collections` module.
.. index::
object: sequence
object: string
object: bytes
object: bytearray
object: tuple
object: list
object: range
Strings contain Unicode characters. Their literals are written in single or
double quotes: ``'xyzzy'``, ``"frobozz"``. See :ref:`strings` for more about
string literals. In addition to the functionality described here, there are
also string-specific methods described in the :ref:`string-methods` section.
Bytes and bytearray objects contain single bytes -- the former is immutable
while the latter is a mutable sequence. Bytes objects can be constructed the
constructor, :func:`bytes`, and from literals; use a ``b`` prefix with normal
string syntax: ``b'xyzzy'``. To construct byte arrays, use the
:func:`bytearray` function.
.. warning::
While string objects are sequences of characters (represented by strings of
length 1), bytes and bytearray objects are sequences of *integers* (between 0
and 255), representing the ASCII value of single bytes. That means that for
a bytes or bytearray object *b*, ``b[0]`` will be an integer, while
``b[0:1]`` will be a bytes or bytearray object of length 1. The
representation of bytes objects uses the literal format (``b'...'``) since it
is generally more useful than e.g. ``bytes([50, 19, 100])``. You can always
convert a bytes object into a list of integers using ``list(b)``.
Also, while in previous Python versions, byte strings and Unicode strings
could be exchanged for each other rather freely (barring encoding issues),
strings and bytes are now completely separate concepts. There's no implicit
en-/decoding if you pass and object of the wrong type. A string always
compares unequal to a bytes or bytearray object.
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,)``.
Objects of type range are created using the :func:`range` 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
operator: in
operator: not in
Notes:
(1)
When *s* is a string object, the ``in`` and ``not in`` operations act like a
substring test.
(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.
.. _string-methods:
String Methods
--------------
.. index:: pair: string; methods
String objects support the methods listed below. 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, see 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.
.. method:: str.center(width[, fillchar])
Return centered in a string of length *width*. Padding is done using the
specified *fillchar* (default is a space).
.. method:: str.count(sub[, start[, end]])
Return the number of occurrences of substring *sub* in the range [*start*,
*end*]. Optional arguments *start* and *end* are interpreted as in slice
notation.
.. 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`.
.. 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.
.. 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.
.. 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.
.. method:: str.isalpha()
Return true if all characters in the string are alphabetic and there is at least
one character, false otherwise.
.. method:: str.isdecimal()
Return true if all characters in the string are decimal
characters and there is at least one character, 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.
.. method:: str.isdigit()
Return true if all characters in the string are digits and there is at least one
character, false otherwise.
.. method:: str.isidentifier()
Return true if the string is a valid identifier according to the language
definition, section :ref:`identifiers`.
.. method:: str.islower()
Return true if all cased characters in the string are lowercase and there is at
least one cased character, false otherwise.
.. method:: str.isnumeric()
Return true if all characters in the string are numeric
characters, and there is at least one character, 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:: str.isprintable()
Return true if all characters in the string are printable or the string is
empty, false otherwise. Nonprintable characters are those characters defined
in the Unicode character database as "Other" or "Separator", excepting the
ASCII space (0x20) which is considered printable. (Note that printable
characters in this context are those which should not be escaped when
:func:`repr` is invoked on a string. It has no bearing on the handling of
strings written to :data:`sys.stdout` or :data:`sys.stderr`.)
.. method:: str.isspace()
Return true if there are only whitespace characters in the string and there is
at least one character, false otherwise.
.. 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.
.. method:: str.isupper()
Return true if all cased characters in the string are uppercase and there is at
least one cased character, false otherwise.
.. method:: str.join(seq)
Return a string which is the concatenation of the strings in the sequence
*seq*. A :exc:`TypeError` will be raised if there are any non-string values
in *seq*, including :class:`bytes` objects. 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)``.
.. method:: str.lower()
Return a copy of the string converted to lowercase.
.. 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'
.. method:: str.maketrans(x[, y[, z]])
This static method returns a translation table usable for :meth:`str.translate`.
If there is only one argument, it must be a dictionary mapping Unicode
ordinals (integers) or characters (strings of length 1) to Unicode ordinals,
strings (of arbitrary lengths) or None. Character keys will then be
converted to ordinals.
If there are two arguments, they must be strings of equal length, and in the
resulting dictionary, each character in x will be mapped to the character at
the same position in y. If there is a third argument, it must be a string,
whose characters will be mapped to None in the result.
.. 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.
.. 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)``.
.. 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.
.. 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.
.. 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'
.. 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.
.. 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'
.. method:: str.swapcase()
Return a copy of the string with uppercase characters converted to lowercase and
vice versa.
.. method:: str.title()
Return a titlecased version of the string: words start with uppercase
characters, all remaining cased characters are lowercase.
.. method:: str.translate(map)
Return a copy of the *s* where all characters have been mapped through the
*map* which must be a dictionary of Unicode ordinals (integers) to Unicode
ordinals, strings or ``None``. Unmapped characters are left untouched.
Characters mapped to ``None`` are deleted.
You can use :meth:`str.maketrans` to create a translation map from
character-to-character mappings in different formats.
.. note::
An even 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.
.. 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)``.
.. _old-string-formatting:
Old 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
.. XXX is the note enough?
.. note::
The formatting operations described here are obsolete and may go away in future
versions of Python. Use the new :ref:`string-formatting` in new code.
String 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), ``%`` 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* 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 positive number (or empty |
| | string) produced by a signed conversion. |
+---------+---------------------------------------------------------------------+
| ``'+'`` | A sign character (``'+'`` or ``'-'``) will precede the conversion |
| | (overrides a "space" flag). |
+---------+---------------------------------------------------------------------+
A length modifier (``h``, ``l``, or ``L``) may be present, but is ignored as it
is not necessary for Python -- so e.g. ``%ld`` is identical to ``%d``.
