# in example.py

def inner_func(x=None):
    if x is None:
        return 1

    return x + 1


def big_func():
    a = inner_func()
    b = inner_func(a)
    c = inner_func(b)

    return c


# in mocks.py
from unittest.mock import Mock

class MyMock(Mock):
    def __call__(self, *args, **kwargs):
        if isinstance(self.return_values, MyMock):
            """
            By default, when you try to access a property of a Mock which is not set
            you will get a Mock object from the same type (in this case MyMock).

            Because of the way unittest treats our mocks, we cannot use neither the self.__class__
            nor type(self) in the if-condition.
            """
            raise ValueError('Provide `return_values` to the mock')

        super().__call__(*args, **kwargs)

        call = self.call_count - 1
        if (len(self.return_values) >= call):
            return self.return_values[-1]

        return self.return_values[call]

# in tests.py

from unittest.mock import patch
from unittest import TestCase

from .example import big_func
from .mocks import MyMock

class BigFuncTests(TestCase):
    @patch('<path_to_inner_func>.inner_func', new_callable=MyMock)
    def test_mega_shano_use_case(self, inner_mock):
        inner_mock.return_values = [1, 2, 10]
        res = big_func()

        self.assertEqual(res, 10)