[issue42915] enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag value

[Ethan Furman]

Ethan Furman  added the comment:

Fixed in 3.10 in issue38250.

Also fixed in my 3rd-party library, aenum 3.0:

   (https://pypi.org/project/aenum/)

--
resolution:  -> wont fix
stage:  -> resolved
status: open -> closed
superseder:  -> enum.Flag should be more set-like
type:  -> behavior

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42915] enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag value

[Ethan Furman]

Ethan Furman  added the comment:

Yes.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42915] enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag value

[Kevin Chen]

Kevin Chen  added the comment:

Awesome thanks! Does the rewrite fix the issue with creating negated flags as 
well?

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42915] enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag value

[Ethan Furman]

Ethan Furman  added the comment:

I just finished a rewrite of Flag for 3.10.  Using your test below I was able 
to tweak the rewrite so the final numbers are:

Took normal 0.148092 seconds.
Took cached 0.017438 seconds.

Your original post had a ratio of nearly 200 -- it is now 8.7ish.

--
assignee:  -> ethan.furman

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42915] enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag value

[Guido van Rossum]

Change by Guido van Rossum :


--
nosy: +ethan.furman

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42915] enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag value

[Kevin Chen]

New submission from Kevin Chen :

Here's a code sample:

```
import time
from enum import Flag, auto


class MyFlag(Flag):
NONE = 0
FLAG_1 = auto()
FLAG_2 = auto()
FLAG_3 = auto()
FLAG_4 = auto()
FLAG_5 = auto()
FLAG_6 = auto()
#
# NOT_FLAG_1_OR_2 = ~FLAG_1 & ~FLAG_2


def test_flag():
f = MyFlag.NONE
inverted = (
~MyFlag.FLAG_1
& ~MyFlag.FLAG_2
& ~MyFlag.FLAG_3
& ~MyFlag.FLAG_4
& ~MyFlag.FLAG_5
& ~MyFlag.FLAG_6
)
return f & inverted


INVERTED = (
~MyFlag.FLAG_1
& ~MyFlag.FLAG_2
& ~MyFlag.FLAG_3
& ~MyFlag.FLAG_4
& ~MyFlag.FLAG_5
& ~MyFlag.FLAG_6
)


def test_flag_cached():
f = MyFlag.NONE
return f & INVERTED


if __name__ == "__main__":
start_time = time.time()
for _ in range(10_000):
test_flag()

elapsed = time.time() - start_time
print(f"Took normal {elapsed:2f} seconds.")

start_time = time.time()
for _ in range(10_000):
test_flag_cached()

elapsed = time.time() - start_time
print(f"Took cached {elapsed:2f} seconds.")
```

And its outputs:
```
Took normal 1.799731 seconds.
Took cached 0.009488 seconds.
```

Basically, bitwise negation is very very slow. From what I can tell, it seems 
that a lot of time is spent here computing powers of two. I've read elsewhere 
that flag values are cached, and it looks like negated Flag values can't be 
cached? This seems related to the second issue, which is that any negated Flag 
value being defined results in `RecursionError: maximum recursion depth 
exceeded` as it searches for a good name for Flag.

Obviously, the simple workaround is just to define a constant variable 
elsewhere with the negated value, but it isn't very obvious anywhere that this 
is necessary, and I wanted to raise this to see if anyone has knowledge of the 
implementation details of Flag for possibly resolving this in the class itself.

--
messages: 384983
nosy: aspin2
priority: normal
severity: normal
status: open
title: enum.Flag ~ bitwise negation is very slow and can't be defined as a Flag 
value
versions: Python 3.8

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com