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.. 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.
.. function:: abs(x)
Return the absolute value of a number. The argument may be an
integer or a floating point number. If the argument is a complex number, its
magnitude is returned.
.. 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
.. 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. The result is a valid Python
expression. If *x* is not a Python :class:`int` object, it has to define an
:meth:`__index__` method that returns an integer.
.. function:: bool([x])
Convert a value to a Boolean, using the standard truth testing procedure. If
*x* is false or omitted, this returns :const:`False`; otherwise it returns
:const:`True`. :class:`bool` is also a class, which is a subclass of
:class:`int`. Class :class:`bool` cannot be subclassed further. Its only
instances are :const:`False` and :const:`True`.
.. index:: pair: Boolean; type
.. function:: bytearray([source[, encoding[, errors]]])
Return a new array of bytes. The :class:`bytearray` type 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 *buffer* interface, 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.
.. function:: bytes([source[, encoding[, errors]]])
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`.
.. function:: chr(i)
Return the string representing a character whose Unicode codepoint is the integer
*i*. For example, ``chr(97)`` returns the string ``'a'``. 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.
Note that on narrow Unicode builds, the result is a string of
length two for *i* greater than 65,535 (0xFFFF in hexadecimal).
.. function:: classmethod(function)
Return a class method for *function*.
A class method receives the class as 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 the description
of function definitions in :ref:`function` for details.
It 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, consult the documentation on the standard
type hierarchy in :ref:`types`.
.. function:: compile(source, filename, mode, flags=0, dont_inherit=False)
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 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 future
statements (see :pep:`236`) affect the compilation of *source*. If neither
is present (or both are zero) the code is compiled with those future
statements that are in effect in the code that is calling compile. If the
*flags* argument is given and *dont_inherit* is not (or is zero) then 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 future statements in effect around the call
to compile are ignored.
Future statements are specified by bits which can be bitwise ORed together to
specify multiple statements. The bitfield required to specify a given feature
can be found as the :attr:`compiler_flag` attribute on the :class:`_Feature`
instance in the :mod:`__future__` module.
This function raises :exc:`SyntaxError` if the compiled source is invalid,
and :exc:`TypeError` if the source contains null bytes.
.. 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.
.. versionchanged:: 3.2
Allowed use of Windows and Mac newlines. Also input in ``'exec'`` mode
does not have to end in a newline anymore.
.. function:: complex([real[, imag]])
Create a complex number with the value *real* + *imag*\*j or convert a string or
number to a complex number. If the first parameter is a string, it will be
interpreted as a complex number and the function must be called without a second
parameter. The second parameter can never be a string. Each argument may be any
numeric type (including complex). If *imag* is omitted, it defaults to zero and
the function serves as a numeric conversion function like :func:`int`
and :func:`float`. If both arguments are omitted, returns ``0j``.
The complex type is described in :ref:`typesnumeric`.
.. 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``.
.. function:: dict([arg])
:noindex:
Create a new data dictionary, optionally with items taken from *arg*.
The dictionary type is described in :ref:`typesmapping`.
For other containers see the built in :class:`list`, :class:`set`, and
:class:`tuple` classes, and the :mod:`collections` module.
.. function:: 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:`__dir__`, this method will be called and
must return the list of attributes. This allows objects that implement a custom
:func:`__getattr__` or :func:`__getattribute__` function to customize the way
:func:`dir` reports their attributes.
If the object does not provide :meth:`__dir__`, the function tries its best to
gather information from the object's :attr:`__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:`__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() # doctest: +SKIP
['__builtins__', '__doc__', '__name__', 'struct']
>>> dir(struct) # doctest: +NORMALIZE_WHITESPACE
['Struct', '__builtins__', '__doc__', '__file__', '__name__',
'__package__', '_clearcache', 'calcsize', 'error', 'pack', 'pack_into',
'unpack', 'unpack_from']
>>> class Foo:
... def __dir__(self):
... return ["kan", "ga", "roo"]
...
>>> f = Foo()
>>> dir(f)
['ga', 'kan', 'roo']
.. 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:`__next__` method of the iterator returned by :func:`enumerate` returns a
tuple containing a count (from *start* which defaults to 0) and the
corresponding value obtained from iterating over *iterable*.
