3.32 __future__ -- Future statement definitions

# 3.32 `__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 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 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__`.