Constructor functions are used to create objects of a given type.
Note
In Python, the type is a constructor function; there's no difference
at all in Python. So you can use the type function, which we
will discuss momentarily, to look up the type of an object, then make
instances of that same type.
First we will look at the constructor functions, which are more
typically used for conversion. This is because there is generally a
convenient literal syntax available, or in the case of bool, there
are only two such constants, True and False.
.. 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`. If no argument is given, this function returns :const:`False`.
.. function:: chr(i) Return a string of one character whose ASCII code is the integer *i*. For example, ``chr(97)`` returns the string ``'a'``. This is the inverse of :func:`ord`. The argument must be in the range [0..255], inclusive; :exc:`ValueError` will be raised if *i* is outside that range. See also :func:`unichr`.
.. 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`, :func:`long` and :func:`float`. If both arguments are omitted, returns ``0j``.
.. 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.
Although there is a convenient literal for creating dict objects:
a_dict = { 'alpha' : 1, 'beta' : 2, 'gamma' : 3 }
It can be more convenient to create them using the dict function:
a_dict = dict(alpha=1, beta=2, gamma=3)
Of course in this latter case, the keys of the entries being created must be valid Python keywords.
.. function:: float([x]) Convert a string or a number to floating point. If the argument is a string, it must contain a possibly signed decimal or floating point number, possibly embedded in whitespace. The argument may also be [+|-]nan or [+|-]inf. Otherwise, the argument may be a plain or long integer or a floating point number, and a floating point number with the same value (within Python's floating point precision) is returned. If no argument is given, returns ``0.0``.
.. 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:: int([x[, radix]]) Convert a string or number to a plain integer. If the argument is a string, it must contain a possibly signed decimal number representable as a Python integer, possibly embedded in whitespace. The *radix* parameter gives the base for the conversion (which is 10 by default) and may be any integer in the range [2, 36], or zero. If *radix* is zero, the proper radix is determined based on the contents of string; the interpretation is the same as for integer literals. (See :ref:`numbers`.) If *radix* is specified and *x* is not a string, :exc:`TypeError` is raised. Otherwise, the argument may be a plain or long integer or a floating point number. Conversion of floating point numbers to integers truncates (towards zero). If the argument is outside the integer range a long object will be returned instead. If no arguments are given, returns ``0``. The integer type is described in :ref:`typesnumeric`.
.. function:: iter(o[, 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, *o* 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 *o* must be a callable object. The iterator created in this case will call *o* 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. .. versionadded:: 2.2
.. 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`. For other containers see the built in :class:`dict`,
:class:`set`, and :class:`tuple` classes, and the :mod:`collections` module.
.. function:: object()
Return a new featureless object. :class:`object` is a base for all new style
classes. It has the methods that are common to all instances of new style
classes.
.. versionadded:: 2.2
.. versionchanged:: 2.3
This function does not accept any arguments. Formerly, it accepted arguments but
ignored them.
.. function:: open(filename[, mode[, bufsize]])
Open a file, returning an object of the :class:`file` type described in
section :ref:`bltin-file-objects`. If the file cannot be opened,
:exc:`IOError` is raised. When opening a file, it's preferable to use
:func:`open` instead of invoking the :class:`file` constructor directly.
The first two arguments are the same as for ``stdio``'s :c:func:`fopen`:
*filename* is the file name to be opened, and *mode* is a string indicating how
the file is to be opened.
The most commonly-used values of *mode* are ``'r'`` for reading, ``'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). If *mode* is omitted, it
defaults to ``'r'``. The default is to use text mode, which may convert
``'\n'`` characters to a platform-specific representation on writing and back
on reading. Thus, when opening a binary file, you should append ``'b'`` to
the *mode* value to open the file in binary mode, which will improve
portability. (Appending ``'b'`` is useful even on systems that don't treat
binary and text files differently, where it serves as documentation.) See below
for more possible values of *mode*.
.. index::
single: line-buffered I/O
single: unbuffered I/O
single: buffer size, I/O
single: I/O control; buffering
The optional *bufsize* argument specifies the file's desired buffer size: 0
means unbuffered, 1 means line buffered, any other positive value means use a
buffer of (approximately) that size. A negative *bufsize* means to use the
system default, which is usually line buffered for tty devices and fully
buffered for other files. If omitted, the system default is used.