:func:`enumerate` is useful for obtaining an indexed series: ``(0, seq[0])``,
``(1, seq[1])``, ``(2, seq[2])``, .... For example:
>>> for i, season in enumerate(['Spring', 'Summer', 'Fall', 'Winter']):
... print(i, season)
0 Spring
1 Summer
2 Fall
3 Winter
.. function:: eval(expression, globals=None, locals=None)
The arguments are a string and optional globals and locals. If provided,
*globals* must be a dictionary. If provided, *locals* can be any mapping
object.
The *expression* argument is parsed and evaluated as a Python expression
(technically speaking, a condition list) using the *globals* and *locals*
dictionaries as global and local namespace. If the *globals* dictionary is
present and lacks '__builtins__', the current globals are copied into *globals*
before *expression* is parsed. This means that *expression* normally has full
access to the standard :mod:`builtins` module and restricted environments are
propagated. If the *locals* dictionary is omitted it defaults to the *globals*
dictionary. If both dictionaries are omitted, the expression is executed in the
environment where :func:`eval` is called. The return value is the result of
the evaluated expression. Syntax errors are reported as exceptions. 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
returns the current global and local dictionary, respectively, which may be
useful to pass around for use by :func:`eval` or :func:`exec`.
See :func:`ast.literal_eval` for a function that can safely evaluate strings
with expressions containing only literals.
.. function:: exec(object[, globals[, locals]])
This function supports dynamic execution of Python code. *object* 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 "File input" in the Reference Manual). Be aware that the
:keyword:`return` and :keyword:`yield` 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, 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.
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`.
.. note::
The built-in functions :func:`globals` and :func:`locals` return the current
global and local dictionary, 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:
modifications to the default *locals* dictionary should not be attempted.
Pass an explicit *locals* dictionary if you need to see effects of the
code on *locals* after function :func:`exec` returns.
.. function:: filter(function, iterable)
Construct an iterator from those elements of *iterable* for which *function*
returns 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* returns false.
.. function:: float([x])
.. index::
single: NaN
single: Infinity
Convert a string or a number to floating point.
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 a positive or negative infinity. More precisely, the
input must conform to the following grammar after leading and trailing
whitespace characters are removed:
.. productionlist::
sign: "+" | "-"
infinity: "Infinity" | "inf"
nan: "nan"
numeric-value: `floatnumber` | `infinity` | `nan`
numeric-string: [`sign`] `numeric-value`
Here ``floatnumber`` is the form of a Python floating-point literal,
described in :ref:`floating`. 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 no argument is given, ``0.0`` is returned.
Examples::
>>> float('+1.23')
1.23
>>> float(' -12345\n')
-12345.0
>>> float('1e-003')
0.001
>>> float('+1E6')
1000000.0
>>> float('-Infinity')
-inf
The float type is described in :ref:`typesnumeric`.
.. function:: format(value[, format_spec])
.. index::
pair: str; format
single: __format__
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`.
.. note::
``format(value, format_spec)`` merely calls
``value.__format__(format_spec)``.
.. function:: frozenset([iterable])
:noindex:
Return a frozenset object, optionally with elements taken from *iterable*.
The frozenset type is described in :ref:`types-set`.
For other containers see the built in :class:`dict`, :class:`list`, and
:class:`tuple` classes, and the :mod:`collections` module.
.. function:: 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.
.. function:: globals()
Return a dictionary representing the current global symbol table. This is always
the dictionary of the current module (inside a function or method, this is the
module where it is defined, not the module from which it 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).
.. function:: help([object])
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.
This function is added to the built-in namespace by the :mod:`site` module.
.. function:: hex(x)
Convert an integer number to a hexadecimal string. The result is a valid Python
expression. If *x* is not a Python :class:`int` object, it has to define an
:meth:`__index__` method that returns an integer.
.. 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.
.. function:: 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('--> ')
--> Monty Python's Flying Circus
>>> s
"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.
