Python Mock Patch multiple methods in a class

Question

Im trying to patch multiple methods in a class. Here is my simplified set up

Hook.py is defined as

class Hook():
    def get_key(self):
        return "Key"

    def get_value(self):
        return "Value"

HookTransfer.py defined as

from Hook import Hook

class HookTransfer():
    def execute(self):
        self.hook = Hook()
        key = self.hook.get_key()
        value = self.hook.get_value()
        print(key)
        print(value)

I want to mock the methods get_key and get_value in the Hook class. The following works ie prints New_Key and New_Value

from HookTransfer import HookTransfer
import unittest
from unittest import mock

class TestMock(unittest.TestCase):
    @mock.patch('HookTransfer.Hook.get_key', return_value="New_Key")
    @mock.patch('HookTransfer.Hook.get_value', return_value="New_Value")
    def test_execute1(self, mock_get_key, mock_get_value):
        HookTransfer().execute()

if __name__ == '__main__':
    unittest.main()

However this does not. It prints <MagicMock name='Hook().get_key()' id='4317706896'> and <MagicMock name='Hook().get_value()' id='4317826128'>

from HookTransfer import HookTransfer
import unittest
from unittest import mock

class TestMock(unittest.TestCase):
    @mock.patch('HookTransfer.Hook', spec=True)
    def test_execute2(self, mock_hook):
        mock_hook.get_key = mock.Mock(return_value="New_Key")
        mock_hook.get_value = mock.Mock(return_value="New_Value")
        HookTransfer().execute()

if __name__ == '__main__':
    unittest.main()

Intuitively it seems like the second one should work too but it doesnt. Could you help explain why it does not. I suspect it has something to do with "where to patch" but Im unable to get clarity.

Answer 1

You can patch multiple methods of a module or a class using patch.multiple() . Something like this should work for your case:

import unittest
from unittest.mock import MagicMock, patch

class TestMock(unittest.TestCase):
    @patch.multiple('HookTransfer.Hook',
                    get_key=MagicMock(return_value='New_Key'),
                    get_value=MagicMock(return_value='New_Value'))
    def test_execute1(self, **mocks):
        HookTransfer().execute()

When patch.multiple() is used as a decorator, the mocks are passed into the decorated function by keyword, and a dictionary is returned when it's used as a context manager.

Answer 2

What you need to is:

mock the class Hook,

from HookTransfer import HookTransfer
from Hook import Hook

import unittest
try:
    import mock
except ImportError:
    from unittest import mock

class TestMock(unittest.TestCase):
    @mock.patch.object(Hook, 'get_key', return_value="New_Key")
    @mock.patch.object(Hook, 'get_value', return_value="New_Value")
    def test_execute1(self, mock_get_key, mock_get_value):
        HookTransfer().execute()

if __name__ == "__main__":
    unittest.main()

Answer 3

After some testing I was able to find the issue.

In the second test case, the patch decorator creates a new instance of a Mock class and passes it via mock_hook argument to test_execute2 function. Lets refer to this as mock1. mock1 replaces the Hook class in HookTransfer.py. When self.hook = Hook() is run, it translates to calling __init__ of mock1. By design this returns yet another Mock instance - lets refer to this as mock2. So self.hook points to mock2. But mock_hook.get_key = mock.Mock(return_value="New_Key") , mocks the methods in mock1.

In order to mock correctly, mock2 needs to be patched. This can be done in 2 ways

1. By mocking the return_value of mock1 (which returns mock2) mock_hook.return_value.get_key = mock.Mock(return_value="New_Key")
2. Mocking the return value of constructor of mock1 (which returns mock2) mock_hook().get_key = mock.Mock(return_value="New_Key")

Under the wraps both options really do the same thing.