A.2 `future` -- Future statement definitions

__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 programatically via importing __future__ and examining its contents.

Each statment in __future__.py is of the form:

FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ")"

where, normally, OptionalRelease is less then 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 MandatoryReleases that have 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().

No feature description will ever be deleted from __future__.

See About this document... for information on suggesting changes.