# `__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
generator_stop | 3.5.0b1 | 3.7 | PEP 479: StopIteration handling inside generators
annotations | 3.7.0b1 | 4.0 | PEP 563: Postponed evaluation of annotations

也參考

Future statements
  How the compiler treats future imports.