Mirror
/
cpython
mirror of https://github.com/python/cpython
4
1
Fork 0
Code Issues Releases Wiki Activity
cpython/Doc/library/functions.rst

2299 lines
94 KiB
ReStructuredText
Raw Blame History

.. XXX document all delegations to __special__ methods
.. _built-in-funcs:
Built-in Functions
==================
The Python interpreter has a number of functions and types built into it that
are always available. They are listed here in alphabetical order.
+---------------------------------------------------------------------------------------------------+
| Built-in Functions |
+=========================+=======================+=======================+=========================+
| | **A** | | **E** | | **L** | | **R** |
| | :func:`abs` | | :func:`enumerate` | | :func:`len` | | |func-range|_ |
| | :func:`aiter` | | :func:`eval` | | |func-list|_ | | :func:`repr` |
| | :func:`all` | | :func:`exec` | | :func:`locals` | | :func:`reversed` |
| | :func:`anext` | | | | | | :func:`round` |
| | :func:`any` | | **F** | | **M** | | |
| | :func:`ascii` | | :func:`filter` | | :func:`map` | | **S** |
| | | | :func:`float` | | :func:`max` | | |func-set|_ |
| | **B** | | :func:`format` | | |func-memoryview|_ | | :func:`setattr` |
| | :func:`bin` | | |func-frozenset|_ | | :func:`min` | | :func:`slice` |
| | :func:`bool` | | | | | | :func:`sorted` |
| | :func:`breakpoint` | | **G** | | **N** | | :func:`staticmethod` |
| | |func-bytearray|_ | | :func:`getattr` | | :func:`next` | | |func-str|_ |
| | |func-bytes|_ | | :func:`globals` | | | | :func:`sum` |
| | | | | | **O** | | :func:`super` |
| | **C** | | **H** | | :func:`object` | | |
| | :func:`callable` | | :func:`hasattr` | | :func:`oct` | | **T** |
| | :func:`chr` | | :func:`hash` | | :func:`open` | | |func-tuple|_ |
| | :func:`classmethod` | | :func:`help` | | :func:`ord` | | :func:`type` |
| | :func:`compile` | | :func:`hex` | | | | |
| | :func:`complex` | | | | **P** | | **V** |
| | | | **I** | | :func:`pow` | | :func:`vars` |
| | **D** | | :func:`id` | | :func:`print` | | |
| | :func:`delattr` | | :func:`input` | | :func:`property` | | **Z** |
| | |func-dict|_ | | :func:`int` | | | | :func:`zip` |
| | :func:`dir` | | :func:`isinstance` | | | | |
| | :func:`divmod` | | :func:`issubclass` | | | | **_** |
| | | | :func:`iter` | | | | :func:`__import__` |
+-------------------------+-----------------------+-----------------------+-------------------------+
.. using :func:`dict` would create a link to another page, so local targets are
used, with replacement texts to make the output in the table consistent
.. |func-dict| replace:: ``dict()``
.. |func-frozenset| replace:: ``frozenset()``
.. |func-memoryview| replace:: ``memoryview()``
.. |func-set| replace:: ``set()``
.. |func-list| replace:: ``list()``
.. |func-str| replace:: ``str()``
.. |func-tuple| replace:: ``tuple()``
.. |func-range| replace:: ``range()``
.. |func-bytearray| replace:: ``bytearray()``
.. |func-bytes| replace:: ``bytes()``
.. function:: abs(x)
Return the absolute value of a number. The argument may be an
integer, a floating-point number, or an object implementing
:meth:`~object.__abs__`.
If the argument is a complex number, its magnitude is returned.
.. function:: aiter(async_iterable)
Return an :term:`asynchronous iterator` for an :term:`asynchronous iterable`.
Equivalent to calling ``x.__aiter__()``.
Note: Unlike :func:`iter`, :func:`aiter` has no 2-argument variant.
.. versionadded:: 3.10
.. function:: all(iterable)
Return ``True`` if all elements of the *iterable* are true (or if the iterable
is empty). Equivalent to::
def all(iterable):
for element in iterable:
if not element:
return False
return True
.. awaitablefunction:: anext(async_iterator)
anext(async_iterator, default)
When awaited, return the next item from the given :term:`asynchronous
iterator`, or *default* if given and the iterator is exhausted.
This is the async variant of the :func:`next` builtin, and behaves
similarly.
This calls the :meth:`~object.__anext__` method of *async_iterator*,
returning an :term:`awaitable`. Awaiting this returns the next value of the
iterator. If *default* is given, it is returned if the iterator is exhausted,
otherwise :exc:`StopAsyncIteration` is raised.
.. versionadded:: 3.10
.. function:: any(iterable)
Return ``True`` if any element of the *iterable* is true. If the iterable
is empty, return ``False``. Equivalent to::
def any(iterable):
for element in iterable:
if element:
return True
return False
.. function:: ascii(object)
As :func:`repr`, return a string containing a printable representation of an
object, but escape the non-ASCII characters in the string returned by
:func:`repr` using ``\x``, ``\u``, or ``\U`` escapes. This generates a string
similar to that returned by :func:`repr` in Python 2.
.. function:: bin(x)
Convert an integer number to a binary string prefixed with "0b". The result
is a valid Python expression. If *x* is not a Python :class:`int` object, it
has to define an :meth:`~object.__index__` method that returns an integer. Some
examples:
>>> bin(3)
'0b11'
>>> bin(-10)
'-0b1010'
If the prefix "0b" is desired or not, you can use either of the following ways.
>>> format(14, '#b'), format(14, 'b')
('0b1110', '1110')
>>> f'{14:#b}', f'{14:b}'
('0b1110', '1110')
See also :func:`format` for more information.
.. class:: bool(object=False, /)
Return a Boolean value, i.e. one of ``True`` or ``False``. The argument
is converted using the standard :ref:`truth testing procedure <truth>`.
If the argument is false
or omitted, this returns ``False``; otherwise, it returns ``True``. The
:class:`bool` class is a subclass of :class:`int` (see :ref:`typesnumeric`).
It cannot be subclassed further. Its only instances are ``False`` and
``True`` (see :ref:`typebool`).
.. index:: pair: Boolean; type
.. versionchanged:: 3.7
The parameter is now positional-only.
.. function:: breakpoint(*args, **kws)
This function drops you into the debugger at the call site. Specifically,
it calls :func:`sys.breakpointhook`, passing ``args`` and ``kws`` straight
through. By default, ``sys.breakpointhook()`` calls
:func:`pdb.set_trace` expecting no arguments. In this case, it is
purely a convenience function so you don't have to explicitly import
:mod:`pdb` or type as much code to enter the debugger. However,
:func:`sys.breakpointhook` can be set to some other function and
:func:`breakpoint` will automatically call that, allowing you to drop into
the debugger of choice.
If :func:`sys.breakpointhook` is not accessible, this function will
raise :exc:`RuntimeError`.
By default, the behavior of :func:`breakpoint` can be changed with
the :envvar:`PYTHONBREAKPOINT` environment variable.
See :func:`sys.breakpointhook` for usage details.
Note that this is not guaranteed if :func:`sys.breakpointhook`
has been replaced.
.. audit-event:: builtins.breakpoint breakpointhook breakpoint
.. versionadded:: 3.7
.. _func-bytearray:
.. class:: bytearray(source=b'')
bytearray(source, encoding)
bytearray(source, encoding, errors)
:noindex:
Return a new array of bytes. The :class:`bytearray` class is a mutable
sequence of integers in the range 0 <= x < 256. It has most of the usual
methods of mutable sequences, described in :ref:`typesseq-mutable`, as well
as most methods that the :class:`bytes` type has, see :ref:`bytes-methods`.
The optional *source* parameter can be used to initialize the array in a few
different ways:
* If it is a *string*, you must also give the *encoding* (and optionally,
*errors*) parameters; :func:`bytearray` then converts the string to
bytes using :meth:`str.encode`.
* If it is an *integer*, the array will have that size and will be
initialized with null bytes.
* If it is an object conforming to the :ref:`buffer interface <bufferobjects>`,
a read-only buffer of the object will be used to initialize the bytes array.
* If it is an *iterable*, it must be an iterable of integers in the range
``0 <= x < 256``, which are used as the initial contents of the array.
Without an argument, an array of size 0 is created.
See also :ref:`binaryseq` and :ref:`typebytearray`.
.. _func-bytes:
.. class:: bytes(source=b'')
bytes(source, encoding)
bytes(source, encoding, errors)
:noindex:
Return a new "bytes" object which is an immutable sequence of integers in
the range ``0 <= x < 256``. :class:`bytes` is an immutable version of
:class:`bytearray` -- it has the same non-mutating methods and the same
indexing and slicing behavior.
Accordingly, constructor arguments are interpreted as for :func:`bytearray`.
Bytes objects can also be created with literals, see :ref:`strings`.
See also :ref:`binaryseq`, :ref:`typebytes`, and :ref:`bytes-methods`.
.. function:: callable(object)
Return :const:`True` if the *object* argument appears callable,
:const:`False` if not. If this returns ``True``, it is still possible that a
call fails, but if it is ``False``, calling *object* will never succeed.
Note that classes are callable (calling a class returns a new instance);
instances are callable if their class has a :meth:`~object.__call__` method.
.. versionadded:: 3.2
This function was first removed in Python 3.0 and then brought back
in Python 3.2.
.. function:: chr(i)
Return the string representing a character whose Unicode code point is the
integer *i*. For example, ``chr(97)`` returns the string ``'a'``, while
``chr(8364)`` returns the string ``'€'``. This is the inverse of :func:`ord`.
The valid range for the argument is from 0 through 1,114,111 (0x10FFFF in
base 16). :exc:`ValueError` will be raised if *i* is outside that range.
.. decorator:: classmethod
Transform a method into a class method.
A class method receives the class as an implicit first argument, just like an
instance method receives the instance. To declare a class method, use this
idiom::
class C:
@classmethod
def f(cls, arg1, arg2): ...