The conversion types are:
+------------+-----------------------------------------------------+-------+
| Conversion | Meaning | Notes |
+============+=====================================================+=======+
| ``'d'`` | Signed integer decimal. | |
+------------+-----------------------------------------------------+-------+
| ``'i'`` | Signed integer decimal. | |
+------------+-----------------------------------------------------+-------+
| ``'o'`` | Signed octal value. | \(1) |
+------------+-----------------------------------------------------+-------+
| ``'u'`` | Obselete type -- it is identical to ``'d'``. | \(7) |
+------------+-----------------------------------------------------+-------+
| ``'x'`` | Signed hexadecimal (lowercase). | \(2) |
+------------+-----------------------------------------------------+-------+
| ``'X'`` | Signed hexadecimal (uppercase). | \(2) |
+------------+-----------------------------------------------------+-------+
| ``'e'`` | Floating point exponential format (lowercase). | \(3) |
+------------+-----------------------------------------------------+-------+
| ``'E'`` | Floating point exponential format (uppercase). | \(3) |
+------------+-----------------------------------------------------+-------+
| ``'f'`` | Floating point decimal format. | \(3) |
+------------+-----------------------------------------------------+-------+
| ``'F'`` | Floating point decimal format. | \(3) |
+------------+-----------------------------------------------------+-------+
| ``'g'`` | Floating point format. Uses lowercase exponential | \(4) |
| | format if exponent is less than -4 or not less than | |
| | precision, decimal format otherwise. | |
+------------+-----------------------------------------------------+-------+
| ``'G'`` | Floating point format. Uses uppercase exponential | \(4) |
| | format if exponent is less than -4 or not less than | |
| | precision, decimal format otherwise. | |
+------------+-----------------------------------------------------+-------+
| ``'c'`` | Single character (accepts integer or single | |
| | character string). | |
+------------+-----------------------------------------------------+-------+
| ``'r'`` | String (converts any python object using | \(5) |
| | :func:`repr`). | |
+------------+-----------------------------------------------------+-------+
| ``'s'`` | String (converts any python object using | |
| | :func:`str`). | |
+------------+-----------------------------------------------------+-------+
| ``'%'`` | No argument is converted, results in a ``'%'`` | |
| | character in the result. | |
+------------+-----------------------------------------------------+-------+
Notes:
(1)
The alternate form causes a leading zero (``'0'``) to be inserted between
left-hand padding and the formatting of the number if the leading character
of the result is not already a zero.
(2)
The alternate form causes a leading ``'0x'`` or ``'0X'`` (depending on whether
the ``'x'`` or ``'X'`` format was used) to be inserted between left-hand padding
and the formatting of the number if the leading character of the result is not
already a zero.
(3)
The alternate form causes the result to always contain a decimal point, even if
no digits follow it.
The precision determines the number of digits after the decimal point and
defaults to 6.
(4)
The alternate form causes the result to always contain a decimal point, and
trailing zeroes are not removed as they would otherwise be.
The precision determines the number of significant digits before and after the
decimal point and defaults to 6.
(5)
The precision determines the maximal number of characters used.
(7)
See :pep:`237`.
Since Python strings have an explicit length, ``%s`` conversions do not assume
that ``'\0'`` is the end of the string.
.. XXX Examples?
For safety reasons, floating point precisions are clipped to 50; ``%f``
conversions for numbers whose absolute value is over 1e25 are replaced by ``%g``
conversions. [#]_ All other errors raise exceptions.
.. index::
module: string
module: re
Additional string operations are defined in standard modules :mod:`string` and
:mod:`re`.
.. _typesseq-range:
Range Type
----------
.. index:: object: range
The :class:`range` type is an immutable sequence which is commonly used for
looping. The advantage of the :class:`range` type is that an :class:`range`
object will always take the same amount of memory, no matter the size of the
range it represents. There are no consistent performance advantages.
Range objects have very little behavior: they only support indexing, iteration,
and the :func:`len` function.
.. _typesseq-mutable:
Mutable Sequence Types
----------------------
.. index::
triple: mutable; sequence; types
object: list
object: bytearray
List and bytearray objects support additional operations that allow in-place
modification of the object. Other mutable sequence types (when added to the
language) should also support these operations. Strings and tuples are
immutable sequence types: such objects cannot be modified once created. The
following operations are defined on mutable sequence types (where *x* is an
arbitrary object).
Note that while lists allow their items to be of any type, bytearray object
"items" are all integers in the range 0 <= x < 256.
+------------------------------+--------------------------------+---------------------+
| Operation | Result | Notes |
+==============================+================================+=====================+
| ``s[i] = x`` | item *i* of *s* is replaced by | |
| | *x* | |
+------------------------------+--------------------------------+---------------------+
| ``s[i:j] = t`` | slice of *s* from *i* to *j* | |
| | is replaced by the contents of | |
| | the iterable *t* | |
+------------------------------+--------------------------------+---------------------+
| ``del s[i:j]`` | same as ``s[i:j] = []`` | |
+------------------------------+--------------------------------+---------------------+
| ``s[i:j:k] = t`` | the elements of ``s[i:j:k]`` | \(1) |
| | are replaced by those of *t* | |
+------------------------------+--------------------------------+---------------------+
| ``del s[i:j:k]`` | removes the elements of | |
| | ``s[i:j:k]`` from the list | |
+------------------------------+--------------------------------+---------------------+
| ``s.append(x)`` | same as ``s[len(s):len(s)] = | |
| | [x]`` | |
+------------------------------+--------------------------------+---------------------+
| ``s.extend(x)`` | same as ``s[len(s):len(s)] = | \(2) |
| | x`` | |
+------------------------------+--------------------------------+---------------------+
| ``s.count(x)`` | return number of *i*'s for | |
| | which ``s[i] == x`` | |
+------------------------------+--------------------------------+---------------------+
| ``s.index(x[, i[, j]])`` | return smallest *k* such that | \(3) |
| | ``s[k] == x`` and ``i <= k < | |
| | j`` | |
+------------------------------+--------------------------------+---------------------+
| ``s.insert(i, x)`` | same as ``s[i:i] = [x]`` | \(4) |
+------------------------------+--------------------------------+---------------------+
| ``s.pop([i])`` | same as ``x = s[i]; del s[i]; | \(5) |
| | return x`` | |
+------------------------------+--------------------------------+---------------------+
| ``s.remove(x)`` | same as ``del s[s.index(x)]`` | \(3) |
+------------------------------+--------------------------------+---------------------+
| ``s.reverse()`` | reverses the items of *s* in | \(6) |
| | place | |
+------------------------------+--------------------------------+---------------------+
| ``s.sort([key[, reverse]])`` | sort the items of *s* in place | (6), (7), (8) |
+------------------------------+--------------------------------+---------------------+
.. index::
triple: operations on; sequence; types
triple: operations on; list; type
pair: subscript; assignment
pair: slice; assignment
statement: del
single: append() (sequence method)
single: extend() (sequence method)
single: count() (sequence method)
single: index() (sequence method)
single: insert() (sequence method)
single: pop() (sequence method)
single: remove() (sequence method)
single: reverse() (sequence method)
single: sort() (sequence method)
Notes:
(1)
*t* must have the same length as the slice it is replacing.
