Remove spy method

[pared]

In some places, we use tests/utils/spy method to mock spied method so that we can observe how particular actions interact with the target method/function. Current implementation:

def spy(method_to_decorate):
    mock = MagicMock()

    def wrapper(self, *args, **kwargs):
        mock(*args, **kwargs)
        return method_to_decorate(self, *args, **kwargs)

    wrapper.mock = mock
    return wrapper

assumes that we are spying on some class method (by having self in wrapper args). This is not always true, and it leads to hiding first argument if we will use it to spy on function (eg file_md5) which results in trimmed call args for functions.

Example:

def test_spy(tmp_dir):
    tmp_dir.gen("file", "file content")

    from tests.utils import spy
    import dvc
    checksum_spy = spy(dvc.utils.file_md5)
    checksum_spy("file")
    assert checksum_spy.mock.call_args_list[0].args == ("file",)

should pass, while it does not because args tuple is empty.

[EDIT]
As @MrOutis noted, pytest's mocker can spy, so it would be good to remove current spy usages and replace them with mocker.spy.

@pawel, there's a mocker.spy already: https://pypi.org/project/pytest-mock/

maybe we could move to that one?

[pared]

@MrOutis Thanks for pointing that out! Renaming the issue.

[fabiosantoscode]

Hey there! I'm on this one :)

[pared]

@fabiosantoscode
Sure, I just would like to note that most use cases of our spy are in UnitTest style tests, so removal might go along with refactoring said tests to pytest style tests.

[fabiosantoscode]

@pared

I've gotten around that with this small TestDvc subclass:

class TestDvcWithMocker(TestDvc):
    @pytest.fixture(autouse=True)
    def use_mocker(self, mocker):
        self.mocker = mocker     

Then subclassing it and accessing self.mocker. I didn't want to go ahead and convert the tests because it felt out of scope, although I do agree that they should be converted.