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False positive unreachable with redefined variable in loop #8865
Description
When checking the following code:
from typing import AbstractSet, Iterable, Optional, Set, TypeVar
T = TypeVar('T')
def intersection(seq: Iterable[AbstractSet[T]]) -> Optional[Set[T]]:
ret = None
for elem in seq:
if ret is None:
ret = set(elem)
else:
ret &= elem
return retmypy reports:
$ mypy --warn-unreachable intersection.py
intersection.py:11: error: Statement is unreachable
This statement is the ret &= elem inside the else block.
It seems that mypy is using the initial type of ret to conclude that ret is None is always true, ignoring the fact that inside the loop ret is redefined to a different type.
If I place a reveal_type just before the return, mypy does report Union[builtins.set[T`-1], None] as the type, so it figures out the redefinition eventually.
I'm using mypy 0.770 on Pyton 3.8.2, with the --warn-unreachable flag.
Activity
cc: Rewrite intersection() in a way mypy understands
beezee commentedon May 22, 2020
Does
ret: Optional[Set] = Noneclear the error?mthuurne commentedon May 22, 2020
Yes, which is further evidence that the problem is in the order in which mypy does things: it seems to flag the code as unreachable before the final type for
rethas been deduced.sbalmer commentedon Mar 9, 2021
I stumbled over this and created a minimal example:
The last line contains an error. The variable
maximumshould be passed, but instead, the string"maximum"is passed. Mypy does not detect this because it assumes the last line is unreachable.Tested with Mypy 0.812 on Python 3.6.9.
mthuurne commentedon Feb 7, 2024
I've got a slightly more minimal test case for what I think is the same problem:
Output of mypy 1.8.0:
Expected output:
It seems mypy is able to correctly infer the type of
prevafter the loop (line 7), but doesn't feed this back into the start of the loop body (line 4).Avoid false `unreachable` and `redundant-expr` warnings in loops.
Avoid false `unreachable` and `redundant-expr` warnings in loops. (py…