python - mock and unit test a chain of methods that change input list

Question

I have a list of filters and each filter changes the input list. How do I create a mock for each filter that will change the input list?

class TwoFilter(object):

    def filter(self, arr):
        arr[:] = [i for i in arr if i % 2 != 0]

class ThreeFilter(object):

    def filter(self, arr):
        arr[:] = [i for i in arr if i % 3 != 0]

class FourFilter(object):

    def filter(self, arr):
        arr[:] = [i for i in arr if i % 3 != 0]

class MyFilters(object):

    def __init__(self):
        self.filters = [TwoFilter(), ThreeFilter(), FourFilter()]

    def apply_filters(self, arr):
        for f in self.filters:
            f.filter(arr)

I want to unit test apply_filters by mocking the filters in self.filters for input [1,2,3,4]. Is there a way to have each mock change the input arr and verify if each subsequent filter was called with this modified arr?

P.S: I am able to get around this problem by having the filters return the arr and use mock.return_value to change the mock's output.

Answer 1

As you spoke of mocking the filter classes, I take it that your approach is to also test the filter classes individually as well? This should certainly still be done, and better done as the first step - if I am not mistaken reading your code, you will likely discover a bug then.

Regarding the test of apply_filters: Why do you want to mock the filter classes at all? There seems to be no justification - you can just call apply_filters with some suitable values for arr to see if the other filters are actually called. And, maybe you find the next bug then (after correcting the first) - just try to find a test case that fails if the third filter is not called.

Mocking should be done for a reason - take a look at When to use mock objects in unit tests.