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__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 statement in __future__.py is of the form:
FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ","
CompilerFlag ")"
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().
CompilerFlag is the (bitfield) flag that should be passed in the fourth argument to the
builtin function compile() to enable the feature in dynamically
compiled code. This flag is stored in the compiler_flag attribute on _Future instances.
No feature description will ever be deleted from __future__.
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