(2)
*x* can be any iterable object.
(3)
Raises :exc:`ValueError` when *x* is not found in *s*. When a negative index is
passed as the second or third parameter to the :meth:`index` method, the sequence
length is added, as for slice indices. If it is still negative, it is truncated
to zero, as for slice indices.
(4)
When a negative index is passed as the first parameter to the :meth:`insert`
method, the sequence length is added, as for slice indices. If it is still
negative, it is truncated to zero, as for slice indices.
(5)
The optional argument *i* defaults to ``-1``, so that by default the last
item is removed and returned.
(6)
The :meth:`sort` and :meth:`reverse` methods modify the sequence in place for
economy of space when sorting or reversing a large sequence. To remind you
that they operate by side effect, they don't return the sorted or reversed
sequence.
(7)
The :meth:`sort` method takes optional arguments for controlling the
comparisons. Each must be specified as a keyword argument.
*key* specifies a function of one argument that is used to extract a comparison
key from each list element: ``key=str.lower``. The default value is ``None``.
*reverse* is a boolean value. If set to ``True``, then the list elements are
sorted as if each comparison were reversed.
The :meth:`sort` method is guaranteed to be stable. A
sort is stable if it guarantees not to change the relative order of elements
that compare equal --- this is helpful for sorting in multiple passes (for
example, sort by department, then by salary grade).
While a list is being sorted, the effect of attempting to mutate, or even
inspect, the list is undefined. The C implementation
makes the list appear empty for the duration, and raises :exc:`ValueError` if it
can detect that the list has been mutated during a sort.
(8)
:meth:`sort` is not supported by :class:`bytearray` objects.
.. _bytes-methods:
Bytes and Byte Array Methods
----------------------------
.. index:: pair: bytes; methods
pair: bytearray; methods
Bytes and bytearray objects, being "strings of bytes", have all methods found on
strings, with the exception of :func:`encode`, :func:`format` and
:func:`isidentifier`, which do not make sense with these types. For converting
the objects to strings, they have a :func:`decode` method.
Wherever one of these methods needs to interpret the bytes as characters
(e.g. the :func:`is...` methods), the ASCII character set is assumed.
.. note::
The methods on bytes and bytearray objects don't accept strings as their
arguments, just as the methods on strings don't accept bytes as their
arguments. For example, you have to write ::
a = "abc"
b = a.replace("a", "f")
and ::
a = b"abc"
b = a.replace(b"a", b"f")
The bytes and bytearray types have an additional class method:
.. method:: bytes.fromhex(string)
bytearray.fromhex(string)
This :class:`bytes` class method returns a bytes or bytearray object,
decoding the given string object. The string must contain two hexadecimal
digits per byte, spaces are ignored.
>>> bytes.fromhex('f0 f1f2 ')
b'\xf0\xf1\xf2'
The translate method differs in semantics from the version available on strings:
.. method:: bytes.translate(table[, delete])
Return a copy of the bytes or bytearray object where all bytes occurring in
the optional argument *delete* are removed, and the remaining bytes have been
mapped through the given translation table, which must be a bytes object of
length 256.
You can use the :func:`string.maketrans` helper function to create a
translation table.
Set the *table* argument to ``None`` for translations that only delete
characters::
>>> b'read this short text'.translate(None, b'aeiou')
b'rd ths shrt txt'
.. _types-set:
Set Types --- :class:`set`, :class:`frozenset`
==============================================
.. index:: object: set
A :dfn:`set` object is an unordered collection of distinct :term:`hashable` objects.
Common uses include membership testing, removing duplicates from a sequence, and
computing mathematical operations such as intersection, union, difference, and
symmetric difference.
(For other containers see the built in :class:`dict`, :class:`list`,
and :class:`tuple` classes, and the :mod:`collections` module.)
Like other collections, sets support ``x in set``, ``len(set)``, and ``for x in
set``. Being an unordered collection, sets do not record element position or
order of insertion. Accordingly, sets do not support indexing, slicing, or
other sequence-like behavior.
There are currently two builtin set types, :class:`set` and :class:`frozenset`.
The :class:`set` type is mutable --- the contents can be changed using methods
like :meth:`add` and :meth:`remove`. Since it is mutable, it has no hash value
and cannot be used as either a dictionary key or as an element of another set.
The :class:`frozenset` type is immutable and :term:`hashable` --- its contents cannot be
altered after it is created; it can therefore be used as a dictionary key or as
an element of another set.
The constructors for both classes work the same:
.. class:: set([iterable])
frozenset([iterable])
Return a new set or frozenset object whose elements are taken from
*iterable*. The elements of a set must be hashable. To represent sets of
sets, the inner sets must be :class:`frozenset` objects. If *iterable* is
not specified, a new empty set is returned.
Instances of :class:`set` and :class:`frozenset` provide the following
operations:
.. describe:: len(s)
Return the cardinality of set *s*.
.. describe:: x in s
Test *x* for membership in *s*.
.. describe:: x not in s
Test *x* for non-membership in *s*.
.. method:: isdisjoint(other)
Return True if the set has no elements in common with *other*. Sets are
disjoint if and only if their intersection is the empty set.
.. method:: issubset(other)
set <= other
Test whether every element in the set is in *other*.
.. method:: set < other
Test whether the set is a true subset of *other*, that is,
``set <= other and set != other``.
.. method:: issuperset(other)
set >= other
Test whether every element in *other* is in the set.
.. method:: set > other
Test whether the set is a true superset of *other*, that is, ``set >=
other and set != other``.
.. method:: union(other, ...)
set | other | ...
Return a new set with elements from both sets.
.. method:: intersection(other, ...)
set & other & ...
Return a new set with elements common to both sets.
.. method:: difference(other, ...)
set - other - ...
