mypy complains Incompatible return value type (got "Optional[str]", expected "str") when a function can only return str

Question

import os
from typing import Optional

_DEFAULT = 'abc'

def _get_value(param: Optional[str]) -> str:
    return param or os.getenv("PARAM", _DEFAULT)

For this function, mypy would complain

Incompatible return value type (got "Optional[str]", expected "str")

but I thought this function will never return None. Am I missing something?

Answer 1

The mypy type checker doesn't seem to be able to parse or conditions. You have to explicitly check for None values:

if param:
    return param
else:
    return os.getenv("PARAM", _DEFAULT)

Edit: The above code technically checks for falsy values instead of None but it is functionally equivalent to your example.