Inheritance + if/elif/else chain of isinstance() doesn't constrain type as expected

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).

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.

In my actual code, f is a method and obj is an attribute of the class which can be other subclasses of A.

…

On Fri, Apr 6, 2018, 4:54 PM Ivan Levkivskyi ***@***.***> wrote: 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. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#4864 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ADgvnq1th1oBsApaSy1rQDEGWtUnYqyVks5tmAAfgaJpZM4TJiqZ> .

This was fixed in #18538

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Inheritance + if/elif/else chain of isinstance() doesn't constrain type as expected #4864

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Inheritance + if/elif/else chain of isinstance() doesn't constrain type as expected #4864

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ilevkivskyi commented on Apr 6, 2018

osandov commented on Apr 7, 2018

hauntsaninja commented on Dec 27, 2025

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