How do you rerun "tox" on python file changes?

I have a tox configuration that runs all my tests, pep8 checks, and coverage checks. I run tox manually pretty much every time I save changes to my code. Tox runs the tests and coverage reports via "nosetests."

How do I make the re-running of tox happen automatically when changes are saved to python files anywhere within the directory structure beneath my tox.ini file?