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Returning optional union from method complains about got "object" #12823

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Garrett-R opened this issue May 20, 2022 · 3 comments
Closed

Returning optional union from method complains about got "object" #12823

Garrett-R opened this issue May 20, 2022 · 3 comments
Labels
bug mypy got something wrong topic-join-v-union Using join vs. using unions topic-type-context Type context / bidirectional inference

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@Garrett-R
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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
@Garrett-R Garrett-R added the bug mypy got something wrong label May 20, 2022
@erictraut
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Looks like another case where a false positive results from using a join operation rather than a union.

@JelleZijlstra JelleZijlstra added the topic-join-v-union Using join vs. using unions label May 20, 2022
@hauntsaninja hauntsaninja added the topic-type-context Type context / bidirectional inference label May 22, 2022
@hauntsaninja
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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
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Fixed in #18538 and passes on current master.

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bug mypy got something wrong topic-join-v-union Using join vs. using unions topic-type-context Type context / bidirectional inference
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@JelleZijlstra @Garrett-R @erictraut @hauntsaninja @sterliakov