Modes ``'r+'``, ``'w+'`` and ``'a+'`` open the file for updating (note that
``'w+'`` truncates the file). Append ``'b'`` to the mode to open the file in
binary mode, on systems that differentiate between binary and text files; on
systems that don't have this distinction, adding the ``'b'`` has no effect.
In addition to the standard :c:func:`fopen` values *mode* may be ``'U'`` or
``'rU'``. Python is usually built with universal newline support; supplying
``'U'`` opens the file as a text file, but lines may be terminated by any of the
following: the Unix end-of-line convention ``'\n'``, the Macintosh convention
``'\r'``, or the Windows convention ``'\r\n'``. All of these external
representations are seen as ``'\n'`` by the Python program. If Python is built
without universal newline support a *mode* with ``'U'`` is the same as normal
text mode. Note that file objects so opened also have an attribute called
:attr:`newlines` which has a value of ``None`` (if no newlines have yet been
seen), ``'\n'``, ``'\r'``, ``'\r\n'``, or a tuple containing all the newline
types seen.
Python enforces that the mode, after stripping ``'U'``, begins with ``'r'``,
``'w'`` or ``'a'``.
Python provides many file handling modules including
:mod:`fileinput`, :mod:`os`, :mod:`os.path`, :mod:`tempfile`, and
:mod:`shutil`.
.. function:: ord(c)
Given a string of length one, return an integer representing the Unicode code
point of the character when the argument is a unicode object, or the value of
the byte when the argument is an 8-bit string. For example, ``ord('a')`` returns
the integer ``97``, ``ord(u'\u2020')`` returns ``8224``. This is the inverse of
:func:`chr` for 8-bit strings and of :func:`unichr` for unicode objects. If a
unicode argument is given and Python was built with UCS2 Unicode, then the
character's code point must be in the range [0..65535] inclusive; otherwise the
string length is two, and a :exc:`TypeError` will be raised.
.. function:: range([start,] stop[, step])
This is a versatile function to create lists containing arithmetic progressions.
It is most often used in :keyword:`for` loops.
However, we recommend the use of xrange instead.
The arguments must be plain
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 a list
of plain 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:
>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(1, 11)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> range(0, 30, 5)
[0, 5, 10, 15, 20, 25]
>>> range(0, 10, 3)
[0, 3, 6, 9]
>>> range(0, -10, -1)
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
>>> range(0)
[]
>>> range(1, 0)
[]
.. function:: set([iterable]) :noindex: Return a new set, optionally with elements are taken from *iterable*. The set 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. .. versionadded:: 2.4
.. function:: str([object]) Return a string containing a nicely printable representation of an object. For strings, this returns 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. If no argument is given, returns the empty string, ``''``. 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 use template strings or the ``%`` operator described in the :ref:`string-formatting` section. In addition see the :ref:`stringservices` section. See also :func:`unicode`.
.. 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`. For other containers see the built in :class:`dict`,
:class:`list`, and :class:`set` classes, and the :mod:`collections` module.
.. 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(object):
... a = 1
...
>>> X = type('X', (object,), dict(a=1))
.. versionadded:: 2.2
.. function:: unichr(i) Return the Unicode string of one character whose Unicode code is the integer *i*. For example, ``unichr(97)`` returns the string ``u'a'``. This is the inverse of :func:`ord` for Unicode strings. The valid range for the argument depends how Python was configured -- it may be either UCS2 [0..0xFFFF] or UCS4 [0..0x10FFFF]. :exc:`ValueError` is raised otherwise. For ASCII and 8-bit strings see :func:`chr`. .. versionadded:: 2.0
.. function:: unicode([object[, encoding [, errors]]]) Return the Unicode string version of *object* using one of the following modes: If *encoding* and/or *errors* are given, ``unicode()`` will decode the object which can either be an 8-bit 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. If no optional parameters are given, ``unicode()`` will mimic the behaviour of ``str()`` except that it returns Unicode strings instead of 8-bit strings. More precisely, if *object* is a Unicode string or subclass it will return that Unicode string without any additional decoding applied. For objects which provide a :meth:`__unicode__` method, it will call this method without arguments to create a Unicode string. For all other objects, the 8-bit string version or representation is requested and then converted to a Unicode string using the codec for the default encoding in ``'strict'`` mode. For more information on Unicode strings see :ref:`typesseq` which describes sequence functionality (Unicode strings are sequences), and also the string-specific methods described in the :ref:`string-methods` section. To output formatted strings use template strings or the ``%`` operator described in the :ref:`string-formatting` section. In addition see the :ref:`stringservices` section. See also :func:`str`.
