What is the type hint for the pytest fixture "capsys"?

Question

When writing pytest tests for a function that is supposed to print something to the console, to verify the output string I am using the capsys fixture and capsys.readouterr().

This is the code I am currently using:

@pytest.mark.parametrize(
    "values,expected",
    [
        ([], "guessed so far: \n"),
        (["single one"], "guessed so far: single one\n"),
        (["one", "two", "three", "4"], "guessed so far: 4, three, two, one\n"),
    ],
)
def test_print_guesses(capsys, values: list, expected: str) -> None:
    hm.print_guesses(values)
    assert capsys.readouterr().out == expected

I am also using the mypy extension in VS Code, so for now I am getting the warning:

Function is missing a type annotation for one or more arguments

I'd like to get rid of that. What is the appropriate type annotation for the capsys argument?

Answer 1

Per the documentation for capsys, it:

Returns an instance of CaptureFixture[str].

This class indeed has a readouterr method (returning a CaptureResult, which has an out attribute). So your test should look like:

@pytest.mark.parametrize(
    # ...
)
def test_print_guesses(capsys: pytest.CaptureFixture[str], values: list, expected: str) -> None:
    hm.print_guesses(values)
    assert capsys.readouterr().out == expected