1:mod:`functools` --- Higher-order functions and operations on callable objects 2============================================================================== 3 4.. module:: functools 5 :synopsis: Higher-order functions and operations on callable objects. 6.. moduleauthor:: Peter Harris <scav@blueyonder.co.uk> 7.. moduleauthor:: Raymond Hettinger <python@rcn.com> 8.. moduleauthor:: Nick Coghlan <ncoghlan@gmail.com> 9.. sectionauthor:: Peter Harris <scav@blueyonder.co.uk> 10 11.. versionadded:: 2.5 12 13**Source code:** :source:`Lib/functools.py` 14 15-------------- 16 17The :mod:`functools` module is for higher-order functions: functions that act on 18or return other functions. In general, any callable object can be treated as a 19function for the purposes of this module. 20 21The :mod:`functools` module defines the following functions: 22 23.. function:: cmp_to_key(func) 24 25 Transform an old-style comparison function to a :term:`key function`. Used 26 with tools that accept key functions (such as :func:`sorted`, :func:`min`, 27 :func:`max`, :func:`heapq.nlargest`, :func:`heapq.nsmallest`, 28 :func:`itertools.groupby`). This function is primarily used as a transition 29 tool for programs being converted to Python 3 where comparison functions are 30 no longer supported. 31 32 A comparison function is any callable that accept two arguments, compares them, 33 and returns a negative number for less-than, zero for equality, or a positive 34 number for greater-than. A key function is a callable that accepts one 35 argument and returns another value to be used as the sort key. 36 37 Example:: 38 39 sorted(iterable, key=cmp_to_key(locale.strcoll)) # locale-aware sort order 40 41 For sorting examples and a brief sorting tutorial, see :ref:`sortinghowto`. 42 43 44 .. versionadded:: 2.7 45 46.. function:: total_ordering(cls) 47 48 Given a class defining one or more rich comparison ordering methods, this 49 class decorator supplies the rest. This simplifies the effort involved 50 in specifying all of the possible rich comparison operations: 51 52 The class must define one of :meth:`__lt__`, :meth:`__le__`, 53 :meth:`__gt__`, or :meth:`__ge__`. 54 In addition, the class should supply an :meth:`__eq__` method. 55 56 For example:: 57 58 @total_ordering 59 class Student: 60 def __eq__(self, other): 61 return ((self.lastname.lower(), self.firstname.lower()) == 62 (other.lastname.lower(), other.firstname.lower())) 63 def __lt__(self, other): 64 return ((self.lastname.lower(), self.firstname.lower()) < 65 (other.lastname.lower(), other.firstname.lower())) 66 67 .. versionadded:: 2.7 68 69.. function:: reduce(function, iterable[, initializer]) 70 71 This is the same function as :func:`reduce`. It is made available in this module 72 to allow writing code more forward-compatible with Python 3. 73 74 .. versionadded:: 2.6 75 76 77.. function:: partial(func[,*args][, **keywords]) 78 79 Return a new :class:`partial` object which when called will behave like *func* 80 called with the positional arguments *args* and keyword arguments *keywords*. If 81 more arguments are supplied to the call, they are appended to *args*. If 82 additional keyword arguments are supplied, they extend and override *keywords*. 83 Roughly equivalent to:: 84 85 def partial(func, *args, **keywords): 86 def newfunc(*fargs, **fkeywords): 87 newkeywords = keywords.copy() 88 newkeywords.update(fkeywords) 89 return func(*(args + fargs), **newkeywords) 90 newfunc.func = func 91 newfunc.args = args 92 newfunc.keywords = keywords 93 return newfunc 94 95 The :func:`partial` is used for partial function application which "freezes" 96 some portion of a function's arguments and/or keywords resulting in a new object 97 with a simplified signature. For example, :func:`partial` can be used to create 98 a callable that behaves like the :func:`int` function where the *base* argument 99 defaults to two: 100 101 >>> from functools import partial 102 >>> basetwo = partial(int, base=2) 103 >>> basetwo.__doc__ = 'Convert base 2 string to an int.' 104 >>> basetwo('10010') 105 18 106 107 108.. function:: update_wrapper(wrapper, wrapped[, assigned][, updated]) 109 110 Update a *wrapper* function to look like the *wrapped* function. The optional 111 arguments are tuples to specify which attributes of the original function are 112 assigned directly to the matching attributes on the wrapper function and which 113 attributes of the wrapper function are updated with the corresponding attributes 114 from the original function. The default values for these arguments are the 115 module level constants *WRAPPER_ASSIGNMENTS* (which assigns to the wrapper 116 function's *__name__*, *__module__* and *__doc__*, the documentation string) and 117 *WRAPPER_UPDATES* (which updates the wrapper function's *__dict__*, i.e. the 118 instance dictionary). 119 120 The main intended use for this function is in :term:`decorator` functions which 121 wrap the decorated function and return the wrapper. If the wrapper function is 122 not updated, the metadata of the returned function will reflect the wrapper 123 definition rather than the original function definition, which is typically less 124 than helpful. 125 126 127.. function:: wraps(wrapped[, assigned][, updated]) 128 129 This is a convenience function for invoking :func:`update_wrapper` as a 130 function decorator when defining a wrapper function. It is equivalent to 131 ``partial(update_wrapper, wrapped=wrapped, assigned=assigned, updated=updated)``. 132 For example:: 133 134 >>> from functools import wraps 135 >>> def my_decorator(f): 136 ... @wraps(f) 137 ... def wrapper(*args, **kwds): 138 ... print 'Calling decorated function' 139 ... return f(*args, **kwds) 140 ... return wrapper 141 ... 142 >>> @my_decorator 143 ... def example(): 144 ... """Docstring""" 145 ... print 'Called example function' 146 ... 147 >>> example() 148 Calling decorated function 149 Called example function 150 >>> example.__name__ 151 'example' 152 >>> example.__doc__ 153 'Docstring' 154 155 Without the use of this decorator factory, the name of the example function 156 would have been ``'wrapper'``, and the docstring of the original :func:`example` 157 would have been lost. 158 159 160.. _partial-objects: 161 162:class:`partial` Objects 163------------------------ 164 165:class:`partial` objects are callable objects created by :func:`partial`. They 166have three read-only attributes: 167 168 169.. attribute:: partial.func 170 171 A callable object or function. Calls to the :class:`partial` object will be 172 forwarded to :attr:`func` with new arguments and keywords. 173 174 175.. attribute:: partial.args 176 177 The leftmost positional arguments that will be prepended to the positional 178 arguments provided to a :class:`partial` object call. 179 180 181.. attribute:: partial.keywords 182 183 The keyword arguments that will be supplied when the :class:`partial` object is 184 called. 185 186:class:`partial` objects are like :class:`function` objects in that they are 187callable, weak referencable, and can have attributes. There are some important 188differences. For instance, the :attr:`~definition.__name__` and :attr:`__doc__` attributes 189are not created automatically. Also, :class:`partial` objects defined in 190classes behave like static methods and do not transform into bound methods 191during instance attribute look-up. 192 193