Use as decorators: classmethod, staticmethod, property
slice is rarely used directly.
super type - 3 arg form compile
Most math functions are defined in math (or cmath for complex math). These are functions that are builtin:
abs, cmp, divmod, pow, round
You may need to use named functions
The next group of builtin functions operate on iterables, which in
Jython also includes all Java objects that implement the
java.util.Iterator interface.
In particular,
.. function:: enumerate(iterable)
.. function:: zip([,iterable, ...])
The zip function creates a list of tuples by stepping through each
iterable. One very common idiom is to use zip to create a
dict where one iterable has the keys, and the other the
values. This is often seen in working with CSV files (from a header
row) or database cursors (from the description
attribute). However, you might want to consider using
collections.namedtuple instead:
.. function:: sorted(iterable[, cmp[, key[, reverse]]])
The sorted function returns a sorted list. Use the optional key
argument to specify a key function to control how it's sorted. So for
example, this will sort the list by the length of the elements in it:
>>> sorted(['Massachusetts', 'Colorado', 'New York', 'California', 'Utah'], key=len) ['Utah', 'Colorado', 'New York', 'California', 'Massachusetts']
And this one will sort a list of Unicode strings without regard to it whether the characters are upper or lowercase:
>>> sorted(['apple', 'Cherry', 'banana']) ['Cherry', 'apple', 'banana'] >>> sorted(['apple', 'Cherry', 'banana'], key=str.upper) ['apple', 'banana', 'Cherry']
Although using a key function requires building a decorated version of the list to be sorted, in practice this uses substantially less overhead than calling a cmp function on every comparison. We recommend you take advantage of a keyed sort.
.. function:: all(iterable), any(iterable)
all and any will also short cut, if possible.
and sum(iterable[, start=0]) are functions that you will find frequent use for.
.. function:: max(iterable[, key]) or max([, arg, ...][, key]); min(iterable[, key]) or min([, arg, ...][, key])
The max and min functions
take a key function as an optional argument.
Although filter, map, and reduce are still useful, their
use is largely superseded by using other functions, in conjunction
with generator expressions. The range function is still useful for
creating a list of a given sequence, but for portability eventualy to
Python 3.x, using list(xrange()) instead is better.
Some advice:
- Generator expressions (or list comprehensions) are easier to use than
filter.- Most interesting but simple uses of
reducecan be implemented throughsum. And anything more complex should likely be written as a generator.
.. function:: all(iterable)
Returns True if all of the elements in the iterable are true, otherwise False and stop the iteration. (If the iterable is empty, this function returns True).
.. function:: any(iterable)
Returns True if any of the elements in the iterable are true, stopping the iteration. Otherwise returns False and stop the iteration. (If the iterable is empty, this function returns True).
.. function:: enumerate(iterable)
.. function:: filter(function, iterable)
.. function:: sum(iterable[, start=0])
namespace - __import__, delattr, dir, getattr, locals, globals, hasattr, reload, setattr, vars
getattr
Note
Java dynamic integration. the supporting special method for getattr is __getattr__. When Jython code is compiled, it actually uses __getattr__ for implementing attribute lookup. So x.y.z is actually compiled to the equivalent chain of x.__getattr__('y').__getattr__('z'). Alternatively for more efficient Java integration, __findattr__ is supported. It returns null instead of throwing an AttributeError if the attribute is not part of a given object. But use __getattr__ if you are going to be chaining method calls together so as to maintain Python exception handling semantics.
If the given Jython class implements a Java interface (or extends a Java class, but this is the less preferrable case in Jython as it is in Java in general), then Java code that uses such instances can statically bind method lookup.