The ``@classmethod`` form is a function :term:`decorator` -- see
:ref:`function` for details.
A class method can be called either on the class (such as ``C.f()``) or on an instance (such
as ``C().f()``). The instance is ignored except for its class. If a class
method is called for a derived class, the derived class object is passed as the
implied first argument.
Class methods are different than C++ or Java static methods. If you want those,
see :func:`staticmethod` in this section.
For more information on class methods, see :ref:`types`.
.. versionchanged:: 3.9
Class methods can now wrap other :term:`descriptors <descriptor>` such as
:func:`property`.
.. versionchanged:: 3.10
Class methods now inherit the method attributes
(:attr:`~function.__module__`, :attr:`~function.__name__`,
:attr:`~function.__qualname__`, :attr:`~function.__doc__` and
:attr:`~function.__annotations__`) and have a new ``__wrapped__``
attribute.
.. deprecated-removed:: 3.11 3.13
Class methods can no longer wrap other :term:`descriptors <descriptor>` such as
:func:`property`.
.. function:: compile(source, filename, mode, flags=0, dont_inherit=False, optimize=-1)
Compile the *source* into a code or AST object. Code objects can be executed
by :func:`exec` or :func:`eval`. *source* can either be a normal string, a
byte string, or an AST object. Refer to the :mod:`ast` module documentation
for information on how to work with AST objects.
The *filename* argument should give the file from which the code was read;
pass some recognizable value if it wasn't read from a file (``'<string>'`` is
commonly used).
The *mode* argument specifies what kind of code must be compiled; it can be
``'exec'`` if *source* consists of a sequence of statements, ``'eval'`` if it
consists of a single expression, or ``'single'`` if it consists of a single
interactive statement (in the latter case, expression statements that
evaluate to something other than ``None`` will be printed).
The optional arguments *flags* and *dont_inherit* control which
:ref:`compiler options <ast-compiler-flags>` should be activated
and which :ref:`future features <future>` should be allowed. If neither
is present (or both are zero) the code is compiled with the same flags that
affect the code that is calling :func:`compile`. If the *flags*
argument is given and *dont_inherit* is not (or is zero) then the compiler
options and the future statements specified by the *flags* argument are used
in addition to those that would be used anyway. If *dont_inherit* is a
non-zero integer then the *flags* argument is it -- the flags (future
features and compiler options) in the surrounding code are ignored.
Compiler options and future statements are specified by bits which can be
bitwise ORed together to specify multiple options. The bitfield required to
specify a given future feature can be found as the
:attr:`~__future__._Feature.compiler_flag` attribute on the
:class:`~__future__._Feature` instance in the :mod:`__future__` module.
:ref:`Compiler flags <ast-compiler-flags>` can be found in :mod:`ast`
module, with ``PyCF_`` prefix.
The argument *optimize* specifies the optimization level of the compiler; the
default value of ``-1`` selects the optimization level of the interpreter as
given by :option:`-O` options. Explicit levels are ``0`` (no optimization;
``__debug__`` is true), ``1`` (asserts are removed, ``__debug__`` is false)
or ``2`` (docstrings are removed too).
This function raises :exc:`SyntaxError` if the compiled source is invalid,
and :exc:`ValueError` if the source contains null bytes.
If you want to parse Python code into its AST representation, see
:func:`ast.parse`.
.. audit-event:: compile source,filename compile
Raises an :ref:`auditing event <auditing>` ``compile`` with arguments
``source`` and ``filename``. This event may also be raised by implicit
compilation.
.. note::
When compiling a string with multi-line code in ``'single'`` or
``'eval'`` mode, input must be terminated by at least one newline
character. This is to facilitate detection of incomplete and complete
statements in the :mod:`code` module.
.. warning::
It is possible to crash the Python interpreter with a
sufficiently large/complex string when compiling to an AST
object due to stack depth limitations in Python's AST compiler.
.. versionchanged:: 3.2
Allowed use of Windows and Mac newlines. Also, input in ``'exec'`` mode
does not have to end in a newline anymore. Added the *optimize* parameter.
.. versionchanged:: 3.5
Previously, :exc:`TypeError` was raised when null bytes were encountered
in *source*.
.. versionadded:: 3.8
``ast.PyCF_ALLOW_TOP_LEVEL_AWAIT`` can now be passed in flags to enable
support for top-level ``await``, ``async for``, and ``async with``.
.. class:: complex(number=0, /)
complex(string, /)
complex(real=0, imag=0)
Convert a single string or number to a complex number, or create a
complex number from real and imaginary parts.
Examples:
.. doctest::
>>> complex('+1.23')
(1.23+0j)
>>> complex('-4.5j')
-4.5j
>>> complex('-1.23+4.5j')
(-1.23+4.5j)
>>> complex('\t( -1.23+4.5J )\n')
(-1.23+4.5j)
>>> complex('-Infinity+NaNj')
(-inf+nanj)
>>> complex(1.23)
(1.23+0j)
>>> complex(imag=-4.5)
-4.5j
>>> complex(-1.23, 4.5)
(-1.23+4.5j)
If the argument is a string, it must contain either a real part (in the
same format as for :func:`float`) or an imaginary part (in the same
format but with a ``'j'`` or ``'J'`` suffix), or both real and imaginary
parts (the sign of the imaginary part is mandatory in this case).
The string can optionally be surrounded by whitespaces and the round
parentheses ``'('`` and ``')'``, which are ignored.
The string must not contain whitespace between ``'+'``, ``'-'``, the
``'j'`` or ``'J'`` suffix, and the decimal number.
For example, ``complex('1+2j')`` is fine, but ``complex('1 + 2j')`` raises
:exc:`ValueError`.
More precisely, the input must conform to the :token:`~float:complexvalue`
production rule in the following grammar, after parentheses and leading and
trailing whitespace characters are removed:
.. productionlist:: float
complexvalue: `floatvalue` |
: `floatvalue` ("j" | "J") |
: `floatvalue` `sign` `absfloatvalue` ("j" | "J")
If the argument is a number, the constructor serves as a numeric
conversion like :class:`int` and :class:`float`.
For a general Python object ``x``, ``complex(x)`` delegates to
``x.__complex__()``.
If :meth:`~object.__complex__` is not defined then it falls back
to :meth:`~object.__float__`.
If :meth:`!__float__` is not defined then it falls back
to :meth:`~object.__index__`.
If two arguments are provided or keyword arguments are used, each argument
may be any numeric type (including complex).
If both arguments are real numbers, return a complex number with the real
component *real* and the imaginary component *imag*.
If both arguments are complex numbers, return a complex number with the real
component ``real.real-imag.imag`` and the imaginary component
``real.imag+imag.real``.
If one of arguments is a real number, only its real component is used in
the above expressions.
See also :meth:`complex.from_number` which only accepts a single numeric argument.
If all arguments are omitted, returns ``0j``.
The complex type is described in :ref:`typesnumeric`.
.. versionchanged:: 3.6
Grouping digits with underscores as in code literals is allowed.
.. versionchanged:: 3.8
Falls back to :meth:`~object.__index__` if :meth:`~object.__complex__` and
:meth:`~object.__float__` are not defined.
.. deprecated:: 3.14
Passing a complex number as the *real* or *imag* argument is now
deprecated; it should only be passed as a single positional argument.
.. function:: delattr(object, name)
This is a relative of :func:`setattr`. The arguments are an object and a
string. The string must be the name of one of the object's attributes. The
function deletes the named attribute, provided the object allows it. For
example, ``delattr(x, 'foobar')`` is equivalent to ``del x.foobar``.
*name* need not be a Python identifier (see :func:`setattr`).
.. _func-dict:
.. class:: dict(**kwarg)
dict(mapping, **kwarg)
dict(iterable, **kwarg)
:noindex:
Create a new dictionary. The :class:`dict` object is the dictionary class.
See :class:`dict` and :ref:`typesmapping` for documentation about this class.
For other containers see the built-in :class:`list`, :class:`set`, and
:class:`tuple` classes, as well as the :mod:`collections` module.
.. function:: dir()
dir(object)
Without arguments, return the list of names in the current local scope. With an
argument, attempt to return a list of valid attributes for that object.
If the object has a method named :meth:`~object.__dir__`,
this method will be called and
must return the list of attributes. This allows objects that implement a custom
:func:`~object.__getattr__` or :func:`~object.__getattribute__` function
to customize the way
:func:`dir` reports their attributes.
If the object does not provide :meth:`~object.__dir__`,
the function tries its best to gather information from the object's
:attr:`~object.__dict__` attribute, if defined, and
from its type object. The resulting list is not necessarily complete and may
be inaccurate when the object has a custom :func:`~object.__getattr__`.
The default :func:`dir` mechanism behaves differently with different types of
objects, as it attempts to produce the most relevant, rather than complete,
information:
* If the object is a module object, the list contains the names of the module's
attributes.
* If the object is a type or class object, the list contains the names of its
attributes, and recursively of the attributes of its bases.
* Otherwise, the list contains the object's attributes' names, the names of its
class's attributes, and recursively of the attributes of its class's base
classes.
The resulting list is sorted alphabetically. For example:
>>> import struct
>>> dir() # show the names in the module namespace # doctest: +SKIP
['__builtins__', '__name__', 'struct']
>>> dir(struct) # show the names in the struct module # doctest: +SKIP
['Struct', '__all__', '__builtins__', '__cached__', '__doc__', '__file__',
'__initializing__', '__loader__', '__name__', '__package__',
'_clearcache', 'calcsize', 'error', 'pack', 'pack_into',
'unpack', 'unpack_from']
>>> class Shape:
... def __dir__(self):
... return ['area', 'perimeter', 'location']
...
>>> s = Shape()
>>> dir(s)
['area', 'location', 'perimeter']
.. note::
Because :func:`dir` is supplied primarily as a convenience for use at an
interactive prompt, it tries to supply an interesting set of names more
than it tries to supply a rigorously or consistently defined set of names,
and its detailed behavior may change across releases. For example,
metaclass attributes are not in the result list when the argument is a
class.
