How to properly mock private members of a class

Question

I am trying to write some unit tests for a method that depends on another private method. - As shown in the example below:

def is_member_of(self, group_name):
  members = self.__get_group_members(group_name)

The private method that I'd like to mock is __get_group_members; I'd also like to mock the private attribute __user_id since it will be used in the is_member_of function (not shown in the example above).

What I have so far:

import unittest
from unittest import mock

class Test(unittest.TestCase):
  group_data = []
  user_id = 'test_user_id'

  def mock_dependencies(self, x):
    x.__user_id = mock.PropertyMock(return_value=self.user_id)
    x.__get_group_members = mock.MagicMock(return_value=self.group_data)

  def first_test(self):
    x = A(('name', 'group'))
    self.mock_dependencies(x)
    x.is_member_of('test_group')

When I invoke x.is_member_of() the mocking doesn't work as anticipated.

Answer 1

You can access a private attribute in Python since private and protected are by convention. - What you're looking for is basically using _ClassName__private_attribute_name since, python carries out the renaming in order to achieve the convention agreed upon.

Example (returning a MagicMock):

with mock.patch.object(Class, '_ClassName__private_attribute_name', return_value='value') as obj_mock:
    pass

Example (returning a raw value):

with mock.patch.object(Class, '_ClassName__private_attribute_name', new_callable=PropertyMock) as obj_mock:
    obj_mock.return_value = 'string value'

Class is a reference to the class itself - not the instance.

Complete Example:

from unittest.mock import patch, PropertyMock
from unittest import TestCase, main


class Private:
    __attr = 'hello'


class PrivateTest(TestCase):
    @patch.object(Private, '_Private__attr', new_callable=PropertyMock)
    def test_private_attribute_value_change_decorator_success(self, private_mock):
        obj = Private()
        private_mock.return_value = 'string'

        self.assertEqual('string', obj._Private__attr)

    def test_private_attribute_value_change_context_manager_success(self):
        with patch.object(Private, '_Private__attr', new_callable=PropertyMock) as o_mock:
            obj = Private()

            o_mock.return_value = 'mocked value'
            self.assertEqual('mocked value', obj._Private__attr)


if __name__ == '__main__':
    main()

Modifications to your example:

from unittest import TestCase, mock, main


class A:
    __user_id = 3

    def __init__(self, user, group):
        """
        Your logic is missing - obviously
        :param user:
        :param group:
        """


    def __get_group_members(self):
        """
        Your logic is missing - obviously
        :return:
        """
        return ['user_1', 'user_2']

    def is_member_of(self, group_name):
        members = self.__get_group_members(group_name)

        # will return if the user is a member of the group
        return self.__user_id in members


class GroupTest(TestCase):
    group_data = [1, 2]
    user_id = 'test_user_id'

    @mock.patch.object(A, '_A__get_group_members')
    @mock.patch.object(A, '_A__user_id', new_callable=mock.PropertyMock)
    def test_this_is_my_first_success(self, user_id_mock: mock.PropertyMock, get_group_members_mock: mock.MagicMock):
        get_group_members_mock.return_value = self.group_data
        user_id_mock.return_value = 3

        x = A('user_3', 'this_group')
        self.assertEqual(False, x.is_member_of('test_group'))

    @mock.patch.object(A, '_A__get_group_members')
    @mock.patch.object(A, '_A__user_id', new_callable=mock.PropertyMock)
    def test_this_is_my_first_failure(self, user_id_mock: mock.PropertyMock, get_group_members_mock: mock.MagicMock):
        get_group_members_mock.return_value = self.group_data
        user_id_mock.return_value = 1

        x = A('user_1', 'this_group')
        self.assertEqual(True, x.is_member_of('test_group'))


if __name__ == '__main__':
    main()

If you know you'll mock these two attributes in all test cases you can add the decorators on the class level and expect the arguments like-wise.

In the case where the attribute is set through the __init__ or any other method, you could simply alter it as shown below.

from unittest import TestCase, mock, main


class A:
    def __init__(self, user, group):
        """
        Your logic is missing - obviously
        :param user:
        :param group:
        """


    def __get_group_members(self):
        """
        Your logic is missing - obviously
        :return:
        """
        return ['user_1', 'user_2']

    def is_member_of(self, group_name):
        members = self.__get_group_members(group_name)

        # will return if the user is a member of the group
        return self.__user_id in members


class GroupTest(TestCase):
    group_data = [1, 2]
    user_id = 'test_user_id'

    @mock.patch.object(A, '_A__get_group_members')
    def test_this_is_my_first_success(self, get_group_members_mock: mock.MagicMock):
        x = A('user_3', 'this_group')

        x._A__user_id = 5
        get_group_members_mock.return_value = self.group_data

        self.assertEqual(False, x.is_member_of('test_group'))

    @mock.patch.object(A, '_A__get_group_members')
    def test_this_is_my_first_failure(self, get_group_members_mock: mock.MagicMock):
        get_group_members_mock.return_value = self.group_data

        x = A('user_1', 'this_group')
        x._A__user_id = 1
        self.assertEqual(True, x.is_member_of('test_group'))


if __name__ == '__main__':
    main()