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

user_856804 0
Viewed 1966
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?

mypy python
2 Answers

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.

user_20111470

mypy seems to be missing some inference of "old style ternary" functions -- those of the form A or B or A and B or C

looking at reveal_type for the three expressions

# reveal_type(param)
t.py:9: note: Revealed type is 'Union[builtins.str, None]'
# reveal_type(os.getenv("PARAM", _DEFAULT)
t.py:10: note: Revealed type is 'builtins.str'
# param or reveal_type(os.getenv("PARAM", _DEFAULT)
t.py:11: note: Revealed type is 'Union[builtins.str, None]'

you can get around this by using a true ternary:

def _get_value(param: Optional[str]) -> str:
    return param if param is not None else os.getenv("PARAM", _DEFAULT)
user_8121830
Related