.. function:: int([number | string[, base]])
Convert a number or string to an integer. If no arguments are given, return
``0``. If a number is given, return ``number.__int__()``. Conversion of
floating point numbers to integers truncates towards zero. A string must be
a base-radix integer literal optionally preceded by '+' or '-' (with no space
in between) and optionally surrounded by whitespace. A base-n literal
consists of the digits 0 to n-1, with 'a' to 'z' (or 'A' to 'Z') having
values 10 to 35. The default *base* is 10. The allowed values are 0 and 2-36.
Base-2, -8, and -16 literals can be optionally prefixed with ``0b``/``0B``,
``0o``/``0O``, or ``0x``/``0X``, as with integer literals in code. Base 0
means to interpret exactly as a code literal, so that the actual base is 2,
8, 10, or 16, and so that ``int('010', 0)`` is not legal, while
``int('010')`` is, as well as ``int('010', 8)``.
The integer type is described in :ref:`typesnumeric`.
.. function:: isinstance(object, classinfo)
Return true if the *object* argument is an instance of the *classinfo*
argument, or of a (direct or indirect) subclass thereof. If *object* is not
an object of the given type, the function always returns false. If
*classinfo* is not a class (type object), it may be a tuple of type objects,
or may recursively contain other such tuples (other sequence types are not
accepted). If *classinfo* is not a type or tuple of types and such tuples,
a :exc:`TypeError` exception is raised.
.. function:: issubclass(class, classinfo)
Return true if *class* is a subclass (direct or indirect) of *classinfo*. A
class is considered a subclass of itself. *classinfo* may be a tuple of class
objects, in which case every entry in *classinfo* will be checked. In any other
case, a :exc:`TypeError` exception is raised.
.. function:: 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
iteration protocol (the :meth:`__iter__` method), or it must support the
sequence protocol (the :meth:`__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:`__next__`
method; if the value returned is equal to *sentinel*, :exc:`StopIteration`
will be raised, otherwise the value will be returned.
One useful application of the second form of :func:`iter` is to read lines of
a file until a certain line is reached. The following example reads a file
until ``"STOP"`` is reached: ::
with open("mydata.txt") as fp:
for line in iter(fp.readline, "STOP"):
process_line(line)
.. function:: len(s)
Return the length (the number of items) of an object. The argument may be a
sequence (string, tuple or list) or a mapping (dictionary).
.. function:: list([iterable])
Return a list whose items are the same and in the same order as *iterable*'s
items. *iterable* may be either a sequence, a container that supports
iteration, or an iterator object. If *iterable* is already a list, a copy is
made and returned, similar to ``iterable[:]``. For instance, ``list('abc')``
returns ``['a', 'b', 'c']`` and ``list( (1, 2, 3) )`` returns ``[1, 2, 3]``.
If no argument is given, returns a new empty list, ``[]``.
:class:`list` is a mutable sequence type, as documented in :ref:`typesseq`.
.. function:: locals()
Update and return a dictionary representing the current local symbol table.
Free variables are returned by :func:`locals` when it is called in function
blocks, but not in class blocks.
.. note::
The contents of this dictionary should not be modified; changes may not
affect the values of local and free variables used by the interpreter.
.. function:: map(function, iterable, ...)
Return an iterator that applies *function* to every item of *iterable*,
yielding the results. If additional *iterable* 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[, args...], *[, key])
With a single argument *iterable*, return the largest item of a non-empty
iterable (such as a string, tuple or list). With more than one argument, return
the largest of the arguments.
The optional keyword-only *key* argument specifies a one-argument ordering
function like that used for :meth:`list.sort`.
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)``.
.. function:: memoryview(obj)
:noindex:
Return a "memory view" object created from the given argument. See
:ref:`typememoryview` for more information.
.. function:: min(iterable[, args...], *[, key])
With a single argument *iterable*, return the smallest item of a non-empty
iterable (such as a string, tuple or list). With more than one argument, return
the smallest of the arguments.
The optional keyword-only *key* argument specifies a one-argument ordering
function like that used for :meth:`list.sort`.
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)``.
.. function:: next(iterator[, default])
Retrieve the next item from the *iterator* by calling its :meth:`__next__`
method. If *default* is given, it is returned if the iterator is exhausted,
otherwise :exc:`StopIteration` is raised.
.. function:: object()
Return a new featureless object. :class:`object` is a base for all classes.
It has the methods that are common to all instances of Python classes. This
function does not accept any arguments.