[The Clamp project supports an alternate way of exposing Java interfaces, such that the interfaces are created from Jython code. I'm not so certain about this approach as a best practice however. Java interfaces in Java are quite precise with respect to interoperability. Other parts are useful, such as AOT compilation of Java proxies for Jython classes.]
compile, eval, exec Creating code objects.
evaluation - eval, execfile, predicates - callable, isinstance, issubclass hex, oct, id, hash, ord, repr len input, rawinput
Just refer to the documentation on these: deprecated functions - apply, buffer, coerce, intern ...
Operators
.. function:: abs(x) Return the absolute value of a number. The argument may be a plain or long 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. Equivalent to::
def all(iterable):
for element in iterable:
if not element:
return False
return True
.. versionadded:: 2.5
.. function:: any(iterable)
Return True if any element of the *iterable* is true. Equivalent to::
def any(iterable):
for element in iterable:
if element:
return True
return False
.. versionadded:: 2.5
.. function:: basestring() This abstract type is the superclass for :class:`str` and :class:`unicode`. It cannot be called or instantiated, but it can be used to test whether an object is an instance of :class:`str` or :class:`unicode`. ``isinstance(obj, basestring)`` is equivalent to ``isinstance(obj, (str, unicode))``. .. versionadded:: 2.3
.. 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. .. versionadded:: 2.6
.. 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); class instances are callable if they have a :meth:`__call__` method.
.. 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`.
.. versionadded:: 2.2
.. versionchanged:: 2.4
Function decorator syntax added.
.. function:: cmp(x, y) Compare the two objects *x* and *y* and return an integer according to the outcome. The return value is negative if ``x < y``, zero if ``x == y`` and strictly positive if ``x > y``.
.. function:: compile(source, filename, mode[, flags[, dont_inherit]])
Compile the *source* into a code or AST object. Code objects can be executed
by an :keyword:`exec` statement or evaluated by a call to :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 else 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 statements, line endings must be
represented by a single newline character (``'\n'``), and the input must
be terminated by at least one newline character. If line endings are
represented by ``'\r\n'``, use :meth:`str.replace` to change them into
``'\n'``.
.. versionchanged:: 2.3
The *flags* and *dont_inherit* arguments were added.
.. versionchanged:: 2.6
Support for compiling AST objects.
.. 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:: 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(object):
... 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 long division. With mixed
operand types, the rules for binary arithmetic operators apply. For plain and
long 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)``.
.. versionchanged:: 2.3
Using :func:`divmod` with complex numbers is deprecated.
.. function:: enumerate(sequence[, start=0])
Return an enumerate object. *sequence* 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
.. versionadded:: 2.3
.. versionadded:: 2.6
The *start* parameter.
.. function:: eval(expression[, globals[, locals]])
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.
.. versionchanged:: 2.4
formerly *locals* was required to be a dictionary.
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:`__builtin__` 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
>>> print 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
*kind* argument, :func:`eval`\'s return value will be ``None``.
Hints: dynamic execution of statements is supported by the :keyword:`exec`
statement. Execution of statements from a file is supported by the
:func:`execfile` 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:`execfile`.
.. function:: execfile(filename[, globals[, locals]])
This function is similar to the :keyword:`exec` statement, but parses a file
instead of a string. It is different from the :keyword:`import` statement in
that it does not use the module administration --- it reads the file
unconditionally and does not create a new module.
The arguments are a file name and two optional dictionaries. The file is parsed
and evaluated as a sequence of Python statements (similarly to a module) using
the *globals* and *locals* dictionaries as global and local namespace. If
provided, *locals* can be any mapping object.
.. versionchanged:: 2.4
formerly *locals* was required to be a dictionary.
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:`execfile` is called. The return value is ``None``.
.. warning::
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:`execfile` returns. :func:`execfile` cannot be
used reliably to modify a function's locals.
.. function:: file(filename[, mode[, bufsize]]) Constructor function for the :class:`file` type, described further in section :ref:`bltin-file-objects`. The constructor's arguments are the same as those of the :func:`open` built-in function described below. When opening a file, it's preferable to use :func:`open` instead of invoking this constructor directly. :class:`file` is more suited to type testing (for example, writing ``isinstance(f, file)``). .. versionadded:: 2.2
.. function:: filter(function, iterable) Construct a list 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 *iterable* is a string or a tuple, the result also has that type; otherwise it is always a list. 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 ``[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:: getattr(object, name[, default]) Return the value of the named attributed 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 exception 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. .. versionadded:: 2.2
.. function:: hex(x)
Convert an integer number (of any size) to a hexadecimal string. The result is a
valid Python expression.
