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Type narrowing on object attribute with match statement doesn't apply #14911
Description
The bug
It seems that doing a "type checking" with the object UI in pattern matching seems to not work properly.
Attribute match is not considered.
Code
class Test:
def __init__(self, a: int | str):
self.a: int | str = a
test = Test(5)
match test:
case Test(a = int()):
print(test.a + 3)
case Test(a = str()):
print(test.a + "3")Expected behavior
By doing a match with Test(a = int()), python will check test is of type Test AND test.a is of type int.
But mypy "only assert" that test if of type Test. It keep the type int | str for test.a.
Reproduce :
On mypy-playground :
https://mypy-play.net/?mypy=latest&python=3.11&gist=6f05124abd1ef68032d59651a7d5c6c3
Reacted by Tom Kuson
Activity
erictraut commentedon Mar 16, 2023
For reference: microsoft/pyright#4775
cdce8p commentedon Aug 27, 2025
This should be fixed with #19736.
tjkuson commentedon Sep 24, 2025
Would it be possible to support narrowing across cases like this as part of this issue, or would it be a separate issue/not possible? I searched the issue tracker and this issue seemed to be the closest to what I'm looking for.
This works if I go back to using
isinstancewith ordinary if statements, but that would mean being unable to use pattern matching features.cdce8p commentedon Sep 24, 2025
@tjkuson With #19736 it would almost work. Just
assert_neverwould still throw an error as I haven't implemented the "else" narrowing. Similarly to how it isn't implemented for just the bound variable yet.... case Test(a=a): reveal_type(a) # int | str