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

Question

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 fro

Answer 1

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