.. versionchanged:: 2.4
Formerly only returned an unsigned literal.
.. function:: id(object) Return the "identity" of an object. This is an integer (or long 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. (Implementation note: this is the address of the object.)
.. function:: input([prompt])
Equivalent to ``eval(raw_input(prompt))``.
.. warning::
This function is not safe from user errors! It expects a valid Python
expression as input; if the input is not syntactically valid, a
:exc:`SyntaxError` will be raised. Other exceptions may be raised if there is an
error during evaluation. (On the other hand, sometimes this is exactly what you
need when writing a quick script for expert use.)
If the :mod:`readline` module was loaded, then :func:`input` will use it to
provide elaborate line editing and history features.
Consider using the :func:`raw_input` function for general input from users.
.. 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. Also return true if *classinfo*
is a type object (new-style class) and *object* is an object of that type or of
a (direct or indirect) subclass thereof. If *object* is not a class instance or
an object of the given type, the function always returns false. If *classinfo*
is neither a class object nor a type object, it may be a tuple of class or type
objects, or may recursively contain other such tuples (other sequence types are
not accepted). If *classinfo* is not a class, type, or tuple of classes, types,
and such tuples, a :exc:`TypeError` exception is raised.
.. versionchanged:: 2.2
Support for a tuple of type information was added.
.. 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.
.. versionchanged:: 2.3
Support for a tuple of type information was added.
.. 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:: locals()
Update and return a dictionary representing the current local symbol table.
.. warning::
The contents of this dictionary should not be modified; changes may not affect
the values of local variables used by the interpreter.
Free variables are returned by :func:`locals` when it is called in a function block.
Modifications of free variables may not affect the values used by the
interpreter. Free variables are not returned in class blocks.
.. function:: long([x[, radix]]) Convert a string or number to a long integer. If the argument is a string, it must contain a possibly signed number of arbitrary size, possibly embedded in whitespace. The *radix* argument is interpreted in the same way as for :func:`int`, and may only be given when *x* is a string. Otherwise, the argument may be a plain or long integer or a floating point number, and a long integer with the same value is returned. Conversion of floating point numbers to integers truncates (towards zero). If no arguments are given, returns ``0L``. The long type is described in :ref:`typesnumeric`.
.. function:: map(function, iterable, ...) Apply *function* to every item of *iterable* and return a list of 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. If one iterable is shorter than another it is assumed to be extended with ``None`` items. If *function* is ``None``, the identity function is assumed; if there are multiple arguments, :func:`map` returns a list consisting of tuples containing the corresponding items from all iterables (a kind of transpose operation). The *iterable* arguments may be a sequence or any iterable object; the result is always a list.
.. 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 *key* argument specifies a one-argument ordering function like that
used for :meth:`list.sort`. The *key* argument, if supplied, must be in keyword
form (for example, ``max(a,b,c,key=func)``).
.. versionchanged:: 2.5
Added support for the optional *key* argument.
.. 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 *key* argument specifies a one-argument ordering function like that
used for :meth:`list.sort`. The *key* argument, if supplied, must be in keyword
form (for example, ``min(a,b,c,key=func)``).
.. versionchanged:: 2.5
Added support for the optional *key* argument.
.. 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. .. versionadded:: 2.6
.. function:: oct(x)
Convert an integer number (of any size) to an octal string. The result is a
valid Python expression.
.. versionchanged:: 2.4
Formerly only returned an unsigned literal.
.. 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 int and long 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``. (This last feature was added in Python 2.2. In Python 2.1 and before, if both arguments were of integer types and the second argument was negative, an exception was raised.) 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. (This restriction was added in Python 2.2. In Python 2.1 and before, floating 3-argument ``pow()`` returned platform-dependent results depending on floating-point rounding accidents.)
.. 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.
.. note::
This function is not normally available as a builtin since the name
``print`` is recognized as the :keyword:`print` statement. To disable the
statement and use the :func:`print` function, use this future statement at
the top of your module::
from __future__ import print_function
.. versionadded:: 2.6
.. function:: property([fget[, fset[, fdel[, doc]]]])
Return a property attribute for :term:`new-style class`\es (classes that
derive from :class:`object`).
