Antony Lee <[email protected]> added the comment:
Actually, microoptimizing _power_of_two is a red herring and the problem is the
quadratic complexity of _decompose (_value2member_map_ becomes bigger and
bigger at each iteration). Replacing the implementation of _decompose by the
following fixes the performance issue (the original snippet executes in ~150ms,
which may be even more optimizable but already a 30x improvement :)), and still
passes test_enum.py (from cpython 3.7.4):
def _decompose(flag, value):
"""Extract all members from the value."""
# _decompose is only called if the value is not named
not_covered = value
members = []
for member in flag:
member_value = member.value
if member_value and member_value & value == member_value:
members.append(member)
not_covered &= ~member_value
if issubclass(flag, IntFlag) and value > 0:
while not_covered:
new_value = 2 ** _high_bit(not_covered)
if new_value not in flag._value2member_map_:
# construct singleton pseudo-members
pseudo_member = int.__new__(flag, value)
pseudo_member._name_ = None
pseudo_member._value_ = value
flag._value2member_map_.setdefault(value, pseudo_member)
members.append(flag(new_value))
not_covered &= ~new_value
if not members and value in flag._value2member_map_:
members.append(flag._value2member_map_[value])
members.sort(key=lambda m: m._value_, reverse=True)
if len(members) > 1 and members[0].value == value:
# we have the breakdown, don't need the value member itself
members.pop(0)
return members, not_covered
Checking for issubclass(flag, IntFlag) is a bit ugly but that's because Flag
and IntFlag really need two separate implementations of _decompose -- the
former wants to error out on uncovered values, the latter should create them
on-the-fly.
----------
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Python tracker <[email protected]>
<https://bugs.python.org/issue38045>
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