How to make fun of a function defined in a separate Python module using mock @patch

I am trying to create a test for a Python application using mock and the @patch decorator.

Given the following directory structure:

  |-- mypackage
  |   |-- mymodule
  |   |   |-- __init__.py
  |   |   \-- somefile.py
  |   \-- myothermodule
  |       |-- tests
  |       |   |-- __init__.py
  |       |   \-- test_func_to_test.py
  |       \-- __init__.py
  \-- __init__.py

If the contents of the files:

#mypackage/mymodule/somefile.py

def some_function():
    return 'A'

#mypackage/myothermodule/__init__.py

from mypackage.mymodule.somefile import some_function

def func_to_test():
    return some_function()

#mypackage/myothermodule/tests/test_func_to_test.py

from unittest import TestCase
from mock import patch

class TestFunc_to_test(TestCase):
    def test_func_to_test(self):
        from mypackage.myothermodule import func_to_test
        self.assertEqual('A', func_to_test())

    @patch('mypackage.mymodule.somefile.some_function')
    def test_func_to_test_mocked(self, some_mock_function):
        from mypackage.myothermodule import func_to_test
        some_mock_function.return_value = 'B'
        self.assertEqual('B', func_to_test())

The problem is that although the first test passes (test_func_to_test), the second test (test_func_to_test_mocked) fails (due to AssertionError).

I was able to mock a function from the "built-in" modules (for example, request.get, for example) using the same approach, however I cannot get @patch to work when trying to fix a function from one of my modules ...

Any help would be appreciated :)