Unfortunately I don't get time to add unit tests or improve on this anymore (as I don't use it much if at all) but there are other forks of fysom that you should definitely check out:
Maximilien Riehl's smoothened fork with 100% test coverage and install scripts. He's also made it available through PyPi (easily installable using
pip install fysom).
Bartosz Ptaszynski's beefed up version with per event+state callbacks and multiple destination event choices.
Below is the list of other ports of this library:
See the pydoc in fysom.py