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Inheritance + if/elif/else chain of isinstance() doesn't constrain type as expected #4864
Description
I have something like the following code:
from typing import Any, Union
class A:
pass
class B(A):
def __init__(self):
self.member = 1
class C(A):
def __init__(self):
self.member = 2
def f(obj: A) -> int:
if isinstance(obj, B):
pass
elif isinstance(obj, C):
pass
else:
raise ValueError()
return obj.member
mypy claims:
test.py:25: error: "A" has no attribute "member"
However, at line 25, it's clear that obj must be either a B or a C. If I remove the elif isinstance(obj, C) branch, then there is no error.
This similar code has a related error:
from typing import Any, Union
class A:
pass
class B(A):
def __init__(self):
self.member = 1
class C(A):
def __init__(self):
self.member = 2
def f(obj: A) -> Union[B, C]:
if isinstance(obj, B):
pass
elif isinstance(obj, C):
pass
else:
raise ValueError()
return obj
test.py:25: error: Incompatible return value type (got "A", expected "Union[B, C]")
Interestingly, if I remove the inheritance so that B and C are no longer subclasses of A, there is no error.
This happens on mypy 0.570 and current git (0.590-dev-3346cfad75374c096b1dd8024df24ce50f18a35c).
Activity
ilevkivskyi commentedon Apr 6, 2018
It looks like another case of join vs union problem plus a complex binder situation. Why can't you just use
obj: Union[B, C]in the signature of your function?I would say this is a relatively low priority. There may be some binder refactoring later, then this will probably be easier to fix.
osandov commentedon Apr 7, 2018
hauntsaninja commentedon Dec 27, 2025
This was fixed in #18538