"__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:

* To avoid confusing existing tools that analyze import statements and
  expect to find the modules they’re importing.

* 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).


Module Contents
===============

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:

+--------------------+---------------+----------------+-----------------------------------------------+
| feature            | optional in   | mandatory in   | effect                                        |
|====================|===============|================|===============================================|
| nested_scopes      | 2.1.0b1       | 2.2            | **PEP 227**: *Statically Nested Scopes*       |
+--------------------+---------------+----------------+-----------------------------------------------+
| generators         | 2.2.0a1       | 2.3            | **PEP 255**: *Simple Generators*              |
+--------------------+---------------+----------------+-----------------------------------------------+
| division           | 2.2.0a2       | 3.0            | **PEP 238**: *Changing the Division Operator* |
+--------------------+---------------+----------------+-----------------------------------------------+
| absolute_import    | 2.5.0a1       | 3.0            | **PEP 328**: *Imports: Multi-Line and         |
|                    |               |                | Absolute/Relative*                            |
+--------------------+---------------+----------------+-----------------------------------------------+
| with_statement     | 2.5.0a1       | 2.6            | **PEP 343**: *The “with” Statement*           |
+--------------------+---------------+----------------+-----------------------------------------------+
| print_function     | 2.6.0a2       | 3.0            | **PEP 3105**: *Make print a function*         |
+--------------------+---------------+----------------+-----------------------------------------------+
| unicode_literals   | 2.6.0a2       | 3.0            | **PEP 3112**: *Bytes literals in Python 3000* |
+--------------------+---------------+----------------+-----------------------------------------------+
| generator_stop     | 3.5.0b1       | 3.7            | **PEP 479**: *StopIteration handling inside   |
|                    |               |                | generators*                                   |
+--------------------+---------------+----------------+-----------------------------------------------+
| annotations        | 3.7.0b1       | TBD [1]        | **PEP 563**: *Postponed evaluation of         |
|                    |               |                | annotations*                                  |
+--------------------+---------------+----------------+-----------------------------------------------+

class __future__._Feature

   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
      )

_Feature.getOptionalRelease()

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

_Feature.getMandatoryRelease()

   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 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 Python Steering Council
    twice decided to delay the change (announcement for Python 3.10;
    announcement for Python 3.11). No final decision has been made
    yet. See also **PEP 563** and **PEP 649**.

See also:

  Future statements
     How the compiler treats future imports.

  **PEP 236** - Back to the __future__
     The original proposal for the __future__ mechanism.