Return a new set with elements in the set that are not in the others.
.. method:: symmetric_difference(other)
set ^ other
Return a new set with elements in either the set or *other* but not both.
.. method:: copy()
Return a new set with a shallow copy of *s*.
Note, the non-operator versions of :meth:`union`, :meth:`intersection`,
:meth:`difference`, and :meth:`symmetric_difference`, :meth:`issubset`, and
:meth:`issuperset` methods will accept any iterable as an argument. In
contrast, their operator based counterparts require their arguments to be
sets. This precludes error-prone constructions like ``set('abc') & 'cbs'``
in favor of the more readable ``set('abc').intersection('cbs')``.
Both :class:`set` and :class:`frozenset` support set to set comparisons. Two
sets are equal if and only if every element of each set is contained in the
other (each is a subset of the other). A set is less than another set if and
only if the first set is a proper subset of the second set (is a subset, but
is not equal). A set is greater than another set if and only if the first set
is a proper superset of the second set (is a superset, but is not equal).
Instances of :class:`set` are compared to instances of :class:`frozenset`
based on their members. For example, ``set('abc') == frozenset('abc')``
returns ``True`` and so does ``set('abc') in set([frozenset('abc')])``.
The subset and equality comparisons do not generalize to a complete ordering
function. For example, any two disjoint sets are not equal and are not
subsets of each other, so *all* of the following return ``False``: ``a<b``,
``a==b``, or ``a>b``.
Since sets only define partial ordering (subset relationships), the output of
the :meth:`list.sort` method is undefined for lists of sets.
Set elements, like dictionary keys, must be :term:`hashable`.
Binary operations that mix :class:`set` instances with :class:`frozenset`
return the type of the first operand. For example: ``frozenset('ab') |
set('bc')`` returns an instance of :class:`frozenset`.
The following table lists operations available for :class:`set` that do not
apply to immutable instances of :class:`frozenset`:
.. method:: update(other, ...)
set |= other | ...
Update the set, adding elements from *other*.
.. method:: intersection_update(other, ...)
set &= other & ...
Update the set, keeping only elements found in it and *other*.
.. method:: difference_update(other, ...)
set -= other | ...
Update the set, removing elements found in others.
.. method:: symmetric_difference_update(other)
set ^= other
Update the set, keeping only elements found in either set, but not in both.
.. method:: add(elem)
Add element *elem* to the set.
.. method:: remove(elem)
Remove element *elem* from the set. Raises :exc:`KeyError` if *elem* is
not contained in the set.
.. method:: discard(elem)
Remove element *elem* from the set if it is present.
.. method:: pop()
Remove and return an arbitrary element from the set. Raises
:exc:`KeyError` if the set is empty.
.. method:: clear()
Remove all elements from the set.
Note, the non-operator versions of the :meth:`update`,
:meth:`intersection_update`, :meth:`difference_update`, and
:meth:`symmetric_difference_update` methods will accept any iterable as an
argument.
Note, the *elem* argument to the :meth:`__contains__`, :meth:`remove`, and
:meth:`discard` methods may be a set. To support searching for an equivalent
frozenset, the *elem* set is temporarily mutated during the search and then
restored. During the search, the *elem* set should not be read or mutated
since it does not have a meaningful value.
.. _typesmapping:
Mapping Types --- :class:`dict`
===============================
.. index::
object: mapping
object: dictionary
triple: operations on; mapping; types
triple: operations on; dictionary; type
statement: del
builtin: len
A :dfn:`mapping` object maps :term:`hashable` values to arbitrary objects.
Mappings are mutable objects. There is currently only one standard mapping
type, the :dfn:`dictionary`. (For other containers see the built in
:class:`list`, :class:`set`, and :class:`tuple` classes, and the
:mod:`collections` module.)
A dictionary's keys are *almost* arbitrary values. Values that are not
:term:`hashable`, that is, values containing lists, dictionaries or other
mutable types (that are compared by value rather than by object identity) may
not be used as keys. Numeric types used for keys obey the normal rules for
numeric comparison: if two numbers compare equal (such as ``1`` and ``1.0``)
then they can be used interchangeably to index the same dictionary entry. (Note
however, that since computers store floating-point numbers as approximations it
is usually unwise to use them as dictionary keys.)
Dictionaries can be created by placing a comma-separated list of ``key: value``
pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
'jack', 4127: 'sjoerd'}``, or by the :class:`dict` constructor.
.. class:: dict([arg])
Return a new dictionary initialized from an optional positional argument or
from a set of keyword arguments. If no arguments are given, return a new
empty dictionary. If the positional argument *arg* is a mapping object,
return a dictionary mapping the same keys to the same values as does the
mapping object. Otherwise the positional argument must be a sequence, a
container that supports iteration, or an iterator object. The elements of
the argument must each also be of one of those kinds, and each must in turn
contain exactly two objects. The first is used as a key in the new
dictionary, and the second as the key's value. If a given key is seen more
than once, the last value associated with it is retained in the new
dictionary.
If keyword arguments are given, the keywords themselves with their associated
values are added as items to the dictionary. If a key is specified both in
the positional argument and as a keyword argument, the value associated with
the keyword is retained in the dictionary. For example, these all return a
dictionary equal to ``{"one": 2, "two": 3}``:
* ``dict(one=2, two=3)``
* ``dict({'one': 2, 'two': 3})``
* ``dict(zip(('one', 'two'), (2, 3)))``
* ``dict([['two', 3], ['one', 2]])``
The first example only works for keys that are valid Python identifiers; the
others work with any valid keys.
These are the operations that dictionaries support (and therefore, custom
mapping types should support too):
.. describe:: len(d)
Return the number of items in the dictionary *d*.
.. describe:: d[key]
Return the item of *d* with key *key*. Raises a :exc:`KeyError` if *key* is
not in the map.
If a subclass of dict defines a method :meth:`__missing__`, if the key *key*
is not present, the ``d[key]`` operation calls that method with the key *key*
as argument. The ``d[key]`` operation then returns or raises whatever is
returned or raised by the ``__missing__(key)`` call if the key is not
present. No other operations or methods invoke :meth:`__missing__`. If
:meth:`__missing__` is not defined, :exc:`KeyError` is raised.
:meth:`__missing__` must be a method; it cannot be an instance variable. For
an example, see :class:`collections.defaultdict`.
