Error If Return Annotation Is Provided (Generics + TypeVar + isinstance) #12873

julzt0244 · 2022-05-26T13:59:05Z

I get this strange error whereby adding a return annotation causes mypy to fail to identify the code after the "isinstance" block.
Mypy gives an error on the last line of ErrorIfReturnIsAnnotated.foo() while it doesn't give any errors for WorksIfNoReturnAnnotation.foo()

The only difference in those two classes are that one of them declares an explicit None return annotation.

Environment

Mypy version used: 0.950 and 0.960
Mypy command-line flags: None
Mypy configuration options from mypy.ini (and other config files): None
Python version used: Python 3.10.4
Operating system and version: Windows 10 (Version 21H2)

Full simplified test code to reproduce the bug

from abc import ABCMeta
from typing import Generic, TypeVar, Union


class BaseEngine(metaclass=ABCMeta):
    pass


class BaseEngineVariantA(BaseEngine):
    pass


class BaseEngineVariantB(BaseEngine):
    pass


class VariantAConcreteEngine(BaseEngineVariantA):
    pass


T = TypeVar('T', bound=Union[BaseEngineVariantA, BaseEngineVariantB])


class IBase(Generic[T], metaclass=ABCMeta):
    def __init__(self, engine: T):
        self.engine = engine


class IAction(IBase[T], metaclass=ABCMeta):
    pass


class ErrorIfReturnIsAnnotated(IBase[T]):
    def __init__(self, something: T, actions_injected: type[IAction[T]]) -> None:
        self.something = something
        self.actions_injected = actions_injected

    def foo(self) -> None:
        if not isinstance(self.something, VariantAConcreteEngine):
            raise Exception("HI")

        self.actions_injected(self.something)  # Gives a mypy error of - error: Argument 1 to "IAction" has incompatible type "VariantAConcreteEngine"; expected "T"


class WorksIfNoReturnAnnotation(IBase[T]):
    def __init__(self, something: T, actions_injected: type[IAction[T]]) -> None:
        self.something = something
        self.actions_injected = actions_injected

    def foo(self):
        if not isinstance(self.something, VariantAConcreteEngine):
            raise Exception("HI")

        self.actions_injected(self.something)  # No errors

The text was updated successfully, but these errors were encountered:

JelleZijlstra · 2022-05-26T14:06:43Z

https://mypy.readthedocs.io/en/stable/common_issues.html#no-errors-reported-for-obviously-wrong-code

julzt0244 · 2022-05-26T14:12:50Z

Sorry can you explain why that is the case? I assume you're referring to "The function containing the error is not annotated" but the code for IAction has a init in its superclass IBase that dictates the first argument to be "T"

JelleZijlstra · 2022-05-26T14:13:47Z

Mypy reports no errors in unannotated functions by default. It doesn't matter whether the function they're calling is annotated.

julzt0244 · 2022-05-26T14:15:20Z

I see thanks, is there a way for me to fix the mypy error for the first class ErrorIfReturnIsAnnotated without having to manually cast "cast(T, self.something)" after the isinstance block then? (That would be an incorrect cast as far as I know)

I would have assumed "actions_injected: type[IAction[T]]" would be happy to take in a concrete version (VariantAConcreteEngine) of one of its bound objects (BaseEngineVariantA) declared in the TypeVar

julzt0244 added the bug mypy got something wrong label May 26, 2022

JelleZijlstra closed this as not planned Won't fix, can't repro, duplicate, stale May 26, 2022

julzt0244 mentioned this issue May 27, 2022

TypeVar error #12882

Closed

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Error If Return Annotation Is Provided (Generics + TypeVar + isinstance) #12873

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