# `functools` --- Higher-order functions and operations on callable objects

Code source : Lib/functools.py

Le module `functools` concerne les fonctions d'ordre supérieur : des fonctions qui agissent sur, ou renvoient, d'autres fonctions. En général, tout objet appelable peut être considéré comme une fonction dans la description de ce module.

Le module `functools` définit les fonctions suivantes :

@functools.cache(user_function)

  Fonction de cache très simple et sans limite de taille. Cette technique est parfois appelée « mémoïsation ».

  Returns the same as `lru_cache(maxsize=None)`, creating a thin wrapper around a dictionary lookup for the function arguments. Because it never needs to evict old values, this is smaller and faster than `lru_cache()` with a size limit.

  Par exemple :

      @cache
      def factorial(n):
          return n * factorial(n-1) if n else 1

      >>> factorial(10)      # no previously cached result, makes 11 recursive calls
      3628800
      >>> factorial(5)       # just looks up cached value result
      120
      >>> factorial(12)      # makes two new recursive calls, the other 10 are cached
      479001600

  The cache is threadsafe so that the wrapped function can be used in multiple threads. This means that the underlying data structure will remain coherent during concurrent updates.

  It is possible for the wrapped function to be called more than once if another thread makes an additional call before the initial call has been completed and cached.

  Ajouté dans la version 3.9.

@functools.cached_property(func)

  Transform a method of a class into a property whose value is computed once and then cached as a normal attribute for the life of the instance. Similar to `property()`, with the addition of caching. Useful for expensive computed properties of instances that are otherwise effectively immutable.

  Exemple :

      class DataSet:

          def __init__(self, sequence_of_numbers):
              self._data = tuple(sequence_of_numbers)

          @cached_property
          def stdev(self):
              return statistics.stdev(self._data)

  The mechanics of `cached_property()` are somewhat different from `property()`. A regular property blocks attribute writes unless a setter is defined. In contrast, a cached_property allows writes.

  The cached_property decorator only runs on lookups and only when an attribute of the same name doesn't exist. When it does run, the cached_property writes to the attribute with the same name. Subsequent attribute reads and writes take precedence over the cached_property method and it works like a normal attribute.

  The cached value can be cleared by deleting the attribute. This allows the cached_property method to run again.

  The cached_property does not prevent a possible race condition in multi-threaded usage. The getter function could run more than once on the same instance, with the latest run setting the cached value. If the cached property is idempotent or otherwise not harmful to run more than once on an instance, this is fine. If synchronization is needed, implement the necessary locking inside the decorated getter function or around the cached property access.

  Note, this decorator interferes with the operation of PEP 412 key-sharing dictionaries. This means that instance dictionaries can take more space than usual.

  Also, this decorator requires that the `__dict__` attribute on each instance be a mutable mapping. This means it will not work with some types, such as metaclasses (since the `__dict__` attributes on type instances are read-only proxies for the class namespace), and those that specify `__slots__` without including `__dict__` as one of the defined slots (as such classes don't provide a `__dict__` attribute at all).

  If a mutable mapping is not available or if space-efficient key sharing is desired, an effect similar to `cached_property()` can also be achieved by stacking `property()` on top of `lru_cache()`. See Comment mettre en cache le résultat d'une méthode ? for more details on how this differs from `cached_property()`.

  Ajouté dans la version 3.8.

  Modifié dans la version 3.12: Prior to Python 3.12, `cached_property` included an undocumented lock to ensure that in multi-threaded usage the getter function was guaranteed to run only once per instance. However, the lock was per-property, not per-instance, which could result in unacceptably high lock contention. In Python 3.12+ this locking is removed.

functools.cmp_to_key(func)

  Transforme une fonction de comparaison à l'ancienne en une fonction clé. Utilisé avec des outils qui acceptent des fonctions clef (comme `sorted()`, `min()`, `max()`, `heapq.nlargest()`, `heapq.nsmallest()`, `itertools.groupby()`). Cette fonction est destinée au portage de fonctions python 2 utilisant des fonctions de comparaison vers Python 3.

  A comparison function is any callable that accepts 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 to be used as the sort key.

  Exemple :

      sorted(iterable, key=cmp_to_key(locale.strcoll))  # locale-aware sort order

  Pour des exemples de tris et un bref tutoriel, consultez Sorting Techniques.

  Ajouté dans la version 3.2.

@functools.lru_cache(user_function)
@functools.lru_cache(maxsize=128, typed=False)

  Décorateur qui englobe une fonction avec un appelable mémoïsant qui enregistre jusqu'à maxsize appels récents. Cela peut gagner du temps quand une fonction coûteuse en ressources est souvent appelée avec les mêmes arguments.

