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Diffstat (limited to 'Doc/library/functools.rst')
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1 files changed, 141 insertions, 1 deletions
diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst index 570f4d2..2316e80 100644 --- a/Doc/library/functools.rst +++ b/Doc/library/functools.rst @@ -8,6 +8,9 @@ .. moduleauthor:: Nick Coghlan <ncoghlan@gmail.com> .. sectionauthor:: Peter Harris <scav@blueyonder.co.uk> +**Source code:** :source:`Lib/functools.py` + +-------------- 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 @@ -15,6 +18,123 @@ function for the purposes of this module. The :mod:`functools` module defines the following functions: +.. function:: cmp_to_key(func) + + Transform an old-style comparison function to a key-function. Used with + tools that accept key functions (such as :func:`sorted`, :func:`min`, + :func:`max`, :func:`heapq.nlargest`, :func:`heapq.nsmallest`, + :func:`itertools.groupby`). This function is primarily used as a transition + tool for programs being converted from Py2.x which supported the use of + comparison functions. + + A compare function is any callable that accept two arguments, compares them, + and returns a negative number for less-than, zero for equality, or a positive + number for greater-than. A key function is a callable that accepts one + argument and returns another value indicating the position in the desired + collation sequence. + + Example:: + + sorted(iterable, key=cmp_to_key(locale.strcoll)) # locale-aware sort order + + .. versionadded:: 3.2 + + +.. decorator:: lru_cache(maxsize=100) + + Decorator to wrap a function with a memoizing callable that saves up to the + *maxsize* most recent calls. It can save time when an expensive or I/O bound + function is periodically called with the same arguments. + + Since a dictionary is used to cache results, the positional and keyword + arguments to the function must be hashable. + + If *maxsize* is set to None, the LRU feature is disabled and the cache + can grow without bound. + + To help measure the effectiveness of the cache and tune the *maxsize* + parameter, the wrapped function is instrumented with a :func:`cache_info` + function that returns a :term:`named tuple` showing *hits*, *misses*, + *maxsize* and *currsize*. In a multi-threaded environment, the hits + and misses are approximate. + + The decorator also provides a :func:`cache_clear` function for clearing or + invalidating the cache. + + The original underlying function is accessible through the + :attr:`__wrapped__` attribute. This is useful for introspection, for + bypassing the cache, or for rewrapping the function with a different cache. + + An `LRU (least recently used) cache + <http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used>`_ works + best when more recent calls are the best predictors of upcoming calls (for + example, the most popular articles on a news server tend to change daily). + The cache's size limit assures that the cache does not grow without bound on + long-running processes such as web servers. + + Example of an LRU cache for static web content:: + + @lru_cache(maxsize=20) + def get_pep(num): + 'Retrieve text of a Python Enhancement Proposal' + resource = 'http://www.python.org/dev/peps/pep-%04d/' % num + try: + with urllib.request.urlopen(resource) as s: + return s.read() + except urllib.error.HTTPError: + return 'Not Found' + + >>> for n in 8, 290, 308, 320, 8, 218, 320, 279, 289, 320, 9991: + ... pep = get_pep(n) + ... print(n, len(pep)) + + >>> print(get_pep.cache_info()) + CacheInfo(hits=3, misses=8, maxsize=20, currsize=8) + + Example of efficiently computing + `Fibonacci numbers <http://en.wikipedia.org/wiki/Fibonacci_number>`_ + using a cache to implement a + `dynamic programming <http://en.wikipedia.org/wiki/Dynamic_programming>`_ + technique:: + + @lru_cache(maxsize=None) + def fib(n): + if n < 2: + return n + return fib(n-1) + fib(n-2) + + >>> print([fib(n) for n in range(16)]) + [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610] + + >>> print(fib.cache_info()) + CacheInfo(hits=28, misses=16, maxsize=None, currsize=16) + + .. versionadded:: 3.2 + +.. decorator:: total_ordering + + Given a class defining one or more rich comparison ordering methods, this + class decorator supplies the rest. This simplifies the effort involved + in specifying all of the possible rich comparison operations: + + The class must define one of :meth:`__lt__`, :meth:`__le__`, + :meth:`__gt__`, or :meth:`__ge__`. + In addition, the class should supply an :meth:`__eq__` method. + + For example:: + + @total_ordering + class Student: + def __eq__(self, other): + return ((self.lastname.lower(), self.firstname.lower()) == + (other.lastname.lower(), other.firstname.lower())) + def __lt__(self, other): + return ((self.lastname.lower(), self.firstname.lower()) < + (other.lastname.lower(), other.firstname.lower())) + + .. versionadded:: 3.2 + + .. function:: partial(func, *args, **keywords) Return a new :class:`partial` object which when called will behave like *func* @@ -70,14 +190,34 @@ The :mod:`functools` module defines the following functions: documentation string) and *WRAPPER_UPDATES* (which updates the wrapper function's *__dict__*, i.e. the instance dictionary). + To allow access to the original function for introspection and other purposes + (e.g. bypassing a caching decorator such as :func:`lru_cache`), this function + automatically adds a __wrapped__ attribute to the wrapper that refers to + the original function. + 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. + :func:`update_wrapper` may be used with callables other than functions. Any + attributes named in *assigned* or *updated* that are missing from the object + being wrapped are ignored (i.e. this function will not attempt to set them + on the wrapper function). :exc:`AttributeError` is still raised if the + wrapper function itself is missing any attributes named in *updated*. + + .. versionadded:: 3.2 + Automatic addition of the ``__wrapped__`` attribute. + + .. versionadded:: 3.2 + Copying of the ``__annotations__`` attribute by default. + + .. versionchanged:: 3.2 + Missing attributes no longer trigger an :exc:`AttributeError`. + -.. function:: wraps(wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES) +.. decorator:: wraps(wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES) This is a convenience function for invoking ``partial(update_wrapper, wrapped=wrapped, assigned=assigned, updated=updated)`` as a function decorator |