Increase Enum performance

[MrMrRobat]

BPO	39102
Nosy	@warsaw, @methane, @ethanfurman, @ilevkivskyi, @jack1142, @MrMrRobat
PRs	bpo-39102: Increase Enum performance up to 10x times (3x average) #17669
Files	benchmark_result.txt: Benchmark-comparison

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = None
created_at = <Date 2019-12-20.02:48:28.850>
labels = ['library', '3.10', 'performance']
title = 'Increase Enum performance'
updated_at = <Date 2021-04-11.15:41:25.877>
user = 'https://github.com/MrMrRobat'

bugs.python.org fields:

activity = <Date 2021-04-11.15:41:25.877>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-12-20.02:48:28.850>
creator = 'MrMrRobat'
dependencies = []
files = ['48793']
hgrepos = []
issue_num = 39102
keywords = ['patch']
message_count = 2.0
messages = ['358694', '358742']
nosy_count = 7.0
nosy_names = ['barry', 'eli.bendersky', 'methane', 'ethan.furman', 'levkivskyi', 'jack1142', 'MrMrRobat']
pr_nums = ['17669']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue39102'
versions = ['Python 3.10']

[MrMrRobat]

Now enum has very poor speed on trying values and attributes access (especially when it comes to accessing members name/value attrs)

There are two major reasons why attrs access is slow:

• All values/names access going through DynamicClassAttribute (x10 slower than accessing attr directly)
• EnumMeta has __getattr__ which is slowing down even direct class attributes access (up to x6 slower)

However, there are no need to use it, as we could just set value and name to newly created enum members without affecting its class.

The main issue with values check is the slow _missing_ hook handling when it raising exception, which happens pretty much always, if value is not valid enum and we talking about vanilla Enum class.

Also I found Flag performance issue being fixed already:
https://bugs.python.org/issue38045
It's also related, because new Flag creation involves many member.name lookups

My proposal:

• I think we should completely get rid of __getattr__ on Enum (~6x speed boost)
• Rework DynamicClassAttribute so it could work without __getattr__ or perhaps completely get rid of it
• Don't use DynamicClassAttribute for member.name and .value (~10x speed boost)
• Think of faster handling of _missing_ hook (~2x speed boost)
• Make other improvements to the code

Proposed changes doesn't require changing public API or behaviour.

By far I were able to implement almost things proposed here and will be happy to make a PR.

[MrMrRobat]

Also, do we need to leave compatibility with python <3.8?
If not, we could use the fact that python 3.8 dicts and sets which are preserve order to speed things up even more. Also I'd replace % string formatting with f-strings, as they also faster.

And another thing to think about: maybe we can calculate values returned by __str__, __repr__ and __invert__ once on member creation, since they not supposed to change during its life?

For example __invert__ on Flag does a lot of work on every call:
def __invert__(self):
cls = self.__class__
members, uncovered = _decompose(cls, self._value_)
inverted = cls(0)
for m in cls:
if m not in members and not (m._value_ & self._value_):
inverted = inverted | m
return cls(inverted)

[iritkatriel]

CC @mdboom .

[ethanfurman]

I believe this issue is no longer relevant as the ideas have been incorporated in other PRs; I'll double check at some point.