  The cache is threadsafe so that the wrapped function can be used in multiple threads. This means that the underlying data structure will remain coherent during concurrent updates.

  It is possible for the wrapped function to be called more than once if another thread makes an additional call before the initial call has been completed and cached.

  Since a dictionary is used to cache results, the positional and keyword arguments to the function must be hashable.

  Distinct argument patterns may be considered to be distinct calls with separate cache entries. For example, `f(a=1, b=2)` and `f(b=2, a=1)` differ in their keyword argument order and may have two separate cache entries.

  Si user_function est défini, ce doit être un appelable. Ceci permet à lru_cache d'être appliqué directement sur une fonction de l'utilisateur, sans préciser maxsize (qui est alors défini à sa valeur par défaut, 128) :

      @lru_cache
      def count_vowels(sentence):
          return sum(sentence.count(vowel) for vowel in 'AEIOUaeiou')

  Si maxsize est à `None`, la fonctionnalité LRU est désactivée et le cache peut grossir sans limite.

  If typed is set to true, function arguments of different types will be cached separately. If typed is false, the implementation will usually regard them as equivalent calls and only cache a single result. (Some types such as str and int may be cached separately even when typed is false.)

  Note, type specificity applies only to the function's immediate arguments rather than their contents. The scalar arguments, `Decimal(42)` and `Fraction(42)` are be treated as distinct calls with distinct results. In contrast, the tuple arguments `('answer', Decimal(42))` and `('answer', Fraction(42))` are treated as equivalent.

  The wrapped function is instrumented with a `cache_parameters()` function that returns a new `dict` showing the values for maxsize and typed. This is for information purposes only. Mutating the values has no effect.

  To help measure the effectiveness of the cache and tune the maxsize parameter, the wrapped function is instrumented with a `cache_info()` function that returns a named tuple showing hits, misses, maxsize and currsize.

  The decorator also provides a `cache_clear()` function for clearing or invalidating the cache.

  La fonction sous-jacente originale est accessible à travers l'attribut `__wrapped__`. Ceci est utile pour l'introspection, pour outrepasser le cache, ou pour ré-englober la fonction avec un cache différent.

  The cache keeps references to the arguments and return values until they age out of the cache or until the cache is cleared.

  If a method is cached, the `self` instance argument is included in the cache. See Comment mettre en cache le résultat d'une méthode ?

  An LRU (least recently used) cache works best when the most recent calls are the best predictors of upcoming calls (for example, the most popular articles on a news server tend to change each day). The cache's size limit assures that the cache does not grow without bound on long-running processes such as web servers.

  In general, the LRU cache should only be used when you want to reuse previously computed values. Accordingly, it doesn't make sense to cache functions with side-effects, functions that need to create distinct mutable objects on each call (such as generators and async functions), or impure functions such as time() or random().

  Exemple d'un cache LRU pour du contenu web statique :

      @lru_cache(maxsize=32)
      def get_pep(num):
          'Retrieve text of a Python Enhancement Proposal'
          resource = f'https://peps.python.org/pep-{num:04d}'
          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))

      >>> get_pep.cache_info()
      CacheInfo(hits=3, misses=8, maxsize=32, currsize=8)

  Exemple de calcul efficace de la suite de Fibonacci en utilisant un cache pour implémenter la technique de programmation dynamique :

      @lru_cache(maxsize=None)
      def fib(n):
          if n < 2:
              return n
          return fib(n-1) + fib(n-2)

      >>> [fib(n) for n in range(16)]
      [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610]

      >>> fib.cache_info()
      CacheInfo(hits=28, misses=16, maxsize=None, currsize=16)

  Ajouté dans la version 3.2.

  Modifié dans la version 3.3: L'option typed a été ajoutée.

  Modifié dans la version 3.8: Ajout de l'option user_function.

  Modifié dans la version 3.9: Added the function `cache_parameters()`

@functools.total_ordering

  A partir d'une classe définissant une ou plusieurs méthodes de comparaison riches, ce décorateur de classe fournit le reste. Ceci simplifie l'effort à fournir dans la spécification de toutes les opérations de comparaison riche :

  The class must define one of `__lt__()`, `__le__()`, `__gt__()`, or `__ge__()`. In addition, the class should supply an `__eq__()` method.

