# 28.11. `__future__` — Définitions des futurs

Source code: Lib/_future_.py

`__future__` is a real module, and serves three purposes:

- To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.
- To ensure that future statements run under releases prior to 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).
- 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.

Each statement in `__future__.py` is of the form:

    FeatureName = _Feature(OptionalRelease, MandatoryRelease,
                           CompilerFlag)

where, normally, OptionalRelease is less than MandatoryRelease, and both are 5-tuples of the same form as `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
    )

OptionalRelease records the first release in which the feature was accepted.

In the case of a MandatoryRelease that has not yet occurred, MandatoryRelease predicts the release in which the feature will become part of the language.

Else MandatoryRelease records when the feature became part of the language; in releases at or after that, modules no longer need a future statement to use the feature in question, but may continue to use such imports.

MandatoryRelease may also be `None`, meaning that a planned feature got dropped.

Instances of class `_Feature` have two corresponding methods, `getOptionalRelease()` and `getMandatoryRelease()`.

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 `compiler_flag` attribute on `_Feature` instances.

Aucune fonctionnalité ne sera jamais supprimée de `__future__`. Depuis son introduction dans Python 2.1, les fonctionnalités suivantes ont trouvé leur places dans le langage utilisant ce mécanisme :

fonctionnalité | optionnel dans | obligatoire dans | effet
--- | --- | --- | ---
nested_scopes | 2.1.0b1 | 2.2 | PEP 227: Statically Nested Scopes
générateurs | 2.2.0a1 | 2.3 | PEP 255 : Générateurs simples
division | 2.2.0a2 | 3.0 | PEP 328 : Changer l’opérateur de division
absolute_import | 2.5.0a1 | 3.0 | PEP 328: Imports: Multi-Line and Absolute/Relative
with_statement | 2.5.0a1 | 2.6 | PEP 343 : L’instruction « with »
print_function | 2.6.0a2 | 3.0 | PEP 3105: Make print a function
unicode_literals | 2.6.0a2 | 3.0 | PEP 3112 : Obtets littéraux en Python 3000

Voir aussi

L’instruction future
  Comment le compilateur gère les imports du futur.