Mock parent class __init__ method

Question

I am trying to write some unittests on a class which is derived from another, but I have some difficulties to mock the parent class init method, which afaik you cannot, so I am looking for suggestion.

Here an example of how are my classes

Imported.py

class Imported():
    def __init__(self, a="I am Imported"):
        print("a:{}".format(a))

Parent.py

from Imported import Imported

class Parent(object):

    parent_list = ["PARENT"]

    def __init__(self, a="I am Parent"):
        imported = Imported(a)

Derived.py

from Parent import Parent

class Derived(Parent):

    Parent.parent_list.append("DERIVED")

In my unittests I want to verify that Parent.parent_list == ["PARENT", "DERIVED"] when I instantiate an object from the Derived class, Derived().

Both of this solution doesn't work

test_Derived.py

import unittest
from mock import patch

from Derived import Derived


class test_Derived(unittest.TestCase):

    @patch("Derived.Parent.__init__")
    def test_init_001(self, mock_parent_init):
        a = Derived("I am Derived")
        mock_parent_init.assert_called_with("I am Derived")
        self.assertEquals(a.parent_list, ["PARENT", "DERIVED"])

    @patch("Derived.Imported.Imported")
    def test_init_002(self, mock_parent_init):
        a = Derived("I am Derived")
        mock_parent_init.assert_called_with("I am Derived")
        self.assertEquals(a.parent_list, ["PARENT", "DERIVED"])

test_init_001 fails with

TypeError: __init__() should return None, not 'MagicMock'

test_init_002 fails with

ImportError: No module named Parent.Imported

Any suggestion?

Solution

For the first solution, change the return value of the __init__ method to None.

@patch("Derived.Parent.__init__")
def test_init_001(self, mock_parent_init):
    mock_parent_init.return_value = None  # <---
    a = Derived("I am Derived")
    mock_parent_init.assert_called_with("I am Derived")
    self.assertEquals(a.parent_list, ["PARENT", "DERIVED"])

For the second solution, patch Parent.Imported:

@patch("Parent.Imported")  # <---
def test_init_002(self, mock_parent_init):
    a = Derived("I am Derived")
    mock_parent_init.assert_called_with("I am Derived")
    self.assertEquals(a.parent_list, ["PARENT", "DERIVED"])

Comments

[Chris]

I was trying to test that a class was initialized with the right parameters, then a method called on that instance correctly, by mocking __init__ and the method (let's call it foo()). The mocked foo method registered no calls until I set the return value of __init__ to None. (I assume this is because the initialization in the code was broken by the mock of __init__, although setting side_effect to the original __init__ method didn't work - the foo() mock still registered no calls.) Thanks this nearly defeated me!

[adrz]

In your first solution instead of the line mock_parent_init.return_value = None you could specify this thing right in the patch definition, ie. @patch("Derived.Parent.__init__", return_value=None).