  Par exemple :

      @total_ordering
      class Student:
          def _is_valid_operand(self, other):
              return (hasattr(other, "lastname") and
                      hasattr(other, "firstname"))
          def __eq__(self, other):
              if not self._is_valid_operand(other):
                  return NotImplemented
              return ((self.lastname.lower(), self.firstname.lower()) ==
                      (other.lastname.lower(), other.firstname.lower()))
          def __lt__(self, other):
              if not self._is_valid_operand(other):
                  return NotImplemented
              return ((self.lastname.lower(), self.firstname.lower()) <
                      (other.lastname.lower(), other.firstname.lower()))

  Note

  Même si ce décorateur permet de créer des types ordonnables facilement, cela vient avec un coût d'exécution et des traces d'exécution complexes pour les méthodes de comparaison dérivées. Si des tests de performances le révèlent comme un goulot d'étranglement, l'implémentation manuelle des six méthodes de comparaison riches résoudra normalement vos problèmes de rapidité.

  Note

  This decorator makes no attempt to override methods that have been declared in the class or its superclasses. Meaning that if a superclass defines a comparison operator, total_ordering will not implement it again, even if the original method is abstract.

  Ajouté dans la version 3.2.

  Modifié dans la version 3.4: Returning `NotImplemented` from the underlying comparison function for unrecognised types is now supported.

functools.partial(func, /, *args, **keywords)

  Retourne un nouvel objet partiel qui, quand il est appelé, fonctionne comme func appelée avec les arguments positionnels args et les arguments nommés keywords. Si plus d'arguments sont fournis à l'appel, ils sont ajoutés à args. Si plus d'arguments nommés sont fournis, ils étendent et surchargent keywords. À peu près équivalent à :

      def partial(func, /, *args, **keywords):
          def newfunc(*fargs, **fkeywords):
              newkeywords = {**keywords, **fkeywords}
              return func(*args, *fargs, **newkeywords)
          newfunc.func = func
          newfunc.args = args
          newfunc.keywords = keywords
          return newfunc

  `partial()` est utilisé pour une application de fonction partielle qui "gèle" une portion des arguments et/ou mots-clés d'une fonction donnant un nouvel objet avec une signature simplifiée. Par exemple, `partial()` peut être utilisé pour créer un appelable qui se comporte comme la fonction `int()` ou l'argument base est deux par défaut :

      >>> from functools import partial
      >>> basetwo = partial(int, base=2)
      >>> basetwo.__doc__ = 'Convert base 2 string to an int.'
      >>> basetwo('10010')
      18

class functools.partialmethod(func, /, *args, **keywords)

  Retourne un nouveau descripteur `partialmethod` qui se comporte comme `partial` sauf qu'il est fait pour être utilisé comme une définition de méthode plutôt que d'être appelé directement.

  func doit être un descriptor ou un appelable (les objets qui sont les deux, comme les fonction normales, sont gérés comme des descripteurs).

  When func is a descriptor (such as a normal Python function, `classmethod()`, `staticmethod()`, `abstractmethod()` or another instance of `partialmethod`), calls to `__get__` are delegated to the underlying descriptor, and an appropriate partial object returned as the result.

  Quand func est un appelable non-descripteur, une méthode liée appropriée est crée dynamiquement. Elle se comporte comme une fonction Python normale quand elle est utilisée comme méthode : l'argument self sera inséré comme premier argument positionnel, avant les args et keywords fournis au constructeur `partialmethod`.

  Exemple :

      >>> class Cell:
      ...     def __init__(self):
      ...         self._alive = False
      ...     @property
      ...     def alive(self):
      ...         return self._alive
      ...     def set_state(self, state):
      ...         self._alive = bool(state)
      ...     set_alive = partialmethod(set_state, True)
      ...     set_dead = partialmethod(set_state, False)
      ...
      >>> c = Cell()
      >>> c.alive
      False
      >>> c.set_alive()
      >>> c.alive
      True

  Ajouté dans la version 3.4.

functools.reduce(function, iterable, [initial, ]/)

  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 initial 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 initial is not given and iterable contains only one item, the first item is returned.

  À peu près équivalent à :

      initial_missing = object()

      def reduce(function, iterable, initial=initial_missing, /):
          it = iter(iterable)
          if initial is initial_missing:
              value = next(it)
          else:
              value = initial
          for element in it:
              value = function(value, element)
          return value

  Voir `itertools.accumulate()` pour un itérateur qui génère toutes les valeurs intermédiaires.

@functools.singledispatch

  Transforme une fonction en une fonction générique single-dispatch.

