Right directory structure for testing

I'm new to Python testing and I want to set up an structure similar to the one you can have when testing Ruby with RSpec. So I followed this blog, but it's not working for me. My current structure is:

/brute-force
    /source
        __init__.py
        graph.py
    /tests
        __init__.py
        test_graph.py

When running nosetests on the root brute-force directory, it says that 0 tests were run.