*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(object):
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 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(object):
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(object):
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.
.. versionadded:: 2.2
.. versionchanged:: 2.5
Use *fget*'s docstring if no *doc* given.
.. versionchanged:: 2.6
The ``getter``, ``setter``, and ``deleter`` attributes were added.
.. function:: raw_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 = raw_input('--> ')
--> Monty Python's Flying Circus
>>> s
"Monty Python's Flying Circus"
If the :mod:`readline` module was loaded, then :func:`raw_input` will use it to
provide elaborate line editing and history features.
.. function:: reduce(function, iterable[, initializer]) Apply *function* of two arguments cumulatively to the items of *iterable*, from left to right, so as to reduce the iterable to a single value. For example, ``reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])`` calculates ``((((1+2)+3)+4)+5)``. The left argument, *x*, is the accumulated value and the right argument, *y*, is the update value from the *iterable*. If the optional *initializer* is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If *initializer* is not given and *iterable* contains only one item, the first item is returned.
.. function:: reload(module)
Reload a previously imported *module*. The argument must be a module object, so
it must have been successfully imported before. This is useful if you have
edited the module source file using an external editor and want to try out the
new version without leaving the Python interpreter. The return value is the
module object (the same as the *module* argument).
When ``reload(module)`` is executed:
* Python modules' code is recompiled and the module-level code reexecuted,
defining a new set of objects which are bound to names in the module's
dictionary. The ``init`` function of extension modules is not called a second
time.
* As with all other objects in Python the old objects are only reclaimed after
their reference counts drop to zero.
* The names in the module namespace are updated to point to any new or changed
objects.
* Other references to the old objects (such as names external to the module) are
not rebound to refer to the new objects and must be updated in each namespace
where they occur if that is desired.
There are a number of other caveats:
If a module is syntactically correct but its initialization fails, the first
:keyword:`import` statement for it does not bind its name locally, but does
store a (partially initialized) module object in ``sys.modules``. To reload the
module you must first :keyword:`import` it again (this will bind the name to the
partially initialized module object) before you can :func:`reload` it.
When a module is reloaded, its dictionary (containing the module's global
variables) is retained. Redefinitions of names will override the old
definitions, so this is generally not a problem. If the new version of a module
does not define a name that was defined by the old version, the old definition
remains. This feature can be used to the module's advantage if it maintains a
global table or cache of objects --- with a :keyword:`try` statement it can test
for the table's presence and skip its initialization if desired::
try:
cache
except NameError:
cache = {}
It is legal though generally not very useful to reload built-in or dynamically
loaded modules, except for :mod:`sys`, :mod:`__main__` and :mod:`__builtin__`.
In many cases, however, extension modules are not designed to be initialized
more than once, and may fail in arbitrary ways when reloaded.
If a module imports objects from another module using :keyword:`from` ...
:keyword:`import` ..., calling :func:`reload` for the other module does not
redefine the objects imported from it --- one way around this is to re-execute
the :keyword:`from` statement, another is to use :keyword:`import` and qualified
names (*module*.*name*) instead.
If a module instantiates instances of a class, reloading the module that defines
the class does not affect the method definitions of the instances --- they
continue to use the old class definition. The same is true for derived classes.
.. function:: repr(object) Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. 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``).
.. versionadded:: 2.4
.. versionchanged:: 2.6
Added the possibility to write a custom :meth:`__reversed__` method.
.. 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. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minus *n*; if two multiples are equally close, rounding is done away from 0 (so. for example, ``round(0.5)`` is ``1.0`` and ``round(-0.5)`` is ``-1.0``).
