Python Mocking a function from an imported module

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I want to understand how to @patch a function from an imported module.

This is where I am so far.

app/mocking.py:

from app.my_module import get_user_name

def test_method():
  return get_user_name()

if __name__ == "__main__":
  print "Starting Program..."
  test_method()

app/my_module/__init__.py:

def get_user_name():
  return "Unmocked User"

test/mock-test.py:

import unittest
from app.mocking import test_method 

def mock_get_user():
  return "Mocked This Silly"

@patch("app.my_module.get_user_name")
class MockingTestTestCase(unittest.TestCase):

  def test_mock_stubs(self, mock_method):
    mock_method.return_value = "Mocked This Silly")
    ret = test_method()
    self.assertEqual(ret, "Mocked This Silly")

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

This does not work as I would expect. The "patched" module simply returns the unmocked value of get_user_name. How do I mock methods from other packages that I am importing into a namespace under test?

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