.. function:: divmod(a, b)
Take two (non-complex) numbers as arguments and return a pair of numbers
consisting of their quotient and remainder when using integer division. With
mixed operand types, the rules for binary arithmetic operators apply. For
integers, the result is the same as ``(a // b, a % b)``. For floating-point
numbers the result is ``(q, a % b)``, where *q* is usually ``math.floor(a /
b)`` but may be 1 less than that. In any case ``q * b + a % b`` is very
close to *a*, if ``a % b`` is non-zero it has the same sign as *b*, and ``0
<= abs(a % b) < abs(b)``.
.. function:: enumerate(iterable, start=0)
Return an enumerate object. *iterable* must be a sequence, an
:term:`iterator`, or some other object which supports iteration.
The :meth:`~iterator.__next__` method of the iterator returned by
:func:`enumerate` returns a tuple containing a count (from *start* which
defaults to 0) and the values obtained from iterating over *iterable*.
>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']
>>> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]
>>> list(enumerate(seasons, start=1))
[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]
Equivalent to::
def enumerate(iterable, start=0):
n = start
for elem in iterable:
yield n, elem
n += 1
.. _func-eval:
.. function:: eval(source, /, globals=None, locals=None)
:param source:
A Python expression.
:type source: :class:`str` | :ref:`code object <code-objects>`
:param globals:
The global namespace (default: ``None``).
:type globals: :class:`dict` | ``None``
:param locals:
The local namespace (default: ``None``).
:type locals: :term:`mapping` | ``None``
:returns: The result of the evaluated expression.
:raises: Syntax errors are reported as exceptions.
.. warning::
This function executes arbitrary code. Calling it with
user-supplied input may lead to security vulnerabilities.
The *expression* argument is parsed and evaluated as a Python expression
(technically speaking, a condition list) using the *globals* and *locals*
mappings as global and local namespace. If the *globals* dictionary is
present and does not contain a value for the key ``__builtins__``, a
reference to the dictionary of the built-in module :mod:`builtins` is
inserted under that key before *expression* is parsed. That way you can
control what builtins are available to the executed code by inserting your
own ``__builtins__`` dictionary into *globals* before passing it to
:func:`eval`. If the *locals* mapping is omitted it defaults to the
*globals* dictionary. If both mappings are omitted, the expression is
executed with the *globals* and *locals* in the environment where
:func:`eval` is called. Note, *eval()* will only have access to the
:term:`nested scopes <nested scope>` (non-locals) in the enclosing
environment if they are already referenced in the scope that is calling
:func:`eval` (e.g. via a :keyword:`nonlocal` statement).
Example:
>>> x = 1
>>> eval('x+1')
2
This function can also be used to execute arbitrary code objects (such as
those created by :func:`compile`). In this case, pass a code object instead
of a string. If the code object has been compiled with ``'exec'`` as the
*mode* argument, :func:`eval`\'s return value will be ``None``.
Hints: dynamic execution of statements is supported by the :func:`exec`
function. The :func:`globals` and :func:`locals` functions
return the current global and local dictionary, respectively, which may be
useful to pass around for use by :func:`eval` or :func:`exec`.
If the given source is a string, then leading and trailing spaces and tabs
are stripped.
See :func:`ast.literal_eval` for a function that can safely evaluate strings
with expressions containing only literals.
.. audit-event:: exec code_object eval
Raises an :ref:`auditing event <auditing>` ``exec`` with the code object
as the argument. Code compilation events may also be raised.
.. versionchanged:: 3.13
The *globals* and *locals* arguments can now be passed as keywords.
.. versionchanged:: 3.13
The semantics of the default *locals* namespace have been adjusted as
described for the :func:`locals` builtin.
.. index:: pair: built-in function; exec
.. function:: exec(source, /, globals=None, locals=None, *, closure=None)
.. warning::
This function executes arbitrary code. Calling it with
user-supplied input may lead to security vulnerabilities.
This function supports dynamic execution of Python code. *source* must be
either a string or a code object. If it is a string, the string is parsed as
a suite of Python statements which is then executed (unless a syntax error
occurs). [#]_ If it is a code object, it is simply executed. In all cases,
the code that's executed is expected to be valid as file input (see the
section :ref:`file-input` in the Reference Manual). Be aware that the
:keyword:`nonlocal`, :keyword:`yield`, and :keyword:`return`
statements may not be used outside of
function definitions even within the context of code passed to the
:func:`exec` function. The return value is ``None``.
In all cases, if the optional parts are omitted, the code is executed in the
current scope. If only *globals* is provided, it must be a dictionary
(and not a subclass of dictionary), which
will be used for both the global and the local variables. If *globals* and
*locals* are given, they are used for the global and local variables,
respectively. If provided, *locals* can be any mapping object. Remember
that at the module level, globals and locals are the same dictionary.
.. note::
When ``exec`` gets two separate objects as *globals* and *locals*, the
code will be executed as if it were embedded in a class definition. This
means functions and classes defined in the executed code will not be able
to access variables assigned at the top level (as the "top level"
variables are treated as class variables in a class definition).
If the *globals* dictionary does not contain a value for the key
``__builtins__``, a reference to the dictionary of the built-in module
:mod:`builtins` is inserted under that key. That way you can control what
builtins are available to the executed code by inserting your own
``__builtins__`` dictionary into *globals* before passing it to :func:`exec`.
The *closure* argument specifies a closure--a tuple of cellvars.
It's only valid when the *object* is a code object containing
:term:`free (closure) variables <closure variable>`.
The length of the tuple must exactly match the length of the code object's
:attr:`~codeobject.co_freevars` attribute.
.. audit-event:: exec code_object exec
Raises an :ref:`auditing event <auditing>` ``exec`` with the code object
as the argument. Code compilation events may also be raised.
.. note::
The built-in functions :func:`globals` and :func:`locals` return the current
global and local namespace, respectively, which may be useful to pass around
for use as the second and third argument to :func:`exec`.
.. note::
The default *locals* act as described for function :func:`locals` below.
Pass an explicit *locals* dictionary if you need to see effects of the
code on *locals* after function :func:`exec` returns.
.. versionchanged:: 3.11
Added the *closure* parameter.
.. versionchanged:: 3.13
The *globals* and *locals* arguments can now be passed as keywords.
.. versionchanged:: 3.13
The semantics of the default *locals* namespace have been adjusted as
described for the :func:`locals` builtin.
.. function:: filter(function, iterable)
Construct an iterator from those elements of *iterable* for which *function*
is true. *iterable* may be either a sequence, a container which
supports iteration, or an iterator. If *function* is ``None``, the identity
function is assumed, that is, all elements of *iterable* that are false are
removed.
Note that ``filter(function, iterable)`` is equivalent to the generator
expression ``(item for item in iterable if function(item))`` if function is
not ``None`` and ``(item for item in iterable if item)`` if function is
``None``.
See :func:`itertools.filterfalse` for the complementary function that returns
elements of *iterable* for which *function* is false.
.. class:: float(number=0.0, /)
float(string, /)
.. index::
single: NaN
single: Infinity
Return a floating-point number constructed from a number or a string.
Examples:
.. doctest::
>>> float('+1.23')
1.23
>>> float(' -12345\n')
-12345.0
>>> float('1e-003')
0.001
>>> float('+1E6')
1000000.0
>>> float('-Infinity')
-inf
If the argument is a string, it should contain a decimal number, optionally
preceded by a sign, and optionally embedded in whitespace. The optional
sign may be ``'+'`` or ``'-'``; a ``'+'`` sign has no effect on the value
produced. The argument may also be a string representing a NaN
(not-a-number), or positive or negative infinity.
More precisely, the input must conform to the :token:`~float:floatvalue`
production rule in the following grammar, after leading and trailing
whitespace characters are removed:
.. productionlist:: float
sign: "+" | "-"
infinity: "Infinity" | "inf"
nan: "nan"
digit: <a Unicode decimal digit, i.e. characters in Unicode general category Nd>
digitpart: `digit` (["_"] `digit`)*
number: [`digitpart`] "." `digitpart` | `digitpart` ["."]
exponent: ("e" | "E") [`sign`] `digitpart`
floatnumber: `number` [`exponent`]
absfloatvalue: `floatnumber` | `infinity` | `nan`
floatvalue: [`sign`] `absfloatvalue`
Case is not significant, so, for example, "inf", "Inf", "INFINITY", and
"iNfINity" are all acceptable spellings for positive infinity.
Otherwise, if the argument is an integer or a floating-point number, a
floating-point number with the same value (within Python's floating-point
precision) is returned. If the argument is outside the range of a Python
float, an :exc:`OverflowError` will be raised.
For a general Python object ``x``, ``float(x)`` delegates to
``x.__float__()``. If :meth:`~object.__float__` is not defined then it falls back
to :meth:`~object.__index__`.
See also :meth:`float.from_number` which only accepts a numeric argument.
If no argument is given, ``0.0`` is returned.
The float type is described in :ref:`typesnumeric`.
.. versionchanged:: 3.6
Grouping digits with underscores as in code literals is allowed.
.. versionchanged:: 3.7
The parameter is now positional-only.
.. versionchanged:: 3.8
Falls back to :meth:`~object.__index__` if :meth:`~object.__float__` is not defined.
.. index::
single: __format__
single: string; format() (built-in function)
.. function:: format(value, format_spec="")
Convert a *value* to a "formatted" representation, as controlled by
*format_spec*. The interpretation of *format_spec* will depend on the type
of the *value* argument; however, there is a standard formatting syntax that
is used by most built-in types: :ref:`formatspec`.
The default *format_spec* is an empty string which usually gives the same
effect as calling :func:`str(value) <str>`.
A call to ``format(value, format_spec)`` is translated to
``type(value).__format__(value, format_spec)`` which bypasses the instance
dictionary when searching for the value's :meth:`~object.__format__` method.
A :exc:`TypeError` exception is raised if the method search reaches
:mod:`object` and the *format_spec* is non-empty, or if either the
*format_spec* or the return value are not strings.
