type test fails to infer correct class in else block about mypy HOT 5 CLOSED

Workaround is to change else: -> elif type(val) is Duration: but this should be unnecessary.

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Using isinstance instead of a type check doesn't report an issue but in this case where int is a parent class of Duration that test is incorrect.

def output(val: Duration | int) -> None:
    if isinstance(val, int): # Invalid due to Duration being a sub class of int
        print(f"type: {type(val)} = int: {val}")
    else:
        print(f"type: {type(val)} = duration: {val.nanoseconds()}") # Never called

from mypy.

Mypy's behavior is correct here. The type cannot be safely narrowed to just Duration in the "else" case. It's possible for other subtypes of int to be passed to the function, and they would not pass the test type(val) is int.

class OtherIntSubclass(int):
    pass

output(OtherIntSubclass(1))

You've already identified the correct way to fix this bug in your code — by using isinstance.

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Thanks for clarification, coming from other languages my expectation was for exact type matching requirement from hint and hence Duration | int meant only Duration or int exactly, so OtherIntSubclass was not valid.

I believe what you're saying is this is not the case and instead Duration or int means those classes or any subclasses, is that correct @erictraut?

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Answering my own question from PEP 484

Expressions whose type is a subtype of a specific argument type are also accepted for that argument.

Capturing for others the fix for this case is to invert the test an look for the more specific type, which is easy in this case given one is base type, not so easy for other types as you need know the types of the entire chain.

def output(val: Duration | int) -> None:
    if isinstance(val, Duration):
        print(f"type: {type(val)} = duration: {val.nanoseconds()}")
    else:
        print(f"type: {type(val)} = int: {val}")

Thanks for the quick response, closing as it is working as intended.

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