28.11. "__future__" — Future statement definitions
**************************************************

**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.

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* |
+--------------------+---------------+----------------+-----------------------------------------------+

也參考:

  Future statements
     How the compiler treats future imports.
