[issue27000] improve document of filter

[Josh Rosenberg]

Josh Rosenberg added the comment:

Franklin said it better: The only difference between documentation and behavior is invisible implementation details, which have no business being documented in any event (since they needlessly tie the hands of maintainers of CPython and other Python interpreters, while providing no useful benefit).

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27000>
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