:mod:`functools` --- Higher order functions and operations on callable objects

==============================================================================

.. module:: functools

   :synopsis: Higher order functions and operations on callable objects.

.. moduleauthor:: Peter Harris <scav@blueyonder.co.uk>

.. moduleauthor:: Raymond Hettinger <python@rcn.com>

.. moduleauthor:: Nick Coghlan <ncoghlan@gmail.com>

.. sectionauthor:: Peter Harris <scav@blueyonder.co.uk>

.. versionadded:: 2.5

The :mod:`functools` module is for higher-order functions: functions that act on

or return other functions. In general, any callable object can be treated as a

function for the purposes of this module.

The :mod:`functools` module defines the following functions:

.. function:: reduce(function, iterable[, initializer])

   This is the same function as :func:`reduce`.  It is made available in this module

   to allow writing code more forward-compatible with Python 3.

   .. versionadded:: 2.6

.. function:: partial(func[,*args][, **keywords])

   Return a new :class:`partial` object which when called will behave like *func*

   called with the positional arguments *args* and keyword arguments *keywords*. If

   more arguments are supplied to the call, they are appended to *args*. If

   additional keyword arguments are supplied, they extend and override *keywords*.

   Roughly equivalent to::

      def partial(func, *args, **keywords):

          def newfunc(*fargs, **fkeywords):

              newkeywords = keywords.copy()

              newkeywords.update(fkeywords)

              return func(*(args + fargs), **newkeywords)

          newfunc.func = func

          newfunc.args = args

          newfunc.keywords = keywords

          return newfunc

   The :func:`partial` is used for partial function application which "freezes"

   some portion of a function's arguments and/or keywords resulting in a new object

   with a simplified signature.  For example, :func:`partial` can be used to create

   a callable that behaves like the :func:`int` function where the *base* argument

   defaults to two:

      >>> from functools import partial

      >>> basetwo = partial(int, base=2)

      >>> basetwo.__doc__ = 'Convert base 2 string to an int.'

      >>> basetwo('10010')

      18

.. function:: update_wrapper(wrapper, wrapped[, assigned][, updated])

   Update a *wrapper* function to look like the *wrapped* function. The optional

   arguments are tuples to specify which attributes of the original function are

   assigned directly to the matching attributes on the wrapper function and which

   attributes of the wrapper function are updated with the corresponding attributes

   from the original function. The default values for these arguments are the

   module level constants *WRAPPER_ASSIGNMENTS* (which assigns to the wrapper

   function's *__name__*, *__module__* and *__doc__*, the documentation string) and

   *WRAPPER_UPDATES* (which updates the wrapper function's *__dict__*, i.e. the

   instance dictionary).

   The main intended use for this function is in :term:`decorator` functions which

   wrap the decorated function and return the wrapper. If the wrapper function is

   not updated, the metadata of the returned function will reflect the wrapper

   definition rather than the original function definition, which is typically less

   than helpful.

.. function:: wraps(wrapped[, assigned][, updated])

   This is a convenience function for invoking ``partial(update_wrapper,

   wrapped=wrapped, assigned=assigned, updated=updated)`` as a function decorator

   when defining a wrapper function. For example:

      >>> from functools import wraps

      >>> def my_decorator(f):

      ...     @wraps(f)

      ...     def wrapper(*args, **kwds):

      ...         print 'Calling decorated function'

      ...         return f(*args, **kwds)

      ...     return wrapper

      ...

      >>> @my_decorator

      ... def example():

      ...     """Docstring"""

      ...     print 'Called example function'

      ...

      >>> example()

      Calling decorated function

      Called example function

      >>> example.__name__

      'example'

      >>> example.__doc__

      'Docstring'

   Without the use of this decorator factory, the name of the example function

   would have been ``'wrapper'``, and the docstring of the original :func:`example`

   would have been lost.

.. _partial-objects:

:class:`partial` Objects

------------------------

:class:`partial` objects are callable objects created by :func:`partial`. They

have three read-only attributes:

.. attribute:: partial.func

   A callable object or function.  Calls to the :class:`partial` object will be

   forwarded to :attr:`func` with new arguments and keywords.

.. attribute:: partial.args

   The leftmost positional arguments that will be prepended to the positional

   arguments provided to a :class:`partial` object call.

.. attribute:: partial.keywords

   The keyword arguments that will be supplied when the :class:`partial` object is

   called.

:class:`partial` objects are like :class:`function` objects in that they are

callable, weak referencable, and can have attributes.  There are some important

differences.  For instance, the :attr:`__name__` and :attr:`__doc__` attributes

are not created automatically.  Also, :class:`partial` objects defined in

classes behave like static methods and do not transform into bound methods

during instance attribute look-up.