[issue38045] enum.Flag instance creation is slow

[Antony Lee]

Antony Lee  added the comment:

Now fixed, I believe.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue38045] enum.Flag instance creation is slow

[Ethan Furman]

Ethan Furman  added the comment:


New changeset 0b41a922f95f62b620d5a197b9f2ed8e4bb70730 by Ethan Furman 
(HongWeipeng) in branch 'master':
bpo-38045: Improve the performance of _decompose() in enum.py (GH-16483)
https://github.com/python/cpython/commit/0b41a922f95f62b620d5a197b9f2ed8e4bb70730


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[issue38045] enum.Flag instance creation is slow

[hongweipeng]

Change by hongweipeng :


--
pull_requests: +16068
pull_request: https://github.com/python/cpython/pull/16483

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[issue38045] enum.Flag instance creation is slow

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +ethan.furman

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[issue38045] enum.Flag instance creation is slow

[Roundup Robot]

Change by Roundup Robot :


--
keywords: +patch
pull_requests: +15876
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/16291

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[issue38045] enum.Flag instance creation is slow

[Antony Lee]

Antony Lee  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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[issue38045] enum.Flag instance creation is slow

[STINNER Victor]

Change by STINNER Victor :


--
title: Flag instance creation is slow -> enum.Flag instance creation is slow

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