  To define a generic function, decorate it with the `@singledispatch` decorator. When defining a function using `@singledispatch`, note that the dispatch happens on the type of the first argument:

      >>> from functools import singledispatch
      >>> @singledispatch
      ... def fun(arg, verbose=False):
      ...     if verbose:
      ...         print("Let me just say,", end=" ")
      ...     print(arg)

  To add overloaded implementations to the function, use the `register()` attribute of the generic function, which can be used as a decorator. For functions annotated with types, the decorator will infer the type of the first argument automatically:

      >>> @fun.register
      ... def _(arg: int, verbose=False):
      ...     if verbose:
      ...         print("Strength in numbers, eh?", end=" ")
      ...     print(arg)
      ...
      >>> @fun.register
      ... def _(arg: list, verbose=False):
      ...     if verbose:
      ...         print("Enumerate this:")
      ...     for i, elem in enumerate(arg):
      ...         print(i, elem)

  `types.UnionType` and `typing.Union` can also be used:

      >>> @fun.register
      ... def _(arg: int | float, verbose=False):
      ...     if verbose:
      ...         print("Strength in numbers, eh?", end=" ")
      ...     print(arg)
      ...
      >>> from typing import Union
      >>> @fun.register
      ... def _(arg: Union[list, set], verbose=False):
      ...     if verbose:
      ...         print("Enumerate this:")
      ...     for i, elem in enumerate(arg):
      ...         print(i, elem)
      ...

  Pour le code qui n’utilise pas les indications de type, le type souhaité peut être passé explicitement en argument au décorateur :

      >>> @fun.register(complex)
      ... def _(arg, verbose=False):
      ...     if verbose:
      ...         print("Better than complicated.", end=" ")
      ...     print(arg.real, arg.imag)
      ...

  For code that dispatches on a collections type (e.g., `list`), but wants to typehint the items of the collection (e.g., `list[int]`), the dispatch type should be passed explicitly to the decorator itself with the typehint going into the function definition:

      >>> @fun.register(list)
      ... def _(arg: list[int], verbose=False):
      ...     if verbose:
      ...         print("Enumerate this:")
      ...     for i, elem in enumerate(arg):
      ...         print(i, elem)

  Note

  At runtime the function will dispatch on an instance of a list regardless of the type contained within the list i.e. `[1,2,3]` will be dispatched the same as `["foo", "bar", "baz"]`. The annotation provided in this example is for static type checkers only and has no runtime impact.

  To enable registering lambdas and pre-existing functions, the `register()` attribute can also be used in a functional form:

      >>> def nothing(arg, verbose=False):
      ...     print("Nothing.")
      ...
      >>> fun.register(type(None), nothing)

  The `register()` attribute returns the undecorated function. This enables decorator stacking, `pickling`, and the creation of unit tests for each variant independently:

      >>> @fun.register(float)
      ... @fun.register(Decimal)
      ... def fun_num(arg, verbose=False):
      ...     if verbose:
      ...         print("Half of your number:", end=" ")
      ...     print(arg / 2)
      ...
      >>> fun_num is fun
      False

  Quand elle est appelée, la fonction générique distribue sur le type du premier argument :

      >>> fun("Hello, world.")
      Hello, world.
      >>> fun("test.", verbose=True)
      Let me just say, test.
      >>> fun(42, verbose=True)
      Strength in numbers, eh? 42
      >>> fun(['spam', 'spam', 'eggs', 'spam'], verbose=True)
      Enumerate this:
      0 spam
      1 spam
      2 eggs
      3 spam
      >>> fun(None)
      Nothing.
      >>> fun(1.23)
      0.615

  Where there is no registered implementation for a specific type, its method resolution order is used to find a more generic implementation. The original function decorated with `@singledispatch` is registered for the base `object` type, which means it is used if no better implementation is found.

  If an implementation is registered to an abstract base class, virtual subclasses of the base class will be dispatched to that implementation:

      >>> from collections.abc import Mapping
      >>> @fun.register
      ... def _(arg: Mapping, verbose=False):
      ...     if verbose:
      ...         print("Keys & Values")
      ...     for key, value in arg.items():
      ...         print(key, "=>", value)
      ...
      >>> fun({"a": "b"})
      a => b

  To check which implementation the generic function will choose for a given type, use the `dispatch()` attribute:

      >>> fun.dispatch(float)
      <function fun_num at 0x1035a2840>
      >>> fun.dispatch(dict)    # note: default implementation
      <function fun at 0x103fe0000>

  Pour accéder à toutes les implémentations enregistrées, utiliser l'attribut en lecture seule `registry` :

      >>> fun.registry.keys()
      dict_keys([<class 'NoneType'>, <class 'int'>, <class 'object'>,
                <class 'decimal.Decimal'>, <class 'list'>,
                <class 'float'>])
      >>> fun.registry[float]
      <function fun_num at 0x1035a2840>
      >>> fun.registry[object]
      <function fun at 0x103fe0000>

  Ajouté dans la version 3.4.

