"__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_impo  | 2.5.0a1                   | 3.0                       | **PEP 328** :             |
| rt                        |                           |                           | *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_liter  | 2.6.0a2                   | 3.0                       | **PEP 3112** : *Chaînes   |
| als                       |                           |                           | 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__".
