How to test function is called with correct arguments with pytest?

Question

I'm learning how testing is done in Python using py.test. I am trying to test a specific situation that is quite common when using other libraries like mock. Specifically, testing that a function or method invokes another callable with the correct arguments. No return value is needed, just a confirmation that the method under test makes the call properly.

Here's an example straight from the docs:

>>> class ProductionClass:
...     def method(self):
...         self.something(1, 2, 3)
...     def something(self, a, b, c):
...         pass
...
>>> real = ProductionClass()
>>> real.something = MagicMock()
>>> real.method()
>>> real.something.assert_called_once_with(1, 2, 3)

Is it possible to do this using monkeypatch or fixtures from py.test, without effectively writing my own mocked class? I have searched for this specific use case, but couldn't find an example. Does py.test encourage an alternative way of exercising code like this?

Answer 1

Well. I've come up with something that seems to work, but I suppose its similar to mock:

@pytest.fixture
def argtest():
    class TestArgs(object):
        def __call__(self, *args): 
            self.args = list(args)
    return TestArgs()

class ProductionClass:
    def method(self):
        self.something(1,2,3)
    def something(self, a, b, c):
        pass

def test_example(monkeypatch, argtest):
    monkeypatch.setattr("test_module.ProductionClass.something", argtest)
    real = ProductionClass()
    real.method()
    assert argtest.args == [1,2,3]