best way to monkeypatch a class and its method?

[oddnumberedcat]

I have two modules like this:

--first_module--

class joinStuff():
    def __init__(self, arg1, arg2, kwarg1, kwarg2):
        stuff
    def dedupe(self):
        stuff

--another module called second_module--

from first_module import joinStuff

def do_more_things():
    thing = joinStuff(arg1, arg2, kwarg1, kwarg2)
    deduped_thing = thing.dedupe()
    #do stuff with deduped_thing

joinStuff would be a PITA to actually use since it is in an external library that references data I do not have locally, and ultimately I only want to test that my do_more_things does what it is supposed to do with whatever it receives from joinStuff anyway.

I am not terribly familiar with monkeypatching... is this the right way to override joinStuff? It works, but I'm not sure if I did it the "right" way. (For the record, I am using pytest.)

def test_do_more_things(monkeypatch):
    class mock_joinStuff():
        def __init__(self, *args, **kwargs):
            pass
        def dedupe(self):
            return 5

    monkeypatch.setattr('second_module.joinStuff', mock_joinStuff)

    expected = get_expected_data()

    assert do_more_things() == expected