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Trying to iterate over typing.Union results in hang #92261
Description
If you accidentally iterate over a Union, you get a hang:
from typing import Union
list(Union)
# ...hangIt looks like it's an infinite loop:
from typing import Union
for i in Union:
print(i)
# 0, 1, 2, ...As far as I can tell, this happens because typing.Union is implemented using typing._SpecialForm, and _SpecialForm is implemented using __getitem__ to support parameterisation like Union[int]. Unfortunately, because _SpecialForm doesn't implement __iter__, __getitem__ is also used for iteration - meaning that when trying to iterate, we effectively do Union[0], Union[1], etc.
Activity
JelleZijlstra commentedon May 3, 2022
We should deprecate
itertools.countbecause it's now redundant.More seriously, agree that we should disallow this.
gh-92261: Disallow iteration of Union (and other special forms) (GH-9…
mrahtz commentedon May 8, 2022
Fixed in 4739997.
pythongh-92261: Disallow iteration of Union (and other special forms) (…
gh-92261: Disallow iteration of Union (and other special forms) (GH-9…