.. note::
:class:`object` does *not* have a :attr:`__dict__`, so you can't assign
arbitrary attributes to an instance of the :class:`object` class.
.. function:: oct(x)
Convert an integer number to an octal string. The result is a valid Python
expression. If *x* is not a Python :class:`int` object, it has to define an
:meth:`__index__` method that returns an integer.
.. function:: open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True)
Open *file* and return a corresponding stream. If the file cannot be opened,
an :exc:`IOError` is raised.
*file* is either a string or bytes 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), 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. (For reading and writing raw bytes use
binary mode and leave *encoding* unspecified.) The available modes are:
========= ===============================================================
Character Meaning
--------- ---------------------------------------------------------------
``'r'`` open for reading (default)
``'w'`` open for writing, truncating the file first
``'a'`` open for writing, appending to the end of the file if it exists
``'b'`` binary mode
``'t'`` text mode (default)
``'+'`` open a disk file for updating (reading and writing)
``'U'`` universal newline mode (for backwards compatibility; should
not be used in new code)
========= ===============================================================
The default mode is ``'r'`` (open for reading text, synonym of ``'rt'``).
For binary read-write access, the mode ``'w+b'`` opens and truncates the file
to 0 bytes. ``'r+b'`` opens the file without 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 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 in text mode), and an integer > 1 to indicate the size
of a fixed-size chunk buffer. 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 :attr:`io.DEFAULT_BUFFER_SIZE`. On many systems,
the buffer will typically be 4096 or 8192 bytes long.
* "Interactive" text files (files for which :meth:`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.getpreferredencoding` returns), but any
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. Pass
``'strict'`` to raise a :exc:`ValueError` exception if there is an encoding
error (the default of ``None`` has the same effect), or pass ``'ignore'`` to
ignore 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. When writing, ``'xmlcharrefreplace'``
(replace with the appropriate XML character reference) or
``'backslashreplace'`` (replace with backslashed escape sequences) can be
used. Any other error handling name that has been registered with
:func:`codecs.register_error` is also valid.
*newline* controls how universal newlines works (it only applies to text
mode). It can be ``None``, ``''``, ``'\n'``, ``'\r'``, and ``'\r\n'``. It
works as follows:
* On input, 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 newline 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.
* On output, if *newline* is ``None``, any ``'\n'`` characters written are
translated to the system default line separator, :data:`os.linesep`. If
*newline* is ``''``, 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* has no effect and must be ``True``
(the default).
The type of 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 a :class:`io.BufferedReader`; in write binary and
append binary modes, it returns a :class:`io.BufferedWriter`, and in
read/write mode, it returns a :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
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`.
.. XXX works for bytes too, but should it?
.. function:: ord(c)
Given a string representing one Uncicode character, return an integer
representing the Unicode code
point of that character. For example, ``ord('a')`` returns the integer ``97``
and ``ord('\u2020')`` returns ``8224``. This is the inverse of :func:`chr`.
On wide Unicode builds, if the argument length is not one, a
:exc:`TypeError` will be raised. On narrow Unicode builds, strings
of length two are accepted when they form a UTF-16 surrogate pair.
.. function:: pow(x, y[, z])
Return *x* to the power *y*; if *z* is present, return *x* to the power *y*,
modulo *z* (computed more efficiently than ``pow(x, y) % z``). The two-argument
form ``pow(x, y)`` is equivalent to using the power operator: ``x**y``.
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, ``10**2``
returns ``100``, but ``10**-2`` returns ``0.01``. If the second argument is
negative, the third argument must be omitted. If *z* is present, *x* and *y*
must be of integer types, and *y* must be non-negative.
.. function:: print([object, ...], *, sep=' ', end='\\n', file=sys.stdout)
Print *object*\(s) to the stream *file*, separated by *sep* and followed by
*end*. *sep*, *end* and *file*, 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 *object* is 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.
.. function:: property(fget=None, fset=None, fdel=None, doc=None)
Return a property attribute.
*fget* is a function for getting an attribute value, likewise *fset* is a
function for setting, and *fdel* a function for del'ing, an attribute. 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 then *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
turns the :meth:`voltage` method into a "getter" for a read-only attribute
with the same name.
