My code is organized as several independent classes. Each dependency is injected by a setter, so in the end I create instances of classes B and C, set their parameters and inject them to class A.

Now I want to test class A in isolation, so my first intuition is to create mocks for B and C. My problem is that methods from B and C are called multiple times with different parameters and return different results. How can I mock that?


b = B()
c = C()
a = A()

    X = self.get_b().method1(x) # should return 1
    Y = self.get_b().method2(y) #s hould return 2
    Z = self.get_c().method1(z) # should return a complicated dataframe
    W = self.get_c().method2(w)# should return yet another complicated dataframe
    # process W X Y Z ...

Everything I read on mocking in python is limited to very simple examples where mocking can happen locally.

I know I can set a sequence of return values, but this is limited to a single method and is not future proof if the code logic changes.

Is it even possible or correct to test such a hierarchy?

  • why did you set_param1 twice with different values for class B and C?
    – Yu Zhang
    Commented Jul 30, 2018 at 22:27
  • Regarding is not future proof if the code logic changes, there is no such thing a future proof in my personal opinion. We will always need to go back to modify our old codes in some way to accommodate future changes.
    – Yu Zhang
    Commented Jul 30, 2018 at 22:32
  • @YuZhang thanks for pointing this out. I corrected the typo. By future proof, I meant that if I use this sequence feature, whenever A calls B methods in a different sequence I'll have to rework on B mock, when in fact functionality should be the same. Also if I have several successive calls with the same parameters, I'll have to copy the returned object multiple times.
    – Samuel
    Commented Jul 31, 2018 at 7:42
  • @Samuel Do you know the side_effect attribute of the class Mock: see documentation of side_effect
    – User051209
    Commented Jul 10 at 12:04

1 Answer 1


While there's not a lot of details, the unittest.mock library should handle this for you? I think this is what you mean when you say "I know I can set a sequence of return values, but this is limited to a single method"

As for future-proofing, if you're trying to test A in isolation, you're verifying that A works with the contracts to B and C not changing. If the interfaces to B and C change, tests there should catch that, and you'll need to update your mocks/tests in A.

  • Please see my comment to @YuZhang in original question. Also, how would I mock two methods of B?
    – Samuel
    Commented Jul 31, 2018 at 7:49
  • For mocking two methods of B, you just define them, something like mockB = B(); mockB.method1 = MagicMock(return_value=1); mockB.method2 = MagicMock(return_value=2)? Make sure to define the mock in the appropriate context. For a fancier solution, you could use patch(), which has notes about scope
    – ernie
    Commented Jul 31, 2018 at 17:35

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