.. describe:: d[key] = value
Set ``d[key]`` to *value*.
.. describe:: del d[key]
Remove ``d[key]`` from *d*. Raises a :exc:`KeyError` if *key* is not in the
map.
.. describe:: key in d
Return ``True`` if *d* has a key *key*, else ``False``.
.. describe:: key not in d
Equivalent to ``not key in d``.
.. method:: clear()
Remove all items from the dictionary.
.. method:: copy()
Return a shallow copy of the dictionary.
.. method:: fromkeys(seq[, value])
Create a new dictionary with keys from *seq* and values set to *value*.
:meth:`fromkeys` is a class method that returns a new dictionary. *value*
defaults to ``None``.
.. method:: get(key[, default])
Return the value for *key* if *key* is in the dictionary, else *default*.
If *default* is not given, it defaults to ``None``, so that this method
never raises a :exc:`KeyError`.
.. method:: items()
Return a new view of the dictionary's items (``(key, value)`` pairs). See
below for documentation of view objects.
.. method:: keys()
Return a new view of the dictionary's keys. See below for documentation of
view objects.
.. method:: pop(key[, default])
If *key* is in the dictionary, remove it and return its value, else return
*default*. If *default* is not given and *key* is not in the dictionary,
a :exc:`KeyError` is raised.
.. method:: popitem()
Remove and return an arbitrary ``(key, value)`` pair from the dictionary.
:meth:`popitem` is useful to destructively iterate over a dictionary, as
often used in set algorithms. If the dictionary is empty, calling
:meth:`popitem` raises a :exc:`KeyError`.
.. method:: setdefault(key[, default])
If *key* is in the dictionary, return its value. If not, insert *key*
with a value of *default* and return *default*. *default* defaults to
``None``.
.. method:: update([other])
Update the dictionary with the key/value pairs from *other*, overwriting
existing keys. Return ``None``.
:meth:`update` accepts either another dictionary object or an iterable of
key/value pairs (as a tuple or other iterable of length two). If keyword
arguments are specified, the dictionary is then is updated with those
key/value pairs: ``d.update(red=1, blue=2)``.
.. method:: values()
Return a new view of the dictionary's values. See below for documentation of
view objects.
.. _dict-views:
Dictionary view objects
-----------------------
The objects returned by :meth:`dict.keys`, :meth:`dict.values` and
:meth:`dict.items` are *view objects*. They provide a dynamic view on the
dictionary's entries, which means that when the dictionary changes, the view
reflects these changes.
Dictionary views can be iterated over to yield their respective data, and
support membership tests:
.. describe:: len(dictview)
Return the number of entries in the dictionary.
.. describe:: iter(dictview)
Return an iterator over the keys, values or items (represented as tuples of
``(key, value)``) in the dictionary.
Keys and values are iterated over in an arbitrary order which is non-random,
varies across Python implementations, and depends on the dictionary's history
of insertions and deletions. If keys, values and items views are iterated
over with no intervening modifications to the dictionary, the order of items
will directly correspond. This allows the creation of ``(value, key)`` pairs
using :func:`zip`: ``pairs = zip(d.values(), d.keys())``. Another way to
create the same list is ``pairs = [(v, k) for (k, v) in d.items()]``.
.. describe:: x in dictview
Return ``True`` if *x* is in the underlying dictionary's keys, values or
items (in the latter case, *x* should be a ``(key, value)`` tuple).
Keys views are set-like since their entries are unique and hashable. If all
values are hashable, so that (key, value) pairs are unique and hashable, then
the items view is also set-like. (Values views are not treated as set-like
since the entries are generally not unique.) Then these set operations are
available ("other" refers either to another view or a set):
.. describe:: dictview & other
Return the intersection of the dictview and the other object as a new set.
.. describe:: dictview | other
Return the union of the dictview and the other object as a new set.
.. describe:: dictview - other
Return the difference between the dictview and the other object (all elements
in *dictview* that aren't in *other*) as a new set.
.. describe:: dictview ^ other
Return the symmetric difference (all elements either in *dictview* or
*other*, but not in both) of the dictview and the other object as a new set.
An example of dictionary view usage::
>>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}
>>> keys = dishes.keys()
>>> values = dishes.values()
>>> # iteration
>>> n = 0
>>> for val in values:
... n += val
>>> print(n)
504
>>> # keys and values are iterated over in the same order
>>> list(keys)
['eggs', 'bacon', 'sausage', 'spam']
>>> list(values)
[2, 1, 1, 500]
>>> # view objects are dynamic and reflect dict changes
>>> del dishes['eggs']
>>> del dishes['sausage']
>>> list(keys)
['spam', 'bacon']
>>> # set operations
>>> keys & {'eggs', 'bacon', 'salad'}
{'bacon'}
.. _bltin-file-objects:
File Objects
============
.. index::
object: file
builtin: file
module: os
module: socket
.. XXX this is quite out of date, must be updated with "io" module
File objects are implemented using C's ``stdio`` package and can be
created with the built-in :func:`open` function. File
objects are also returned by some other built-in functions and methods,
such as :func:`os.popen` and :func:`os.fdopen` and the :meth:`makefile`
method of socket objects. Temporary files can be created using the
:mod:`tempfile` module, and high-level file operations such as copying,
moving, and deleting files and directories can be achieved with the
:mod:`shutil` module.
When a file operation fails for an I/O-related reason, the exception
:exc:`IOError` is raised. This includes situations where the operation is not
defined for some reason, like :meth:`seek` on a tty device or writing a file
opened for reading.
Files have the following methods:
.. method:: file.close()
Close the file. A closed file cannot be read or written any more. Any operation
which requires that the file be open will raise a :exc:`ValueError` after the
file has been closed. Calling :meth:`close` more than once is allowed.
You can avoid having to call this method explicitly if you use
the :keyword:`with` statement. For example, the following code will
automatically close *f* when the :keyword:`with` block is exited::
from __future__ import with_statement # This isn't required in Python 2.6
with open("hello.txt") as f:
for line in f:
print(line)
In older versions of Python, you would have needed to do this to get the same
effect::
f = open("hello.txt")
try:
for line in f:
print(line)
finally:
f.close()
.. note::
Not all "file-like" types in Python support use as a context manager for the
:keyword:`with` statement. If your code is intended to work with any file-like
object, you can use the function :func:`contextlib.closing` instead of using
the object directly.
