How to make work typing annotation on mypy with unpacking params for pytest parameterized test?

Question

I have a simple method on my_module.py with the signature:

def my_method(value: float, extra: str = "something") -> str:
    pass

Then I have a parameterized test on test_my_module.py for it:

from typing import List
from typing import Union

import pytest

import my_module

@pytest.mark.parametrize(
    "params, expected",
    [
        ([1.0], "result1"),
        ([2.0], "result2"),
        ([2.0, "extra"], "result3")
    ],
)
def test_my_method(params: List[Union[int, str]], expected: str) -> None:
    assert my_module.my_method(*params) == expected

When I run dynamic type tests such as typeguard it passes. But with mypy I get these [arg-type] errors:

tests/test_my_module.py: note: In function "test_my_method":
tests/test_my_module.py:50:30: error: Argument 1 to "my_method" has incompatible type "*List[Union[int, str]]"; expected "float" 
[arg-type]
        assert my_module.my_method(*params) == expected
                                   ^
tests/test_my_module.py:50:30: error: Argument 1 to "my_method" has incompatible type "*List[Union[int, str]]"; expected "str" 
[arg-type]
        assert my_module.my_method(*params) == expected
                                   ^

Not really sure how should I annotate these types to make it pass. Note that this is a minimal example, so I want to stick unpacking the list of arguments params in the list I create to parameterize the tests.

Any thoughts?

Answer 1

You can annotate with Tuple instead of List:

@pytest.mark.parametrize(
    "params, expected",
    [
        ([1.0], "result1"),
        ([2.0], "result2"),
        ([2.0, "extra"], "result3")
    ],
)
def test_my_method(params: Union[Tuple[float], Tuple[float, str]], expected: str) -> None:
    assert my_module.my_method(*params) == expected

This promises a fixed number of elements and hence the static type checker can infer that the signature is compatible.