Returning optional union from method complains about `got "object"`

[Garrett-R]

Bug Report

Mypy thinks my method returns "object" which is not right.

To Reproduce

Run Mypy on this file:

from typing import Optional


class Cat:
 pass


class Dog:
 pass


class MyClass:
 def __init__(self) -> None:
 self.pet: Optional[Dog | Cat] = None

 def get_pet(self) -> Optional[Dog | Cat]:
 if input() == 'yes':
 self.pet = Cat()
 else:
 self.pet = Dog()
 return self.pet # (incorrect Mypy error on this line!)

Expected Behavior

This should have no errors.

Actual Behavior

It yields:

error: Incompatible return value type (got "object", expected "Union[Dog, Cat, None]")

Your Environment

• Mypy version used: v0.950
• Mypy command-line flags: none
• Mypy configuration options: none
• Python version used: 3.10.4
• Operating system and version: Ubuntu 20.04

[erictraut]

Looks like another case where a false positive results from using a join operation rather than a union.

[hauntsaninja]

Note the following works:

 def get_pet(self) -> Dog | Cat | None:
 pet: Dog | Cat | None = None
 if input() == 'yes':
 pet = Cat()
 else:
 pet = Dog()
 return pet

Looks like we're losing the type context of the instance variable.

[sterliakov]

Fixed in #18538 and passes on current master.