.. versionchanged:: 3.4
``object().__format__(format_spec)`` raises :exc:`TypeError`
if *format_spec* is not an empty string.
.. _func-frozenset:
.. class:: frozenset(iterable=set())
:noindex:
Return a new :class:`frozenset` object, optionally with elements taken from
*iterable*. ``frozenset`` is a built-in class. See :class:`frozenset` and
:ref:`types-set` for documentation about this class.
For other containers see the built-in :class:`set`, :class:`list`,
:class:`tuple`, and :class:`dict` classes, as well as the :mod:`collections`
module.
.. function:: getattr(object, name)
getattr(object, name, default)
Return the value of the named attribute of *object*. *name* must be a string.
If the string is the name of one of the object's attributes, the result is the
value of that attribute. For example, ``getattr(x, 'foobar')`` is equivalent to
``x.foobar``. If the named attribute does not exist, *default* is returned if
provided, otherwise :exc:`AttributeError` is raised.
*name* need not be a Python identifier (see :func:`setattr`).
.. note::
Since :ref:`private name mangling <private-name-mangling>` happens at
compilation time, one must manually mangle a private attribute's
(attributes with two leading underscores) name in order to retrieve it with
:func:`getattr`.
.. function:: globals()
Return the dictionary implementing the current module namespace. For code within
functions, this is set when the function is defined and remains the same
regardless of where the function is called.
.. function:: hasattr(object, name)
The arguments are an object and a string. The result is ``True`` if the
string is the name of one of the object's attributes, ``False`` if not. (This
is implemented by calling ``getattr(object, name)`` and seeing whether it
raises an :exc:`AttributeError` or not.)
.. function:: hash(object)
Return the hash value of the object (if it has one). Hash values are
integers. They are used to quickly compare dictionary keys during a
dictionary lookup. Numeric values that compare equal have the same hash
value (even if they are of different types, as is the case for 1 and 1.0).
.. note::
For objects with custom :meth:`~object.__hash__` methods,
note that :func:`hash`
truncates the return value based on the bit width of the host machine.
.. function:: help()
help(request)
Invoke the built-in help system. (This function is intended for interactive
use.) If no argument is given, the interactive help system starts on the
interpreter console. If the argument is a string, then the string is looked up
as the name of a module, function, class, method, keyword, or documentation
topic, and a help page is printed on the console. If the argument is any other
kind of object, a help page on the object is generated.
Note that if a slash(/) appears in the parameter list of a function when
invoking :func:`help`, it means that the parameters prior to the slash are
positional-only. For more info, see
:ref:`the FAQ entry on positional-only parameters <faq-positional-only-arguments>`.
This function is added to the built-in namespace by the :mod:`site` module.
.. versionchanged:: 3.4
Changes to :mod:`pydoc` and :mod:`inspect` mean that the reported
signatures for callables are now more comprehensive and consistent.
.. function:: hex(x)
Convert an integer number to a lowercase hexadecimal string prefixed with
"0x". If *x* is not a Python :class:`int` object, it has to define an
:meth:`~object.__index__` method that returns an integer. Some examples:
>>> hex(255)
'0xff'
>>> hex(-42)
'-0x2a'
If you want to convert an integer number to an uppercase or lower hexadecimal
string with prefix or not, you can use either of the following ways:
>>> '%#x' % 255, '%x' % 255, '%X' % 255
('0xff', 'ff', 'FF')
>>> format(255, '#x'), format(255, 'x'), format(255, 'X')
('0xff', 'ff', 'FF')
>>> f'{255:#x}', f'{255:x}', f'{255:X}'
('0xff', 'ff', 'FF')
See also :func:`format` for more information.
See also :func:`int` for converting a hexadecimal string to an
integer using a base of 16.
.. note::
To obtain a hexadecimal string representation for a float, use the
:meth:`float.hex` method.
.. function:: id(object)
Return the "identity" of an object. This is an integer which
is guaranteed to be unique and constant for this object during its lifetime.
Two objects with non-overlapping lifetimes may have the same :func:`id`
value.
.. impl-detail:: This is the address of the object in memory.
.. audit-event:: builtins.id id id
.. function:: input()
input(prompt)
If the *prompt* argument is present, it is written to standard output without
a trailing newline. The function then reads a line from input, converts it
to a string (stripping a trailing newline), and returns that. When EOF is
read, :exc:`EOFError` is raised. Example::
>>> s = input('--> ') # doctest: +SKIP
--> Monty Python's Flying Circus
>>> s # doctest: +SKIP
"Monty Python's Flying Circus"
If the :mod:`readline` module was loaded, then :func:`input` will use it
to provide elaborate line editing and history features.
.. audit-event:: builtins.input prompt input
Raises an :ref:`auditing event <auditing>` ``builtins.input`` with
argument ``prompt`` before reading input
.. audit-event:: builtins.input/result result input
Raises an :ref:`auditing event <auditing>` ``builtins.input/result``
with the result after successfully reading input.
.. class:: int(number=0, /)
int(string, /, base=10)
Return an integer object constructed from a number or a string, or return
``0`` if no arguments are given.
Examples:
.. doctest::
>>> int(123.45)
123
>>> int('123')
123
>>> int(' -12_345\n')
-12345
>>> int('FACE', 16)
64206
>>> int('0xface', 0)
64206
>>> int('01110011', base=2)
115
If the argument defines :meth:`~object.__int__`,
``int(x)`` returns ``x.__int__()``. If the argument defines
:meth:`~object.__index__`, it returns ``x.__index__()``.
For floating-point numbers, this truncates towards zero.
If the argument is not a number or if *base* is given, then it must be a string,
:class:`bytes`, or :class:`bytearray` instance representing an integer
in radix *base*. Optionally, the string can be preceded by ``+`` or ``-``
(with no space in between), have leading zeros, be surrounded by whitespace,
and have single underscores interspersed between digits.
A base-n integer string contains digits, each representing a value from 0 to
n-1. The values 0--9 can be represented by any Unicode decimal digit. The
values 10--35 can be represented by ``a`` to ``z`` (or ``A`` to ``Z``). The
default *base* is 10. The allowed bases are 0 and 2--36. Base-2, -8, and -16
strings can be optionally prefixed with ``0b``/``0B``, ``0o``/``0O``, or
``0x``/``0X``, as with integer literals in code. For base 0, the string is
interpreted in a similar way to an :ref:`integer literal in code <integers>`,
in that the actual base is 2, 8, 10, or 16 as determined by the prefix. Base
0 also disallows leading zeros: ``int('010', 0)`` is not legal, while
``int('010')`` and ``int('010', 8)`` are.
The integer type is described in :ref:`typesnumeric`.
.. versionchanged:: 3.4
If *base* is not an instance of :class:`int` and the *base* object has a
:meth:`base.__index__ <object.__index__>` method, that method is called
to obtain an integer for the base. Previous versions used
:meth:`base.__int__ <object.__int__>` instead of :meth:`base.__index__
<object.__index__>`.
.. versionchanged:: 3.6
Grouping digits with underscores as in code literals is allowed.
.. versionchanged:: 3.7
The first parameter is now positional-only.
.. versionchanged:: 3.8
Falls back to :meth:`~object.__index__` if :meth:`~object.__int__` is not defined.
.. versionchanged:: 3.11
:class:`int` string inputs and string representations can be limited to
help avoid denial of service attacks. A :exc:`ValueError` is raised when
the limit is exceeded while converting a string to an :class:`int` or
when converting an :class:`int` into a string would exceed the limit.
See the :ref:`integer string conversion length limitation
<int_max_str_digits>` documentation.
.. versionchanged:: 3.14
:func:`int` no longer delegates to the :meth:`~object.__trunc__` method.
.. function:: isinstance(object, classinfo)
Return ``True`` if the *object* argument is an instance of the *classinfo*
argument, or of a (direct, indirect, or :term:`virtual <abstract base
class>`) subclass thereof. If *object* is not
an object of the given type, the function always returns ``False``.
If *classinfo* is a tuple of type objects (or recursively, other such
tuples) or a :ref:`types-union` of multiple types, return ``True`` if
*object* is an instance of any of the types.
If *classinfo* is not a type or tuple of types and such tuples,
a :exc:`TypeError` exception is raised. :exc:`TypeError` may not be
raised for an invalid type if an earlier check succeeds.
.. versionchanged:: 3.10
*classinfo* can be a :ref:`types-union`.
.. function:: issubclass(class, classinfo)
Return ``True`` if *class* is a subclass (direct, indirect, or :term:`virtual
<abstract base class>`) of *classinfo*. A
class is considered a subclass of itself. *classinfo* may be a tuple of class
objects (or recursively, other such tuples)
or a :ref:`types-union`, in which case return ``True`` if *class* is a
subclass of any entry in *classinfo*. In any other case, a :exc:`TypeError`
exception is raised.
.. versionchanged:: 3.10
*classinfo* can be a :ref:`types-union`.
.. function:: iter(object)
iter(object, sentinel)
Return an :term:`iterator` object. The first argument is interpreted very
differently depending on the presence of the second argument. Without a
second argument, *object* must be a collection object which supports the
:term:`iterable` protocol (the :meth:`~object.__iter__` method),
or it must support
the sequence protocol (the :meth:`~object.__getitem__` method with integer arguments
starting at ``0``). If it does not support either of those protocols,
:exc:`TypeError` is raised. If the second argument, *sentinel*, is given,
then *object* must be a callable object. The iterator created in this case
will call *object* with no arguments for each call to its
:meth:`~iterator.__next__` method; if the value returned is equal to
*sentinel*, :exc:`StopIteration` will be raised, otherwise the value will
be returned.
See also :ref:`typeiter`.
One useful application of the second form of :func:`iter` is to build a
block-reader. For example, reading fixed-width blocks from a binary
database file until the end of file is reached::
from functools import partial
with open('mydata.db', 'rb') as f:
for block in iter(partial(f.read, 64), b''):
process_block(block)
.. function:: len(s)
Return the length (the number of items) of an object. The argument may be a
sequence (such as a string, bytes, tuple, list, or range) or a collection
(such as a dictionary, set, or frozen set).
