I don't understand what you mean by "will always work, even if it doesn't accomplish what you want." Mocking os.listdir will be useless if your product code's imports don't match it. How is this "one rule" to use?
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.
