New submission from Kevin Chen <kevinjche...@gmail.com>:
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 <rep...@bugs.python.org>
<https://bugs.python.org/issue42915>
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