.. impl-detail::
``len`` raises :exc:`OverflowError` on lengths larger than
:data:`sys.maxsize`, such as :class:`range(2 ** 100) <range>`.
.. _func-list:
.. class:: list()
list(iterable)
:noindex:
Rather than being a function, :class:`list` is actually a mutable
sequence type, as documented in :ref:`typesseq-list` and :ref:`typesseq`.
.. function:: locals()
Return a mapping object representing the current local symbol table, with
variable names as the keys, and their currently bound references as the
values.
At module scope, as well as when using :func:`exec` or :func:`eval` with
a single namespace, this function returns the same namespace as
:func:`globals`.
At class scope, it returns the namespace that will be passed to the
metaclass constructor.
When using ``exec()`` or ``eval()`` with separate local and global
arguments, it returns the local namespace passed in to the function call.
In all of the above cases, each call to ``locals()`` in a given frame of
execution will return the *same* mapping object. Changes made through
the mapping object returned from ``locals()`` will be visible as assigned,
reassigned, or deleted local variables, and assigning, reassigning, or
deleting local variables will immediately affect the contents of the
returned mapping object.
In an :term:`optimized scope` (including functions, generators, and
coroutines), each call to ``locals()`` instead returns a fresh dictionary
containing the current bindings of the function's local variables and any
nonlocal cell references. In this case, name binding changes made via the
returned dict are *not* written back to the corresponding local variables
or nonlocal cell references, and assigning, reassigning, or deleting local
variables and nonlocal cell references does *not* affect the contents
of previously returned dictionaries.
Calling ``locals()`` as part of a comprehension in a function, generator, or
coroutine is equivalent to calling it in the containing scope, except that
the comprehension's initialised iteration variables will be included. In
other scopes, it behaves as if the comprehension were running as a nested
function.
Calling ``locals()`` as part of a generator expression is equivalent to
calling it in a nested generator function.
.. versionchanged:: 3.12
The behaviour of ``locals()`` in a comprehension has been updated as
described in :pep:`709`.
.. versionchanged:: 3.13
As part of :pep:`667`, the semantics of mutating the mapping objects
returned from this function are now defined. The behavior in
:term:`optimized scopes <optimized scope>` is now as described above.
Aside from being defined, the behaviour in other scopes remains
unchanged from previous versions.
.. function:: map(function, iterable, *iterables)
Return an iterator that applies *function* to every item of *iterable*,
yielding the results. If additional *iterables* arguments are passed,
*function* must take that many arguments and is applied to the items from all
iterables in parallel. With multiple iterables, the iterator stops when the
shortest iterable is exhausted. For cases where the function inputs are
already arranged into argument tuples, see :func:`itertools.starmap`\.
.. function:: max(iterable, *, key=None)
max(iterable, *, default, key=None)
max(arg1, arg2, *args, key=None)
Return the largest item in an iterable or the largest of two or more
arguments.
If one positional argument is provided, it should be an :term:`iterable`.
The largest item in the iterable is returned. If two or more positional
arguments are provided, the largest of the positional arguments is
returned.
There are two optional keyword-only arguments. The *key* argument specifies
a one-argument ordering function like that used for :meth:`list.sort`. The
*default* argument specifies an object to return if the provided iterable is
empty. If the iterable is empty and *default* is not provided, a
:exc:`ValueError` is raised.
If multiple items are maximal, the function returns the first one
encountered. This is consistent with other sort-stability preserving tools
such as ``sorted(iterable, key=keyfunc, reverse=True)[0]`` and
``heapq.nlargest(1, iterable, key=keyfunc)``.
.. versionchanged:: 3.4
Added the *default* keyword-only parameter.
.. versionchanged:: 3.8
The *key* can be ``None``.
.. _func-memoryview:
.. class:: memoryview(object)
:noindex:
Return a "memory view" object created from the given argument. See
:ref:`typememoryview` for more information.
.. function:: min(iterable, *, key=None)
min(iterable, *, default, key=None)
min(arg1, arg2, *args, key=None)
Return the smallest item in an iterable or the smallest of two or more
arguments.
If one positional argument is provided, it should be an :term:`iterable`.
The smallest item in the iterable is returned. If two or more positional
arguments are provided, the smallest of the positional arguments is
returned.
There are two optional keyword-only arguments. The *key* argument specifies
a one-argument ordering function like that used for :meth:`list.sort`. The
*default* argument specifies an object to return if the provided iterable is
empty. If the iterable is empty and *default* is not provided, a
:exc:`ValueError` is raised.
If multiple items are minimal, the function returns the first one
encountered. This is consistent with other sort-stability preserving tools
such as ``sorted(iterable, key=keyfunc)[0]`` and ``heapq.nsmallest(1,
iterable, key=keyfunc)``.
.. versionchanged:: 3.4
Added the *default* keyword-only parameter.
.. versionchanged:: 3.8
The *key* can be ``None``.
.. function:: next(iterator)
next(iterator, default)
Retrieve the next item from the :term:`iterator` by calling its
:meth:`~iterator.__next__` method. If *default* is given, it is returned
if the iterator is exhausted, otherwise :exc:`StopIteration` is raised.
.. class:: object()
This is the ultimate base class of all other classes. It has methods
that are common to all instances of Python classes. When the constructor
is called, it returns a new featureless object. The constructor does not
accept any arguments.
.. note::
:class:`object` instances do *not* have :attr:`~object.__dict__`
attributes, so you can't assign arbitrary attributes to an instance of
:class:`object`.
.. function:: oct(x)
Convert an integer number to an octal string prefixed with "0o". The result
is a valid Python expression. If *x* is not a Python :class:`int` object, it
has to define an :meth:`~object.__index__` method that returns an integer. For
example:
>>> oct(8)
'0o10'
>>> oct(-56)
'-0o70'
If you want to convert an integer number to an octal string either with the prefix
"0o" or not, you can use either of the following ways.
>>> '%#o' % 10, '%o' % 10
('0o12', '12')
>>> format(10, '#o'), format(10, 'o')
('0o12', '12')
>>> f'{10:#o}', f'{10:o}'
('0o12', '12')
See also :func:`format` for more information.
.. index::
single: file object; open() built-in function
.. function:: open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
Open *file* and return a corresponding :term:`file object`. If the file
cannot be opened, an :exc:`OSError` is raised. See
:ref:`tut-files` for more examples of how to use this function.
*file* is a :term:`path-like object` giving the pathname (absolute or
relative to the current working directory) of the file to be opened or an
integer file descriptor of the file to be wrapped. (If a file descriptor is
given, it is closed when the returned I/O object is closed unless *closefd*
is set to ``False``.)
*mode* is an optional string that specifies the mode in which the file is
opened. It defaults to ``'r'`` which means open for reading in text mode.
Other common values are ``'w'`` for writing (truncating the file if it
already exists), ``'x'`` for exclusive creation, and ``'a'`` for appending
(which on *some* Unix systems, means that *all* writes append to the end of
the file regardless of the current seek position). In text mode, if
*encoding* is not specified the encoding used is platform-dependent:
:func:`locale.getencoding` is called to get the current locale encoding.
(For reading and writing raw bytes use binary mode and leave
*encoding* unspecified.) The available modes are:
.. _filemodes:
.. index::
pair: file; modes
========= ===============================================================
Character Meaning
========= ===============================================================
``'r'`` open for reading (default)
``'w'`` open for writing, truncating the file first
``'x'`` open for exclusive creation, failing if the file already exists
``'a'`` open for writing, appending to the end of file if it exists
``'b'`` binary mode
``'t'`` text mode (default)
``'+'`` open for updating (reading and writing)
========= ===============================================================
The default mode is ``'r'`` (open for reading text, a synonym of ``'rt'``).
Modes ``'w+'`` and ``'w+b'`` open and truncate the file. Modes ``'r+'``
and ``'r+b'`` open the file with no truncation.
As mentioned in the :ref:`io-overview`, Python distinguishes between binary
and text I/O. Files opened in binary mode (including ``'b'`` in the *mode*
argument) return contents as :class:`bytes` objects without any decoding. In
text mode (the default, or when ``'t'`` is included in the *mode* argument),
the contents of the file are returned as :class:`str`, the bytes having been
first decoded using a platform-dependent encoding or using the specified
*encoding* if given.
.. note::
Python doesn't depend on the underlying operating system's notion of text
files; all the processing is done by Python itself, and is therefore
platform-independent.
*buffering* is an optional integer used to set the buffering policy. Pass 0
to switch buffering off (only allowed in binary mode), 1 to select line
buffering (only usable when writing in text mode), and an integer > 1 to indicate the size
in bytes of a fixed-size chunk buffer. Note that specifying a buffer size this
way applies for binary buffered I/O, but ``TextIOWrapper`` (i.e., files opened
with ``mode='r+'``) would have another buffering. To disable buffering in
``TextIOWrapper``, consider using the ``write_through`` flag for
:func:`io.TextIOWrapper.reconfigure`. When no *buffering* argument is
given, the default buffering policy works as follows:
* Binary files are buffered in fixed-size chunks; the size of the buffer is
chosen using a heuristic trying to determine the underlying device's "block
size" and falling back on :const:`io.DEFAULT_BUFFER_SIZE`. On many systems,
the buffer will typically be 4096 or 8192 bytes long.
* "Interactive" text files (files for which :meth:`~io.IOBase.isatty`
returns ``True``) use line buffering. Other text files use the policy
described above for binary files.
*encoding* is the name of the encoding used to decode or encode the file.
This should only be used in text mode. The default encoding is platform
dependent (whatever :func:`locale.getencoding` returns), but any
:term:`text encoding` supported by Python can be used.
See the :mod:`codecs` module for the list of supported encodings.
*errors* is an optional string that specifies how encoding and decoding
errors are to be handled—this cannot be used in binary mode.