.. method:: file.flush()
Flush the internal buffer, like ``stdio``'s :cfunc:`fflush`. This may be a
no-op on some file-like objects.
.. method:: file.fileno()
.. index::
pair: file; descriptor
module: fcntl
Return the integer "file descriptor" that is used by the underlying
implementation to request I/O operations from the operating system. This can be
useful for other, lower level interfaces that use file descriptors, such as the
:mod:`fcntl` module or :func:`os.read` and friends.
.. note::
File-like objects which do not have a real file descriptor should *not* provide
this method!
.. method:: file.isatty()
Return ``True`` if the file is connected to a tty(-like) device, else ``False``.
.. note::
If a file-like object is not associated with a real file, this method should
*not* be implemented.
.. method:: file.__next__()
A file object is its own iterator, for example ``iter(f)`` returns *f* (unless
*f* is closed). When a file is used as an iterator, typically in a
:keyword:`for` loop (for example, ``for line in f: print(line)``), the
:meth:`__next__` method is called repeatedly. This method returns the next
input line, or raises :exc:`StopIteration` when EOF is hit when the file is open
for reading (behavior is undefined when the file is open for writing). In order
to make a :keyword:`for` loop the most efficient way of looping over the lines
of a file (a very common operation), the :meth:`__next__` method uses a hidden
read-ahead buffer. As a consequence of using a read-ahead buffer, combining
:meth:`__next__` with other file methods (like :meth:`readline`) does not work
right. However, using :meth:`seek` to reposition the file to an absolute
position will flush the read-ahead buffer.
.. method:: file.read([size])
Read at most *size* bytes from the file (less if the read hits EOF before
obtaining *size* bytes). If the *size* argument is negative or omitted, read
all data until EOF is reached. The bytes are returned as a string object. An
empty string is returned when EOF is encountered immediately. (For certain
files, like ttys, it makes sense to continue reading after an EOF is hit.) Note
that this method may call the underlying C function :cfunc:`fread` more than
once in an effort to acquire as close to *size* bytes as possible. Also note
that when in non-blocking mode, less data than was requested may be
returned, even if no *size* parameter was given.
.. method:: file.readline([size])
Read one entire line from the file. A trailing newline character is kept in the
string (but may be absent when a file ends with an incomplete line). [#]_ If
the *size* argument is present and non-negative, it is a maximum byte count
(including the trailing newline) and an incomplete line may be returned. An
empty string is returned *only* when EOF is encountered immediately.
.. note::
Unlike ``stdio``'s :cfunc:`fgets`, the returned string contains null characters
(``'\0'``) if they occurred in the input.
.. method:: file.readlines([sizehint])
Read until EOF using :meth:`readline` and return a list containing the lines
thus read. If the optional *sizehint* argument is present, instead of
reading up to EOF, whole lines totalling approximately *sizehint* bytes
(possibly after rounding up to an internal buffer size) are read. Objects
implementing a file-like interface may choose to ignore *sizehint* if it
cannot be implemented, or cannot be implemented efficiently.
.. method:: file.seek(offset[, whence])
Set the file's current position, like ``stdio``'s :cfunc:`fseek`. The *whence*
argument is optional and defaults to ``os.SEEK_SET`` or ``0`` (absolute file
positioning); other values are ``os.SEEK_CUR`` or ``1`` (seek relative to the
current position) and ``os.SEEK_END`` or ``2`` (seek relative to the file's
end). There is no return value.
For example, ``f.seek(2, os.SEEK_CUR)`` advances the position by two and
``f.seek(-3, os.SEEK_END)`` sets the position to the third to last.
Note that if the file is opened for appending
(mode ``'a'`` or ``'a+'``), any :meth:`seek` operations will be undone at the
next write. If the file is only opened for writing in append mode (mode
``'a'``), this method is essentially a no-op, but it remains useful for files
opened in append mode with reading enabled (mode ``'a+'``). If the file is
opened in text mode (without ``'b'``), only offsets returned by :meth:`tell` are
legal. Use of other offsets causes undefined behavior.
Note that not all file objects are seekable.
.. method:: file.tell()
Return the file's current position, like ``stdio``'s :cfunc:`ftell`.
.. note::
On Windows, :meth:`tell` can return illegal values (after an :cfunc:`fgets`)
when reading files with Unix-style line-endings. Use binary mode (``'rb'``) to
circumvent this problem.
.. method:: file.truncate([size])
Truncate the file's size. If the optional *size* argument is present, the file
is truncated to (at most) that size. The size defaults to the current position.
The current file position is not changed. Note that if a specified size exceeds
the file's current size, the result is platform-dependent: possibilities
include that the file may remain unchanged, increase to the specified size as if
zero-filled, or increase to the specified size with undefined new content.
Availability: Windows, many Unix variants.
.. method:: file.write(str)
Write a string to the file. Due to buffering, the string may not actually
show up in the file until the :meth:`flush` or :meth:`close` method is
called.
The meaning of the return value is not defined for every file-like object.
Some (mostly low-level) file-like objects may return the number of bytes
actually written, others return ``None``.
.. method:: file.writelines(sequence)
Write a sequence of strings to the file. The sequence can be any iterable
object producing strings, typically a list of strings. There is no return value.
(The name is intended to match :meth:`readlines`; :meth:`writelines` does not
add line separators.)
Files support the iterator protocol. Each iteration returns the same result as
``file.readline()``, and iteration ends when the :meth:`readline` method returns
an empty string.
File objects also offer a number of other interesting attributes. These are not
required for file-like objects, but should be implemented if they make sense for
the particular object.
.. attribute:: file.closed
bool indicating the current state of the file object. This is a read-only
attribute; the :meth:`close` method changes the value. It may not be available
on all file-like objects.
.. XXX does this still apply?
.. attribute:: file.encoding
The encoding that this file uses. When strings are written to a file,
they will be converted to byte strings using this encoding. In addition, when
the file is connected to a terminal, the attribute gives the encoding that the
terminal is likely to use (that information might be incorrect if the user has
misconfigured the terminal). The attribute is read-only and may not be present
on all file-like objects. It may also be ``None``, in which case the file uses
the system default encoding for converting strings.