A property object has :attr:`getter`, :attr:`setter`, and :attr:`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::
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 also has the attributes ``fget``, ``fset``, and
``fdel`` corresponding to the constructor arguments.
.. XXX does accept objects with __index__ too
.. function:: range([start,] stop[, step])
This is a versatile function to create iterables yielding arithmetic
progressions. It is most often used in :keyword:`for` loops. The arguments
must be integers. If the *step* argument is omitted, it defaults to ``1``.
If the *start* argument is omitted, it defaults to ``0``. The full form
returns an iterable of integers ``[start, start + step, start + 2 * step,
...]``. If *step* is positive, the last element is the largest ``start + i *
step`` less than *stop*; if *step* is negative, the last element is the
smallest ``start + i * step`` greater than *stop*. *step* must not be zero
(or else :exc:`ValueError` is raised). Example:
>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(range(1, 11))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> list(range(0, 30, 5))
[0, 5, 10, 15, 20, 25]
>>> list(range(0, 10, 3))
[0, 3, 6, 9]
>>> list(range(0, -10, -1))
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
>>> list(range(0))
[]
>>> list(range(1, 0))
[]
.. versionchanged:: 3.2
Testing integers for membership takes constant time instead of iterating
through all items.
.. 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:`__repr__` method.
.. function:: reversed(seq)
Return a reverse :term:`iterator`. *seq* must be an object which has
a :meth:`__reversed__` method or supports the sequence protocol (the
:meth:`__len__` method and the :meth:`__getitem__` method with integer
arguments starting at ``0``).
.. function:: round(x[, n])
Return the floating point value *x* rounded to *n* digits after the decimal
point. If *n* is omitted, it defaults to zero. Delegates to
``x.__round__(n)``.
For the built-in types supporting :func:`round`, values are rounded to the
closest multiple of 10 to the power minus *n*; 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``).
The return value is an integer if called with one argument, otherwise of the
same type as *x*.
.. 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.
.. function:: set([iterable])
:noindex:
Return a new set, optionally with elements taken from *iterable*.
The set type is described in :ref:`types-set`.
.. 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``.
.. function:: slice([start,] stop[, step])
.. index:: single: Numerical Python
Return a :term:`slice` object representing the set of indices specified by
``range(start, stop, step)``. The *start* and *step* arguments default to
``None``. 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
Numerical Python and other third party extensions. 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 iterator.
.. function:: sorted(iterable[, key][, reverse])
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 list element: ``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.
For sorting examples and a brief sorting tutorial, see `Sorting HowTo
<http://wiki.python.org/moin/HowTo/Sorting/>`_\.
.. function:: staticmethod(function)
Return a static method for *function*.
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, ...): ...
The ``@staticmethod`` form is a function :term:`decorator` -- see the
description of function definitions in :ref:`function` for details.
It 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.
Static methods in Python are similar to those found in Java or C++. For a more
advanced concept, see :func:`classmethod` in this section.
For more information on static methods, consult the documentation on the
standard type hierarchy in :ref:`types`.
.. function:: str([object[, encoding[, errors]]])
Return a string version of an object, using one of the following modes:
If *encoding* and/or *errors* are given, :func:`str` will decode the
*object* which can either be a byte string or a character buffer using
the codec for *encoding*. The *encoding* parameter is a string giving
the name of an encoding; if the encoding is not known, :exc:`LookupError`
is raised. Error handling is done according to *errors*; this specifies the
treatment of characters which are invalid in the input encoding. If
*errors* is ``'strict'`` (the default), a :exc:`ValueError` is raised on
errors, while a value of ``'ignore'`` causes errors to be silently ignored,
and a value of ``'replace'`` causes the official Unicode replacement character,
U+FFFD, to be used to replace input characters which cannot be decoded.
See also the :mod:`codecs` module.
When only *object* is given, this returns its nicely printable representation.
For strings, this is the string itself. The difference with ``repr(object)``
is that ``str(object)`` does not always attempt to return a string that is
acceptable to :func:`eval`; its goal is to return a printable string.
With no arguments, this returns the empty string.
Objects can specify what ``str(object)`` returns by defining a :meth:`__str__`
special method.