  Modifié dans la version 3.7: The `register()` attribute now supports using type annotations.

  Modifié dans la version 3.11: The `register()` attribute now supports `types.UnionType` and `typing.Union` as type annotations.

class functools.singledispatchmethod(func)

  Transforme une méthode en une fonction générique single-dispatch.

  To define a generic method, decorate it with the `@singledispatchmethod` decorator. When defining a function using `@singledispatchmethod`, note that the dispatch happens on the type of the first non-self or non-cls argument:

      class Negator:
          @singledispatchmethod
          def neg(self, arg):
              raise NotImplementedError("Cannot negate a")

          @neg.register
          def _(self, arg: int):
              return -arg

          @neg.register
          def _(self, arg: bool):
              return not arg

  `@singledispatchmethod` supports nesting with other decorators such as `@classmethod`. Note that to allow for `dispatcher.register`, `singledispatchmethod` must be the outer most decorator. Here is the `Negator` class with the `neg` methods bound to the class, rather than an instance of the class:

      class Negator:
          @singledispatchmethod
          @classmethod
          def neg(cls, arg):
              raise NotImplementedError("Cannot negate a")

          @neg.register
          @classmethod
          def _(cls, arg: int):
              return -arg

          @neg.register
          @classmethod
          def _(cls, arg: bool):
              return not arg

  The same pattern can be used for other similar decorators: `@staticmethod`, `@abstractmethod`, and others.

  Ajouté dans la version 3.8.

functools.update_wrapper(wrapper, wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES)

  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 `__module__`, `__name__`, `__qualname__`, `__annotations__`, `__type_params__`, and `__doc__`, the documentation string) and `WRAPPER_UPDATES` (which updates the wrapper function's `__dict__`, i.e. the instance dictionary).

  Pour autoriser l'accès à la fonction originale pour l'introspection ou à d'autres fins (par ex. outrepasser l'accès à un décorateur de cache comme `lru_cache()`), cette fonction ajoute automatiquement un attribut `__wrapped__` qui référence la fonction englobée.

  La principale utilisation de cette fonction est dans les décorateurs qui renvoient une nouvelle fonction. Si la fonction crée n'est pas mise à jour, ses métadonnées refléteront sa définition dans le décorateur, au lieu de la définition originale, métadonnées souvent bien moins utiles.

  `update_wrapper()` peut être utilisé avec des appelables autres que des fonctions. Tout attribut défini dans assigned ou updated qui ne sont pas l'objet englobé sont ignorés (cette fonction n'essaiera pas de les définir dans la fonction englobante). `AttributeError` est toujours levée si le fonction englobante elle même a des attributs non existants dans updated.

  Modifié dans la version 3.2: The `__wrapped__` attribute is now automatically added. The `__annotations__` attribute is now copied by default. Missing attributes no longer trigger an `AttributeError`.

  Modifié dans la version 3.4: L'attribut `__wrapped__` renvoie toujours la fonction englobée, même si cette fonction définit un attribut `__wrapped__`. (voir bpo-17482)

  Modifié dans la version 3.12: The `__type_params__` attribute is now copied by default.

@functools.wraps(wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES)

  Ceci est une fonction d'aide pour appeler `update_wrapper()` comme décorateur de fonction lors de la définition d'une fonction englobante. C'est équivalent à `partial(update_wrapper, wrapped=wrapped, assigned=assigned, updated=updated)`. Par exemple :

      >>> 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 `example()` would have been lost.

## Objets `partial`

Les objets `partial` sont des objets appelables créés par `partial()`. Ils ont trois attributs en lecture seule :

partial.func

  Un objet ou une fonction appelable. Les appels à l'objet `partial` seront transmis à `func` avec les nouveaux arguments et mots-clés.

partial.args

  Les arguments positionnels qui seront ajoutés avant les arguments fournis lors de l'appel d'un objet `partial`.

partial.keywords

  Les arguments nommés qui seront fournis quand l'objet `partial` est appelé.

`partial` objects are like function objects in that they are callable, weak referenceable, and can have attributes. There are some important differences. For instance, the `__name__` and `function.__doc__` attributes are not created automatically. Also, `partial` objects defined in classes behave like static methods and do not transform into bound methods during instance attribute look-up.