bpo-38045: Avoid quadratic complexity when creating composite IntFlags. #16291
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When one creates all possible combos of an IntFlag, e.g. with
the previous version would exhibit quadratic complexity because
_value2member_map_
would grow bigger over each iteration.Iterate over "true" flag members rather than
_value2member_map_
,to avoid this problem. For a 13-member flag (so 8192 combinations),
the speedup is more than 100x.
https://bugs.python.org/issue38045