A variety of standard error handlers are available
(listed under :ref:`error-handlers`), though any
error handling name that has been registered with
:func:`codecs.register_error` is also valid. The standard names
include:
* ``'strict'`` to raise a :exc:`ValueError` exception if there is
an encoding error. The default value of ``None`` has the same
effect.
* ``'ignore'`` ignores errors. Note that ignoring encoding errors
can lead to data loss.
* ``'replace'`` causes a replacement marker (such as ``'?'``) to be inserted
where there is malformed data.
* ``'surrogateescape'`` will represent any incorrect bytes as low
surrogate code units ranging from U+DC80 to U+DCFF.
These surrogate code units will then be turned back into
the same bytes when the ``surrogateescape`` error handler is used
when writing data. This is useful for processing files in an
unknown encoding.
* ``'xmlcharrefreplace'`` is only supported when writing to a file.
Characters not supported by the encoding are replaced with the
appropriate XML character reference :samp:`&#{nnn};`.
* ``'backslashreplace'`` replaces malformed data by Python's backslashed
escape sequences.
* ``'namereplace'`` (also only supported when writing)
replaces unsupported characters with ``\N{...}`` escape sequences.
.. index::
single: universal newlines; open() built-in function
.. _open-newline-parameter:
*newline* determines how to parse newline characters from the stream.
It can be ``None``, ``''``, ``'\n'``, ``'\r'``, and
``'\r\n'``. It works as follows:
* When reading input from the stream, if *newline* is ``None``, universal
newlines mode is enabled. Lines in the input can end in ``'\n'``,
``'\r'``, or ``'\r\n'``, and these are translated into ``'\n'`` before
being returned to the caller. If it is ``''``, universal newlines mode is
enabled, but line endings are returned to the caller untranslated. If it
has any of the other legal values, input lines are only terminated by the
given string, and the line ending is returned to the caller untranslated.
* When writing output to the stream, if *newline* is ``None``, any ``'\n'``
characters written are translated to the system default line separator,
:data:`os.linesep`. If *newline* is ``''`` or ``'\n'``, no translation
takes place. If *newline* is any of the other legal values, any ``'\n'``
characters written are translated to the given string.
If *closefd* is ``False`` and a file descriptor rather than a filename was
given, the underlying file descriptor will be kept open when the file is
closed. If a filename is given *closefd* must be ``True`` (the default);
otherwise, an error will be raised.
A custom opener can be used by passing a callable as *opener*. The underlying
file descriptor for the file object is then obtained by calling *opener* with
(*file*, *flags*). *opener* must return an open file descriptor (passing
:mod:`os.open` as *opener* results in functionality similar to passing
``None``).
The newly created file is :ref:`non-inheritable <fd_inheritance>`.
The following example uses the :ref:`dir_fd <dir_fd>` parameter of the
:func:`os.open` function to open a file relative to a given directory::
>>> import os
>>> dir_fd = os.open('somedir', os.O_RDONLY)
>>> def opener(path, flags):
... return os.open(path, flags, dir_fd=dir_fd)
...
>>> with open('spamspam.txt', 'w', opener=opener) as f:
... print('This will be written to somedir/spamspam.txt', file=f)
...
>>> os.close(dir_fd) # don't leak a file descriptor
The type of :term:`file object` returned by the :func:`open` function
depends on the mode. When :func:`open` is used to open a file in a text
mode (``'w'``, ``'r'``, ``'wt'``, ``'rt'``, etc.), it returns a subclass of
:class:`io.TextIOBase` (specifically :class:`io.TextIOWrapper`). When used
to open a file in a binary mode with buffering, the returned class is a
subclass of :class:`io.BufferedIOBase`. The exact class varies: in read
binary mode, it returns an :class:`io.BufferedReader`; in write binary and
append binary modes, it returns an :class:`io.BufferedWriter`, and in
read/write mode, it returns an :class:`io.BufferedRandom`. When buffering is
disabled, the raw stream, a subclass of :class:`io.RawIOBase`,
:class:`io.FileIO`, is returned.
.. index::
single: line-buffered I/O
single: unbuffered I/O
single: buffer size, I/O
single: I/O control; buffering
single: binary mode
single: text mode
pair: module; sys
See also the file handling modules, such as :mod:`fileinput`, :mod:`io`
(where :func:`open` is declared), :mod:`os`, :mod:`os.path`, :mod:`tempfile`,
and :mod:`shutil`.
.. audit-event:: open path,mode,flags open
The ``mode`` and ``flags`` arguments may have been modified or inferred from
the original call.
.. versionchanged:: 3.3
* The *opener* parameter was added.
* The ``'x'`` mode was added.
* :exc:`IOError` used to be raised, it is now an alias of :exc:`OSError`.
* :exc:`FileExistsError` is now raised if the file opened in exclusive
creation mode (``'x'``) already exists.
.. versionchanged:: 3.4
* The file is now non-inheritable.
.. versionchanged:: 3.5
* If the system call is interrupted and the signal handler does not raise an
exception, the function now retries the system call instead of raising an
:exc:`InterruptedError` exception (see :pep:`475` for the rationale).
* The ``'namereplace'`` error handler was added.
.. versionchanged:: 3.6
* Support added to accept objects implementing :class:`os.PathLike`.
* On Windows, opening a console buffer may return a subclass of
:class:`io.RawIOBase` other than :class:`io.FileIO`.
.. versionchanged:: 3.11
The ``'U'`` mode has been removed.
.. function:: ord(c)
Given a string representing one Unicode character, return an integer
representing the Unicode code point of that character. For example,
``ord('a')`` returns the integer ``97`` and ``ord('€')`` (Euro sign)
returns ``8364``. This is the inverse of :func:`chr`.
.. function:: pow(base, exp, mod=None)
Return *base* to the power *exp*; if *mod* is present, return *base* to the
power *exp*, modulo *mod* (computed more efficiently than
``pow(base, exp) % mod``). The two-argument form ``pow(base, exp)`` is
equivalent to using the power operator: ``base**exp``.
The arguments must have numeric types. With mixed operand types, the
coercion rules for binary arithmetic operators apply. For :class:`int`
operands, the result has the same type as the operands (after coercion)
unless the second argument is negative; in that case, all arguments are
converted to float and a float result is delivered. For example, ``pow(10, 2)``
returns ``100``, but ``pow(10, -2)`` returns ``0.01``. For a negative base of
type :class:`int` or :class:`float` and a non-integral exponent, a complex
result is delivered. For example, ``pow(-9, 0.5)`` returns a value close
to ``3j``. Whereas, for a negative base of type :class:`int` or :class:`float`
with an integral exponent, a float result is delivered. For example,
``pow(-9, 2.0)`` returns ``81.0``.
For :class:`int` operands *base* and *exp*, if *mod* is present, *mod* must
also be of integer type and *mod* must be nonzero. If *mod* is present and
*exp* is negative, *base* must be relatively prime to *mod*. In that case,
``pow(inv_base, -exp, mod)`` is returned, where *inv_base* is an inverse to
*base* modulo *mod*.
Here's an example of computing an inverse for ``38`` modulo ``97``::
>>> pow(38, -1, mod=97)
23
>>> 23 * 38 % 97 == 1
True
.. versionchanged:: 3.8
For :class:`int` operands, the three-argument form of ``pow`` now allows
the second argument to be negative, permitting computation of modular
inverses.
.. versionchanged:: 3.8
Allow keyword arguments. Formerly, only positional arguments were
supported.
.. function:: print(*objects, sep=' ', end='\n', file=None, flush=False)
Print *objects* to the text stream *file*, separated by *sep* and followed
by *end*. *sep*, *end*, *file*, and *flush*, if present, must be given as keyword
arguments.
All non-keyword arguments are converted to strings like :func:`str` does and
written to the stream, separated by *sep* and followed by *end*. Both *sep*
and *end* must be strings; they can also be ``None``, which means to use the
default values. If no *objects* are given, :func:`print` will just write
*end*.
The *file* argument must be an object with a ``write(string)`` method; if it
is not present or ``None``, :data:`sys.stdout` will be used. Since printed
arguments are converted to text strings, :func:`print` cannot be used with
binary mode file objects. For these, use ``file.write(...)`` instead.
Output buffering is usually determined by *file*.
However, if *flush* is true, the stream is forcibly flushed.
.. versionchanged:: 3.3
Added the *flush* keyword argument.
.. class:: property(fget=None, fset=None, fdel=None, doc=None)
Return a property attribute.
*fget* is a function for getting an attribute value. *fset* is a function
for setting an attribute value. *fdel* is a function for deleting an attribute
value. And *doc* creates a docstring for the attribute.
A typical use is to define a managed attribute ``x``::
class C:
def __init__(self):
self._x = None
def getx(self):
return self._x
def setx(self, value):
self._x = value
def delx(self):
del self._x
x = property(getx, setx, delx, "I'm the 'x' property.")
If *c* is an instance of *C*, ``c.x`` will invoke the getter,
``c.x = value`` will invoke the setter, and ``del c.x`` the deleter.
If given, *doc* will be the docstring of the property attribute. Otherwise, the
property will copy *fget*'s docstring (if it exists). This makes it possible to
create read-only properties easily using :func:`property` as a :term:`decorator`::
class Parrot:
def __init__(self):
self._voltage = 100000
@property
def voltage(self):
"""Get the current voltage."""
return self._voltage
The ``@property`` decorator turns the :meth:`!voltage` method into a "getter"
for a read-only attribute with the same name, and it sets the docstring for
*voltage* to "Get the current voltage."
.. decorator:: property.getter
.. decorator:: property.setter
.. decorator:: property.deleter
A property object has ``getter``, ``setter``,
and ``deleter`` methods usable as decorators that create a
copy of the property with the corresponding accessor function set to the
decorated function. This is best explained with an example:
.. testcode::
class C:
def __init__(self):
self._x = None
@property
def x(self):
"""I'm the 'x' property."""
return self._x
@x.setter
def x(self, value):
self._x = value
@x.deleter
def x(self):
del self._x
This code is exactly equivalent to the first example. Be sure to give the
additional functions the same name as the original property (``x`` in this
case.)