.. 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[, cmp[, key[, reverse]]]) Return a new sorted list from the items in *iterable*. The optional arguments *cmp*, *key*, and *reverse* have the same meaning as those for the :meth:`list.sort` method (described in section :ref:`typesseq-mutable`). *cmp* specifies a custom comparison function of two arguments (iterable elements) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument: ``cmp=lambda x,y: cmp(x.lower(), y.lower())``. The default value is ``None``. *key* specifies a function of one argument that is used to extract a comparison key from each list element: ``key=str.lower``. The default value is ``None``. *reverse* is a boolean value. If set to ``True``, then the list elements are sorted as if each comparison were reversed. In general, the *key* and *reverse* conversion processes are much faster than specifying an equivalent *cmp* function. This is because *cmp* is called multiple times for each list element while *key* and *reverse* touch each element only once. To convert an old-style *cmp* function to a *key* function, see the `CmpToKey recipe in the ASPN cookbook <http://code.activestate.com/recipes/576653/>`_\. .. versionadded:: 2.4
.. 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`.
.. versionadded:: 2.2
.. versionchanged:: 2.4
Function decorator syntax added.
.. 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 are not allowed to be strings. The fast, correct way to concatenate a sequence of strings is by calling ``''.join(sequence)``. Note that ``sum(range(n), m)`` is equivalent to ``reduce(operator.add, range(n), m)`` To add floating point values with extended precision, see :func:`math.fsum`\. .. versionadded:: 2.3
.. 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).
.. note::
:func:`super` only works for :term:`new-style class`\es.
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(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.
.. versionadded:: 2.2
.. function:: type(object) .. index:: object: type Return the type of an *object*. The return value is a type object. The :func:`isinstance` built-in function is recommended for testing the type of an object. With three arguments, :func:`type` functions as a constructor as detailed below.
.. function:: vars([object])
Without arguments, return a dictionary corresponding to the current local symbol
table. With a module, class or class instance object as argument (or anything
else that has a :attr:`__dict__` attribute), returns a dictionary corresponding
to the object's symbol table.
.. warning::
The returned dictionary should not be modified:
the effects on the corresponding symbol table are undefined.
.. function:: xrange([start,] stop[, step])
This function is very similar to :func:`range`, but returns an "xrange object"
instead of a list. This is an opaque sequence type which yields the same values
as the corresponding list, without actually storing them all simultaneously.
The advantage of :func:`xrange` over :func:`range` is minimal (since
:func:`xrange` still has to create the values when asked for them) except when a
very large range is used on a memory-starved machine or when all of the range's
elements are never used (such as when the loop is usually terminated with
:keyword:`break`).
.. note::
:func:`xrange` is intended to be simple and fast. Implementations may impose
restrictions to achieve this. The C implementation of Python restricts all
arguments to native C longs ("short" Python integers), and also requires that
the number of elements fit in a native C long. If a larger range is needed,
an alternate version can be crafted using the :mod:`itertools` module:
``islice(count(start, step), (stop-start+step-1)//step)``.
.. function:: zip([iterable, ...])
This function returns a list of tuples, where the *i*-th tuple contains the
*i*-th element from each of the argument sequences or iterables. The returned
list is truncated in length to the length of the shortest argument sequence.
When there are multiple arguments which are all of the same length, :func:`zip`
is similar to :func:`map` with an initial argument of ``None``. With a single
sequence argument, it returns a list of 1-tuples. With no arguments, it returns
an empty list.
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` in conjunction with the ``*`` operator can be used to unzip a
list::
>>> x = [1, 2, 3]
>>> y = [4, 5, 6]
>>> zipped = zip(x, y)
>>> zipped
[(1, 4), (2, 5), (3, 6)]
>>> x2, y2 = zip(*zipped)
>>> x == x2, y == y2
True
.. versionadded:: 2.0
.. versionchanged:: 2.4
Formerly, :func:`zip` required at least one argument and ``zip()`` raised a
:exc:`TypeError` instead of returning an empty list.
.. function:: __import__(name[, globals[, locals[, fromlist[, level]]]])
.. 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. The default
is ``-1`` which indicates both absolute and relative imports will be
attempted. ``0`` 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(), [], -1)
The statement ``import spam.ham`` results in this call::
spam = __import__('spam.ham', globals(), locals(), [], -1)
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'], -1)
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 get it from :data:`sys.modules`::
>>> import sys
>>> name = 'foo.bar.baz'
>>> __import__(name)
<module 'foo' from ...>
>>> baz = sys.modules[name]
>>> baz
<module 'foo.bar.baz' from ...>
.. versionchanged:: 2.5
The level parameter was added.
.. versionchanged:: 2.5
Keyword support for parameters was added.