.. attribute:: file.errors
The Unicode error handler used along with the encoding.
.. attribute:: file.mode
The I/O mode for the file. If the file was created using the :func:`open`
built-in function, this will be the value of the *mode* parameter. This is a
read-only attribute and may not be present on all file-like objects.
.. attribute:: file.name
If the file object was created using :func:`open`, the name of the file.
Otherwise, some string that indicates the source of the file object, of the
form ``<...>``. This is a read-only attribute and may not be present on all
file-like objects.
.. attribute:: file.newlines
If Python was built with the :option:`--with-universal-newlines` option to
:program:`configure` (the default) this read-only attribute exists, and for
files opened in universal newline read mode it keeps track of the types of
newlines encountered while reading the file. The values it can take are
``'\r'``, ``'\n'``, ``'\r\n'``, ``None`` (unknown, no newlines read yet) or a
tuple containing all the newline types seen, to indicate that multiple newline
conventions were encountered. For files not opened in universal newline read
mode the value of this attribute will be ``None``.
.. _typememoryview:
memoryview Types
================
:class:`memoryview`\s allow Python code to access the internal data of an object
that supports the buffer protocol without copying. Memory can be interpreted as
simple bytes or complex data structures.
.. class:: memoryview(obj)
Create a :class:`memoryview` that references *obj*. *obj* must support the
buffer protocol. Builtin objects that support the buffer protocol include
:class:`bytes` and :class:`bytearray`.
``len(view)`` returns the total number of bytes in the memoryview, *view*.
A :class:`memoryview` supports slicing to expose its data. Taking a single
index will return a single byte. Full slicing will result in a subview::
>>> v = memoryview(b'abcefg')
>>> v[1]
b'b'
>>> v[-1]
b'g'
>>> v[1:4]
<memory at 0x77ab28>
>>> bytes(v[1:4])
b'bce'
>>> v[3:-1]
<memory at 0x744f18>
>>> bytes(v[4:-1])
If the object the memory view is over supports changing its data, the
memoryview supports slice assignment::
>>> data = bytearray(b'abcefg')
>>> v = memoryview(data)
>>> v.readonly
False
>>> v[0] = 'z'
>>> data
bytearray(b'zbcefg')
>>> v[1:4] = b'123'
>>> data
bytearray(b'a123fg')
>>> v[2] = b'spam'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: cannot modify size of memoryview object
Notice how the size of the memoryview object can not be changed.
:class:`memoryview` has two methods:
.. method:: tobytes()
Return the data in the buffer as a bytestring.
.. method:: tolist()
Return the data in the buffer as a list of integers. ::
>>> memoryview(b'abc').tolist()
[97, 98, 99]
There are also several readonly attributes available:
.. attribute:: format
A string containing the format (in :mod:`struct` module style) for each
element in the view. This defaults to ``'B'``, a simple bytestring.
.. attribute:: itemsize
The size in bytes of each element of the memoryview.
.. attribute:: shape
A tuple of integers the length of :attr:`ndim` giving the shape of the
memory as a N-dimensional array.
.. attribute:: ndim
An integer indicating how many dimensions of a multi-dimensional array the
memory represents.
.. attribute:: strides
A tuple of integers the length of :attr:`ndim` giving the size in bytes to
access each element for each dimension of the array.
.. memoryview.suboffsets isn't documented because it only seems useful for C
.. _typecontextmanager:
Context Manager Types
=====================
.. index::
single: context manager
single: context management protocol
single: protocol; context management
Python's :keyword:`with` statement supports the concept of a runtime context
defined by a context manager. This is implemented using two separate methods
that allow user-defined classes to define a runtime context that is entered
before the statement body is executed and exited when the statement ends.
The :dfn:`context management protocol` consists of a pair of methods that need
to be provided for a context manager object to define a runtime context:
.. method:: contextmanager.__enter__()
Enter the runtime context and return either this object or another object
related to the runtime context. The value returned by this method is bound to
the identifier in the :keyword:`as` clause of :keyword:`with` statements using
this context manager.
An example of a context manager that returns itself is a file object. File
objects return themselves from __enter__() to allow :func:`open` to be used as
the context expression in a :keyword:`with` statement.
An example of a context manager that returns a related object is the one
returned by :func:`decimal.localcontext`. These managers set the active
decimal context to a copy of the original decimal context and then return the
copy. This allows changes to be made to the current decimal context in the body
of the :keyword:`with` statement without affecting code outside the
:keyword:`with` statement.
.. method:: contextmanager.__exit__(exc_type, exc_val, exc_tb)
Exit the runtime context and return a Boolean flag indicating if any exception
that occurred should be suppressed. If an exception occurred while executing the
body of the :keyword:`with` statement, the arguments contain the exception type,
value and traceback information. Otherwise, all three arguments are ``None``.
Returning a true value from this method will cause the :keyword:`with` statement
to suppress the exception and continue execution with the statement immediately
following the :keyword:`with` statement. Otherwise the exception continues
propagating after this method has finished executing. Exceptions that occur
during execution of this method will replace any exception that occurred in the
body of the :keyword:`with` statement.
The exception passed in should never be reraised explicitly - instead, this
method should return a false value to indicate that the method completed
successfully and does not want to suppress the raised exception. This allows
context management code (such as ``contextlib.nested``) to easily detect whether
or not an :meth:`__exit__` method has actually failed.
Python defines several context managers to support easy thread synchronisation,
prompt closure of files or other objects, and simpler manipulation of the active
decimal arithmetic context. The specific types are not treated specially beyond
their implementation of the context management protocol. See the
:mod:`contextlib` module for some examples.
Python's :term:`generator`\s and the ``contextlib.contextfactory`` :term:`decorator`
provide a convenient way to implement these protocols. If a generator function is
decorated with the ``contextlib.contextfactory`` decorator, it will return a
context manager implementing the necessary :meth:`__enter__` and
:meth:`__exit__` methods, rather than the iterator produced by an undecorated
generator function.
Note that there is no specific slot for any of these methods in the type
structure for Python objects in the Python/C API. Extension types wanting to
define these methods must provide them as a normal Python accessible method.
Compared to the overhead of setting up the runtime context, the overhead of a
single class dictionary lookup is negligible.