The returned property object also has the attributes ``fget``, ``fset``, and
``fdel`` corresponding to the constructor arguments.
.. versionchanged:: 3.5
The docstrings of property objects are now writeable.
.. attribute:: __name__
Attribute holding the name of the property. The name of the property
can be changed at runtime.
.. versionadded:: 3.13
.. _func-range:
.. class:: range(stop)
range(start, stop, step=1)
:noindex:
Rather than being a function, :class:`range` is actually an immutable
sequence type, as documented in :ref:`typesseq-range` and :ref:`typesseq`.
.. function:: repr(object)
Return a string containing a printable representation of an object. For many
types, this function makes an attempt to return a string that would yield an
object with the same value when passed to :func:`eval`; otherwise, the
representation is a string enclosed in angle brackets that contains the name
of the type of the object together with additional information often
including the name and address of the object. A class can control what this
function returns for its instances
by defining a :meth:`~object.__repr__` method.
If :func:`sys.displayhook` is not accessible, this function will raise
:exc:`RuntimeError`.
This class has a custom representation that can be evaluated::
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __repr__(self):
return f"Person('{self.name}', {self.age})"
.. function:: reversed(seq)
Return a reverse :term:`iterator`. *seq* must be an object which has
a :meth:`~object.__reversed__` method or supports the sequence protocol (the
:meth:`~object.__len__` method and the :meth:`~object.__getitem__` method
with integer arguments starting at ``0``).
.. function:: round(number, ndigits=None)
Return *number* rounded to *ndigits* precision after the decimal
point. If *ndigits* is omitted or is ``None``, it returns the
nearest integer to its input.
For the built-in types supporting :func:`round`, values are rounded to the
closest multiple of 10 to the power minus *ndigits*; if two multiples are
equally close, rounding is done toward the even choice (so, for example,
both ``round(0.5)`` and ``round(-0.5)`` are ``0``, and ``round(1.5)`` is
``2``). Any integer value is valid for *ndigits* (positive, zero, or
negative). The return value is an integer if *ndigits* is omitted or
``None``.
Otherwise, the return value has the same type as *number*.
For a general Python object ``number``, ``round`` delegates to
``number.__round__``.
.. note::
The behavior of :func:`round` for floats can be surprising: for example,
``round(2.675, 2)`` gives ``2.67`` instead of the expected ``2.68``.
This is not a bug: it's a result of the fact that most decimal fractions
can't be represented exactly as a float. See :ref:`tut-fp-issues` for
more information.
.. _func-set:
.. class:: set()
set(iterable)
:noindex:
Return a new :class:`set` object, optionally with elements taken from
*iterable*. ``set`` is a built-in class. See :class:`set` and
:ref:`types-set` for documentation about this class.
For other containers see the built-in :class:`frozenset`, :class:`list`,
:class:`tuple`, and :class:`dict` classes, as well as the :mod:`collections`
module.
.. function:: setattr(object, name, value)
This is the counterpart of :func:`getattr`. The arguments are an object, a
string, and an arbitrary value. The string may name an existing attribute or a
new attribute. The function assigns the value to the attribute, provided the
object allows it. For example, ``setattr(x, 'foobar', 123)`` is equivalent to
``x.foobar = 123``.
*name* need not be a Python identifier as defined in :ref:`identifiers`
unless the object chooses to enforce that, for example in a custom
:meth:`~object.__getattribute__` or via :attr:`~object.__slots__`.
An attribute whose name is not an identifier will not be accessible using
the dot notation, but is accessible through :func:`getattr` etc..
.. note::
Since :ref:`private name mangling <private-name-mangling>` happens at
compilation time, one must manually mangle a private attribute's
(attributes with two leading underscores) name in order to set it with
:func:`setattr`.
.. class:: slice(stop)
slice(start, stop, step=None)
Return a :term:`slice` object representing the set of indices specified by
``range(start, stop, step)``. The *start* and *step* arguments default to
``None``.
.. attribute:: slice.start
.. attribute:: slice.stop
.. attribute:: slice.step
Slice objects have read-only data attributes :attr:`!start`,
:attr:`!stop`, and :attr:`!step` which merely return the argument
values (or their default). They have no other explicit functionality;
however, they are used by NumPy and other third-party packages.
Slice objects are also generated when extended indexing syntax is used. For
example: ``a[start:stop:step]`` or ``a[start:stop, i]``. See
:func:`itertools.islice` for an alternate version that returns an
:term:`iterator`.
.. versionchanged:: 3.12
Slice objects are now :term:`hashable` (provided :attr:`~slice.start`,
:attr:`~slice.stop`, and :attr:`~slice.step` are hashable).
.. function:: sorted(iterable, /, *, key=None, reverse=False)
Return a new sorted list from the items in *iterable*.
Has two optional arguments which must be specified as keyword arguments.
*key* specifies a function of one argument that is used to extract a comparison
key from each element in *iterable* (for example, ``key=str.lower``). The
default value is ``None`` (compare the elements directly).
*reverse* is a boolean value. If set to ``True``, then the list elements are
sorted as if each comparison were reversed.
Use :func:`functools.cmp_to_key` to convert an old-style *cmp* function to a
*key* function.
The built-in :func:`sorted` function 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).
The sort algorithm uses only ``<`` comparisons between items. While
defining an :meth:`~object.__lt__` method will suffice for sorting,
:PEP:`8` recommends that all six :ref:`rich comparisons
<comparisons>` be implemented. This will help avoid bugs when using
the same data with other ordering tools such as :func:`max` that rely
on a different underlying method. Implementing all six comparisons
also helps avoid confusion for mixed type comparisons which can call
reflected the :meth:`~object.__gt__` method.
For sorting examples and a brief sorting tutorial, see :ref:`sortinghowto`.
.. decorator:: staticmethod
Transform a method into a static method.
A static method does not receive an implicit first argument. To declare a static
method, use this idiom::
class C:
@staticmethod
def f(arg1, arg2, argN): ...
The ``@staticmethod`` form is a function :term:`decorator` -- see
:ref:`function` for details.
A static method can be called either on the class (such as ``C.f()``) or on
an instance (such as ``C().f()``).
Moreover, the static method :term:`descriptor` is also callable, so it can
be used in the class definition (such as ``f()``).
Static methods in Python are similar to those found in Java or C++. Also, see
:func:`classmethod` for a variant that is useful for creating alternate class
constructors.
Like all decorators, it is also possible to call ``staticmethod`` as
a regular function and do something with its result. This is needed
in some cases where you need a reference to a function from a class
body and you want to avoid the automatic transformation to instance
method. For these cases, use this idiom::
def regular_function():
...
class C:
method = staticmethod(regular_function)
For more information on static methods, see :ref:`types`.
.. versionchanged:: 3.10
Static methods now inherit the method attributes
(:attr:`~function.__module__`, :attr:`~function.__name__`,
:attr:`~function.__qualname__`, :attr:`~function.__doc__` and
:attr:`~function.__annotations__`), have a new ``__wrapped__`` attribute,
and are now callable as regular functions.
.. index::
single: string; str() (built-in function)
.. _func-str:
.. class:: str(object='')
str(object=b'', encoding='utf-8', errors='strict')
:noindex:
Return a :class:`str` version of *object*. See :func:`str` for details.
``str`` is the built-in string :term:`class`. For general information
about strings, see :ref:`textseq`.
.. function:: sum(iterable, /, start=0)
Sums *start* and the items of an *iterable* from left to right and returns the
total. The *iterable*'s items are normally numbers, and the start value is not
allowed to be a string.
For some use cases, there are good alternatives to :func:`sum`.
The preferred, fast way to concatenate a sequence of strings is by calling
``''.join(sequence)``. To add floating-point values with extended precision,
see :func:`math.fsum`\. To concatenate a series of iterables, consider using
:func:`itertools.chain`.
.. versionchanged:: 3.8
The *start* parameter can be specified as a keyword argument.
.. versionchanged:: 3.12 Summation of floats switched to an algorithm
that gives higher accuracy and better commutativity on most builds.
.. versionchanged:: 3.14
Added specialization for summation of complexes,
using same algorithm as for summation of floats.
.. class:: super()
super(type, object_or_type=None)
Return a proxy object that delegates method calls to a parent or sibling
class of *type*. This is useful for accessing inherited methods that have
been overridden in a class.
The *object_or_type* determines the :term:`method resolution order`
to be searched. The search starts from the class right after the
*type*.
For example, if :attr:`~type.__mro__` of *object_or_type* is
``D -> B -> C -> A -> object`` and the value of *type* is ``B``,
then :func:`super` searches ``C -> A -> object``.
The :attr:`~type.__mro__` attribute of the class corresponding to
*object_or_type* lists the method resolution search order used by both
:func:`getattr` and :func:`super`. The attribute is dynamic and can change
whenever the inheritance hierarchy is updated.
If the second argument is omitted, the super object returned is unbound. If
the second argument is an object, ``isinstance(obj, type)`` must be true. If
the second argument is a type, ``issubclass(type2, type)`` must be true (this
is useful for classmethods).
When called directly within an ordinary method of a class, both arguments may
be omitted ("zero-argument :func:`!super`"). In this case, *type* will be the
enclosing class, and *obj* will be the first argument of the immediately
enclosing function (typically ``self``). (This means that zero-argument
:func:`!super` will not work as expected within nested functions, including
generator expressions, which implicitly create nested functions.)
There are two typical use cases for *super*. In a class hierarchy with
single inheritance, *super* can be used to refer to parent classes without
naming them explicitly, thus making the code more maintainable. This use
closely parallels the use of *super* in other programming languages.
The second use case is to support cooperative multiple inheritance in a
dynamic execution environment. This use case is unique to Python and is
not found in statically compiled languages or languages that only support
single inheritance. This makes it possible to implement "diamond diagrams"
where multiple base classes implement the same method. Good design dictates
that such implementations have the same calling signature in every case (because the
order of calls is determined at runtime, because that order adapts
to changes in the class hierarchy, and because that order can include
sibling classes that are unknown prior to runtime).
