Return None from python function annotated with mypy, multiple return types

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I come from a Typescript background. I'm bringing static type checking into a python project I'm working on (using mypy).

In Typescript, it is valid to return null from a function that is annotated to return something else, i.e. a string:

function test(flag: boolean): string {
    if(flag) {
        return 'success';
    } else {
        return null;
    }
}

It is also valid to annotate your function to have multiple potential return types, i.e. string or boolean:

function test(flag: boolean): string | boolean {
    if(flag) {
        return 'success';
    } else {
        return false;
    }
}

But, in python using mypy, I'm disallowed from returning None from a function that is annotated to return str.

def test(flag: bool) -> str:
    if flag:
        return 'success'
    else:
        return None
        # [mypy] error:Incompatible return value type (got "None", expected "str")

Furthermore, I don't see a way to annotate multiple return types, i.e. str | None.

How should I approach something like this using mypy? Functions that return None from the error state are all over my codebase.

mypy return-type static-typing function python
1 Answers

Okay, I found what I was missing in the documentation thanks to @zsol on the mypy gitter!

Two helpful mypy features are the Optional and Union types that can be imported from python's typing module. Documentation here.

If you want to annotate that the function can potentially return None in addition to the primary type, e.g. str, use Optional:

from typing import Optional

def test(flag: bool) -> Optional[str]:
    if flag:
        return 'success'
    else:
        return None

If you want to annotate that the function can potentially return multiple types, e.g. str | bool, use Union:

from typing import Union

def test(flag: bool) -> Union[str, bool]:
    if flag:
        return 'success'
    else:
        return False
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