.. _typesother:
Other Built-in Types
====================
The interpreter supports several other kinds of objects. Most of these support
only one or two operations.
.. _typesmodules:
Modules
-------
The only special operation on a module is attribute access: ``m.name``, where
*m* is a module and *name* accesses a name defined in *m*'s symbol table.
Module attributes can be assigned to. (Note that the :keyword:`import`
statement is not, strictly speaking, an operation on a module object; ``import
foo`` does not require a module object named *foo* to exist, rather it requires
an (external) *definition* for a module named *foo* somewhere.)
A special member of every module is :attr:`__dict__`. This is the dictionary
containing the module's symbol table. Modifying this dictionary will actually
change the module's symbol table, but direct assignment to the :attr:`__dict__`
attribute is not possible (you can write ``m.__dict__['a'] = 1``, which defines
``m.a`` to be ``1``, but you can't write ``m.__dict__ = {}``). Modifying
:attr:`__dict__` directly is not recommended.
Modules built into the interpreter are written like this: ``<module 'sys'
(built-in)>``. If loaded from a file, they are written as ``<module 'os' from
'/usr/local/lib/pythonX.Y/os.pyc'>``.
.. _typesobjects:
Classes and Class Instances
---------------------------
See :ref:`objects` and :ref:`class` for these.
.. _typesfunctions:
Functions
---------
Function objects are created by function definitions. The only operation on a
function object is to call it: ``func(argument-list)``.
There are really two flavors of function objects: built-in functions and
user-defined functions. Both support the same operation (to call the function),
but the implementation is different, hence the different object types.
See :ref:`function` for more information.
.. _typesmethods:
Methods
-------
.. index:: object: method
Methods are functions that are called using the attribute notation. There are
two flavors: built-in methods (such as :meth:`append` on lists) and class
instance methods. Built-in methods are described with the types that support
them.
If you access a method (a function defined in a class namespace) through an
instance, you get a special object: a :dfn:`bound method` (also called
:dfn:`instance method`) object. When called, it will add the ``self`` argument
to the argument list. Bound methods have two special read-only attributes:
``m.__self__`` is the object on which the method operates, and ``m.__func__`` is
the function implementing the method. Calling ``m(arg-1, arg-2, ..., arg-n)``
is completely equivalent to calling ``m.__func__(m.__self__, arg-1, arg-2, ...,
arg-n)``.
Like function objects, bound method objects support getting arbitrary
attributes. However, since method attributes are actually stored on the
underlying function object (``meth.__func__``), setting method attributes on
bound methods is disallowed. Attempting to set a method attribute results in a
:exc:`TypeError` being raised. In order to set a method attribute, you need to
explicitly set it on the underlying function object::
class C:
def method(self):
pass
c = C()
c.method.__func__.whoami = 'my name is c'
See :ref:`types` for more information.
.. _bltin-code-objects:
Code Objects
------------
.. index:: object: code
.. index::
builtin: compile
single: __code__ (function object attribute)
Code objects are used by the implementation to represent "pseudo-compiled"
executable Python code such as a function body. They differ from function
objects because they don't contain a reference to their global execution
environment. Code objects are returned by the built-in :func:`compile` function
and can be extracted from function objects through their :attr:`__code__`
attribute. See also the :mod:`code` module.
.. index::
builtin: exec
builtin: eval
A code object can be executed or evaluated by passing it (instead of a source
string) to the :func:`exec` or :func:`eval` built-in functions.
See :ref:`types` for more information.
.. _bltin-type-objects:
Type Objects
------------
.. index::
builtin: type
module: types
Type objects represent the various object types. An object's type is accessed
by the built-in function :func:`type`. There are no special operations on
types. The standard module :mod:`types` defines names for all standard built-in
types.
Types are written like this: ``<class 'int'>``.
.. _bltin-null-object:
The Null Object
---------------
This object is returned by functions that don't explicitly return a value. It
supports no special operations. There is exactly one null object, named
``None`` (a built-in name).
It is written as ``None``.
.. _bltin-ellipsis-object:
The Ellipsis Object
-------------------
This object is commonly used by slicing (see :ref:`slicings`). It supports no
special operations. There is exactly one ellipsis object, named
:const:`Ellipsis` (a built-in name).
It is written as ``Ellipsis`` or ``...``.
Boolean Values
--------------
Boolean values are the two constant objects ``False`` and ``True``. They are
used to represent truth values (although other values can also be considered
false or true). In numeric contexts (for example when used as the argument to
an arithmetic operator), they behave like the integers 0 and 1, respectively.
The built-in function :func:`bool` can be used to cast any value to a Boolean,
if the value can be interpreted as a truth value (see section Truth Value
Testing above).
.. index::
single: False
single: True
pair: Boolean; values
They are written as ``False`` and ``True``, respectively.
.. _typesinternal:
Internal Objects
----------------
See :ref:`types` for this information. It describes stack frame objects,
traceback objects, and slice objects.
.. _specialattrs:
Special Attributes
==================
The implementation adds a few special read-only attributes to several object
types, where they are relevant. Some of these are not reported by the
:func:`dir` built-in function.
.. attribute:: object.__dict__
A dictionary or other mapping object used to store an object's (writable)
attributes.
.. attribute:: instance.__class__
The class to which a class instance belongs.
.. attribute:: class.__bases__
The tuple of base classes of a class object. If there are no base classes, this
will be an empty tuple.
.. attribute:: class.__name__
The name of the class or type.
.. rubric:: Footnotes
.. [#] Additional information on these special methods may be found in the Python
Reference Manual (:ref:`customization`).
.. [#] As a consequence, the list ``[1, 2]`` is considered equal to ``[1.0, 2.0]``, and
similarly for tuples.
.. [#] They must have since the parser can't tell the type of the operands.
.. [#] To format only a tuple you should therefore provide a singleton tuple whose only
element is the tuple to be formatted.
.. [#] These numbers are fairly arbitrary. They are intended to avoid printing endless
strings of meaningless digits without hampering correct use and without having
to know the exact precision of floating point values on a particular machine.
.. [#] The advantage of leaving the newline on is that returning an empty string is
then an unambiguous EOF indication. It is also possible (in cases where it
might matter, for example, if you want to make an exact copy of a file while
scanning its lines) to tell whether the last line of a file ended in a newline
or not (yes this happens!).
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