The os module ships with python, so the function call mocking os.listdir will always succeed even if the code being tested does not use os.listdir By mocking m

The os module ships with python, so the function call mocking os.listdir will always succeed even if the code being tested does not use os.listdir By mocking mymodule.os.listdir you add a requirement that mymodule actually import the os module and take advantage of mock.patch failing loudly if it does not.