| :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 function: |
| |
| |
| .. 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:: |
| |
| >>> basetwo = partial(int, base=2) |
| >>> basetwo.__doc__ = 'Convert base 2 string to an int.' |
| >>> basetwo('10010') |
| 18 |
| |
| |
| .. function:: reduce(function, sequence[, initializer]) |
| |
| Apply *function* of two arguments cumulatively to the items of *sequence*, from |
| left to right, so as to reduce the sequence 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 *sequence*. If the optional *initializer* is present, |
| it is placed before the items of the sequence in the calculation, and serves as |
| a default when the sequence is empty. If *initializer* is not given and |
| *sequence* contains only one item, the first item is returned. |
| |
| |
| .. 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 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:: |
| |
| >>> 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. |
| |