For both use cases, a typical superclass call looks like this::
class C(B):
def method(self, arg):
super().method(arg) # This does the same thing as:
# super(C, self).method(arg)
In addition to method lookups, :func:`super` also works for attribute
lookups. One possible use case for this is calling :term:`descriptors <descriptor>`
in a parent or sibling class.
Note that :func:`super` is implemented as part of the binding process for
explicit dotted attribute lookups such as ``super().__getitem__(name)``.
It does so by implementing its own :meth:`~object.__getattribute__` method
for searching
classes in a predictable order that supports cooperative multiple inheritance.
Accordingly, :func:`super` is undefined for implicit lookups using statements or
operators such as ``super()[name]``.
Also note that, aside from the zero argument form, :func:`super` is not
limited to use inside methods. The two argument form specifies the
arguments exactly and makes the appropriate references. The zero
argument form only works inside a class definition, as the compiler fills
in the necessary details to correctly retrieve the class being defined,
as well as accessing the current instance for ordinary methods.
For practical suggestions on how to design cooperative classes using
:func:`super`, see `guide to using super()
<https://rhettinger.wordpress.com/2011/05/26/super-considered-super/>`_.
.. versionchanged:: 3.14
:class:`super` objects are now :mod:`pickleable <pickle>` and
:mod:`copyable <copy>`.
.. _func-tuple:
.. class:: tuple()
tuple(iterable)
:noindex:
Rather than being a function, :class:`tuple` is actually an immutable
sequence type, as documented in :ref:`typesseq-tuple` and :ref:`typesseq`.
.. class:: type(object)
type(name, bases, dict, **kwds)
.. index:: pair: object; type
With one argument, return the type of an *object*. The return value is a
type object and generally the same object as returned by
:attr:`object.__class__`.
The :func:`isinstance` built-in function is recommended for testing the type
of an object, because it takes subclasses into account.
With three arguments, return a new type object. This is essentially a
dynamic form of the :keyword:`class` statement. The *name* string is
the class name and becomes the :attr:`~type.__name__` attribute.
The *bases* tuple contains the base classes and becomes the
:attr:`~type.__bases__` attribute; if empty, :class:`object`, the
ultimate base of all classes, is added. The *dict* dictionary contains
attribute and method definitions for the class body; it may be copied
or wrapped before becoming the :attr:`~type.__dict__` attribute.
The following two statements create identical :class:`!type` objects:
>>> class X:
... a = 1
...
>>> X = type('X', (), dict(a=1))
See also:
* :ref:`Documentation on attributes and methods on classes <class-attrs-and-methods>`.
* :ref:`bltin-type-objects`
Keyword arguments provided to the three argument form are passed to the
appropriate metaclass machinery (usually :meth:`~object.__init_subclass__`)
in the same way that keywords in a class
definition (besides *metaclass*) would.
See also :ref:`class-customization`.
.. versionchanged:: 3.6
Subclasses of :class:`!type` which don't override ``type.__new__`` may no
longer use the one-argument form to get the type of an object.
.. function:: vars()
vars(object)
Return the :attr:`~object.__dict__` attribute for a module, class, instance,
or any other object with a :attr:`!__dict__` attribute.
Objects such as modules and instances have an updateable :attr:`~object.__dict__`
attribute; however, other objects may have write restrictions on their
:attr:`!__dict__` attributes (for example, classes use a
:class:`types.MappingProxyType` to prevent direct dictionary updates).
Without an argument, :func:`vars` acts like :func:`locals`.
A :exc:`TypeError` exception is raised if an object is specified but
it doesn't have a :attr:`~object.__dict__` attribute (for example, if
its class defines the :attr:`~object.__slots__` attribute).
.. versionchanged:: 3.13
The result of calling this function without an argument has been
updated as described for the :func:`locals` builtin.
.. function:: zip(*iterables, strict=False)
Iterate over several iterables in parallel, producing tuples with an item
from each one.
Example::
>>> for item in zip([1, 2, 3], ['sugar', 'spice', 'everything nice']):
... print(item)
...
(1, 'sugar')
(2, 'spice')
(3, 'everything nice')
More formally: :func:`zip` returns an iterator of tuples, where the *i*-th
tuple contains the *i*-th element from each of the argument iterables.
Another way to think of :func:`zip` is that it turns rows into columns, and
columns into rows. This is similar to `transposing a matrix
<https://en.wikipedia.org/wiki/Transpose>`_.
:func:`zip` is lazy: The elements won't be processed until the iterable is
iterated on, e.g. by a :keyword:`!for` loop or by wrapping in a
:class:`list`.
One thing to consider is that the iterables passed to :func:`zip` could have
different lengths; sometimes by design, and sometimes because of a bug in
the code that prepared these iterables. Python offers three different
approaches to dealing with this issue:
* By default, :func:`zip` stops when the shortest iterable is exhausted.
It will ignore the remaining items in the longer iterables, cutting off
the result to the length of the shortest iterable::
>>> list(zip(range(3), ['fee', 'fi', 'fo', 'fum']))
[(0, 'fee'), (1, 'fi'), (2, 'fo')]
* :func:`zip` is often used in cases where the iterables are assumed to be
of equal length. In such cases, it's recommended to use the ``strict=True``
option. Its output is the same as regular :func:`zip`::
>>> list(zip(('a', 'b', 'c'), (1, 2, 3), strict=True))
[('a', 1), ('b', 2), ('c', 3)]
Unlike the default behavior, it raises a :exc:`ValueError` if one iterable
is exhausted before the others:
>>> for item in zip(range(3), ['fee', 'fi', 'fo', 'fum'], strict=True): # doctest: +SKIP
... print(item)
...
(0, 'fee')
(1, 'fi')
(2, 'fo')
Traceback (most recent call last):
...
ValueError: zip() argument 2 is longer than argument 1
..
This doctest is disabled because doctest does not support capturing
output and exceptions in the same code unit.
https://github.com/python/cpython/issues/65382
Without the ``strict=True`` argument, any bug that results in iterables of
different lengths will be silenced, possibly manifesting as a hard-to-find
bug in another part of the program.
* Shorter iterables can be padded with a constant value to make all the
iterables have the same length. This is done by
:func:`itertools.zip_longest`.
Edge cases: With a single iterable argument, :func:`zip` returns an
iterator of 1-tuples. With no arguments, it returns an empty iterator.
Tips and tricks:
* The left-to-right evaluation order of the iterables is guaranteed. This
makes possible an idiom for clustering a data series into n-length groups
using ``zip(*[iter(s)]*n, strict=True)``. This repeats the *same* iterator
``n`` times so that each output tuple has the result of ``n`` calls to the
iterator. This has the effect of dividing the input into n-length chunks.
* :func:`zip` in conjunction with the ``*`` operator can be used to unzip a
list::
>>> x = [1, 2, 3]
>>> y = [4, 5, 6]
>>> list(zip(x, y))
[(1, 4), (2, 5), (3, 6)]
>>> x2, y2 = zip(*zip(x, y))
>>> x == list(x2) and y == list(y2)
True
.. versionchanged:: 3.10
Added the ``strict`` argument.
.. function:: __import__(name, globals=None, locals=None, fromlist=(), level=0)
.. index::
pair: statement; import
pair: module; builtins
.. note::
This is an advanced function that is not needed in everyday Python
programming, unlike :func:`importlib.import_module`.
This function is invoked by the :keyword:`import` statement. It can be
replaced (by importing the :mod:`builtins` module and assigning to
``builtins.__import__``) in order to change semantics of the
:keyword:`!import` statement, but doing so is **strongly** discouraged as it
is usually simpler to use import hooks (see :pep:`302`) to attain the same
goals and does not cause issues with code which assumes the default import
implementation is in use. Direct use of :func:`__import__` is also
discouraged in favor of :func:`importlib.import_module`.
The function imports the module *name*, potentially using the given *globals*
and *locals* to determine how to interpret the name in a package context.
The *fromlist* gives the names of objects or submodules that should be
imported from the module given by *name*. The standard implementation does
not use its *locals* argument at all and uses its *globals* only to
determine the package context of the :keyword:`import` statement.
*level* specifies whether to use absolute or relative imports. ``0`` (the
default) means only perform absolute imports. Positive values for
*level* indicate the number of parent directories to search relative to the
directory of the module calling :func:`__import__` (see :pep:`328` for the
details).
When the *name* variable is of the form ``package.module``, normally, the
top-level package (the name up till the first dot) is returned, *not* the
module named by *name*. However, when a non-empty *fromlist* argument is
given, the module named by *name* is returned.
For example, the statement ``import spam`` results in bytecode resembling the
following code::
spam = __import__('spam', globals(), locals(), [], 0)
The statement ``import spam.ham`` results in this call::
spam = __import__('spam.ham', globals(), locals(), [], 0)
Note how :func:`__import__` returns the toplevel module here because this is
the object that is bound to a name by the :keyword:`import` statement.
On the other hand, the statement ``from spam.ham import eggs, sausage as
saus`` results in ::
_temp = __import__('spam.ham', globals(), locals(), ['eggs', 'sausage'], 0)
eggs = _temp.eggs
saus = _temp.sausage
Here, the ``spam.ham`` module is returned from :func:`__import__`. From this
object, the names to import are retrieved and assigned to their respective
names.
If you simply want to import a module (potentially within a package) by name,
use :func:`importlib.import_module`.
.. versionchanged:: 3.3
Negative values for *level* are no longer supported (which also changes
the default value to 0).
.. versionchanged:: 3.9
When the command line options :option:`-E` or :option:`-I` are being used,
the environment variable :envvar:`PYTHONCASEOK` is now ignored.
.. rubric:: Footnotes
.. [#] Note that the parser only accepts the Unix-style end of line convention.
If you are reading the code from a file, make sure to use newline conversion
mode to convert Windows or Mac-style newlines.
Reference in New Issue View Git Blame Copy Permalink