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(python2.1-ref.info)__future__

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Future statement definitions
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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__'.