# `__future__` --- Future statement definitions

Source code: Lib/_future_.py

Imports of the form `from __future__ import feature` are called future statements. These are special-cased by the Python compiler to allow the use of new Python features in modules containing the future statement before the release in which the feature becomes standard.

While these future statements are given additional special meaning by the Python compiler, they are still executed like any other import statement and the `__future__` exists and is handled by the import system the same way any other Python module would be. This design serves three purposes:

- éviter de dérouter les outils existants qui analysent les instructions d'importation et s'attendent à trouver les modules qu'ils importent ;

- To document when incompatible changes were introduced, and when they will be --- or were --- made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing `__future__` and examining its contents.

- To ensure that future statements run under releases prior to Python 2.1 at least yield runtime exceptions (the import of `__future__` will fail, because there was no module of that name prior to 2.1).

## Contenu du module

No feature description will ever be deleted from `__future__`. Since its introduction in Python 2.1 the following features have found their way into the language using this mechanism:

fonctionnalité | optionnel dans | obligatoire dans | effet
--- | --- | --- | ---
__future__.nested_scopes | 2.1.0b1 | 2.2 | PEP 227 : Portées imbriquées
__future__.generators | 2.2.0a1 | 2.3 | PEP 255 : Générateurs simples
__future__.division | 2.2.0a2 | 3.0 | PEP 238 : Changement de l'opérateur de division
__future__.absolute_import | 2.5.0a1 | 3.0 | PEP 328 : Importations : multilignes et absolues/relatives (ressource en anglais)
__future__.with_statement | 2.5.0a1 | 2.6 | PEP 343: The “with” Statement
__future__.print_function | 2.6.0a2 | 3.0 | PEP 3105 : Transformation de print en fonction
__future__.unicode_literals | 2.6.0a2 | 3.0 | PEP 3112 : Chaînes d'octets littéraux en Python 3000
__future__.generator_stop | 3.5.0b1 | 3.7 | PEP 479 : Gestion de *StopIteration à l’intérieur des générateurs*
__future__.annotations | 3.7.0b1 | Never [1] | PEP 563: Postponed evaluation of annotations, PEP 649: Deferred evaluation of annotations using descriptors

class __future__._Feature

  Chaque instruction dans `__future__.py` est de la forme :

      FeatureName = _Feature(OptionalRelease, MandatoryRelease,
                             CompilerFlag)

  où, normalement, OptionalRelease est inférieur à MandatoryRelease, et les deux sont des quintuplets de la même forme que `sys.version_info` :

      (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
       PY_MINOR_VERSION, # the 1; an int
       PY_MICRO_VERSION, # the 0; an int
       PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
       PY_RELEASE_SERIAL # the 3; an int
      )

_Feature.getOptionalRelease()

  OptionalRelease enregistre la première version dans laquelle la fonctionnalité a été acceptée.

_Feature.getMandatoryRelease()

  Dans le cas d'un MandatoryRelease qui n'a pas encore eu lieu, MandatoryRelease prédit la release dans laquelle la fonctionnalité deviendra un élément du langage.

  Sinon MandatoryRelease enregistre lorsque la fonctionnalité est devenue une partie du langage ; dans cette version ou les suivantes, les modules n'ont plus besoin d'une déclaration future pour utiliser la fonctionnalité en question, mais ils peuvent continuer à utiliser ces importations.

  MandatoryRelease may also be `None`, meaning that a planned feature got dropped or that it is not yet decided.

_Feature.compiler_flag

  CompilerFlag is the (bitfield) flag that should be passed in the fourth argument to the built-in function `compile()` to enable the feature in dynamically compiled code. This flag is stored in the `_Feature.compiler_flag` attribute on `_Feature` instances.

[1]

`from __future__ import annotations` was previously scheduled to become mandatory in Python 3.10, but the change was delayed and ultimately canceled. This feature will eventually be deprecated and removed. See PEP 649 and PEP 749.

Voir aussi

L'instruction future

  Comment le compilateur gère les importations « futures ».

PEP 236 — retour vers le `__future__`

  La proposition originale pour le mécanisme de `__future__`.