For more information on strings see :ref:`typesseq` which describes sequence
functionality (strings are sequences), and also the string-specific methods
described in the :ref:`string-methods` section. To output formatted strings,
see the :ref:`string-formatting` section. In addition see the
:ref:`stringservices` section.
.. function:: sum(iterable[, start])
Sums *start* and the items of an *iterable* from left to right and returns the
total. *start* defaults to ``0``. 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`.
.. function:: super([type[, object-or-type]])
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 search order is same as that used by
:func:`getattr` except that the *type* itself is skipped.
The :attr:`__mro__` attribute of the *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).
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 this method 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)
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:`__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 :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 automatically searches the stack frame
for the class (``__class__``) and the first argument.
.. function:: tuple([iterable])
Return a tuple whose items are the same and in the same order as *iterable*'s
items. *iterable* may be a sequence, a container that supports iteration, or an
iterator object. If *iterable* is already a tuple, it is returned unchanged.
For instance, ``tuple('abc')`` returns ``('a', 'b', 'c')`` and ``tuple([1, 2,
3])`` returns ``(1, 2, 3)``. If no argument is given, returns a new empty
tuple, ``()``.
:class:`tuple` is an immutable sequence type, as documented in :ref:`typesseq`.
.. function:: type(object)
.. index:: object: type
Return the type of an *object*. The return value is a type object and
generally the same object as returned by ``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, :func:`type` functions as a constructor as detailed
below.
.. function:: type(name, bases, dict)
:noindex:
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:`__name__` attribute; the *bases* tuple itemizes the base classes and
becomes the :attr:`__bases__` attribute; and the *dict* dictionary is the
namespace containing definitions for class body and becomes the :attr:`__dict__`
attribute. For example, the following two statements create identical
:class:`type` objects:
>>> class X:
... a = 1
...
>>> X = type('X', (object,), dict(a=1))
.. function:: vars([object])
Without an argument, act like :func:`locals`.
With a module, class or class instance object as argument (or anything else that
has a :attr:`__dict__` attribute), return that attribute.
.. note::
The returned dictionary should not be modified:
the effects on the corresponding symbol table are undefined. [#]_
.. function:: zip(*iterables)
Make an iterator that aggregates elements from each of the iterables.
Returns an iterator of tuples, where the *i*-th tuple contains
the *i*-th element from each of the argument sequences or iterables. The
iterator stops when the shortest input iterable is exhausted. With a single
iterable argument, it returns an iterator of 1-tuples. With no arguments,
it returns an empty iterator. Equivalent to::
def zip(*iterables):
# zip('ABCD', 'xy') --> Ax By
sentinel = object()
iterables = [iter(it) for it in iterables]
while iterables:
result = []
for it in iterables:
elem = next(it, sentinel)
if elem is sentinel:
return
result.append(elem)
yield tuple(result)
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)``.
:func:`zip` should only be used with unequal length inputs when you don't
care about trailing, unmatched values from the longer iterables. If those
values are important, use :func:`itertools.zip_longest` instead.
:func:`zip` in conjunction with the ``*`` operator can be used to unzip a
list::
>>> x = [1, 2, 3]
>>> y = [4, 5, 6]
>>> zipped = zip(x, y)
>>> list(zipped)
[(1, 4), (2, 5), (3, 6)]
>>> x2, y2 = zip(*zip(x, y))
>>> x == list(x2) and y == list(y2)
True
.. function:: __import__(name, globals={}, locals={}, fromlist=[], level=0)
.. index::
statement: import
module: imp
.. note::
This is an advanced function that is not needed in everyday Python
programming.
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 nowadays it is usually simpler to use import
hooks (see :pep:`302`). Direct use of :func:`__import__` is rare, except in
cases where you want to import a module whose name is only known at runtime.
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__`.
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,
you can call :func:`__import__` and then look it up in :data:`sys.modules`::
>>> import sys
>>> name = 'foo.bar.baz'
>>> __import__(name)
<module 'foo' from ...>
>>> baz = sys.modules[name]
>>> baz
<module 'foo.bar.baz' from ...>
.. 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.
.. [#] In the current implementation, local variable bindings cannot normally be
affected this way, but variables retrieved from other scopes (such as modules)
can be. This may change.
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