John Guttag Book - Finger Exercise 4 - need help to make the code better

[Larz60+]

There's a tool that helps, unittest: https://docs.python.org/3.6/library/unittest.html
but I don't know of any tool that's smart enough to test all cases. If you can identify all possible
inputs, and know all possible outputs, a truth table works well.
There is also a coverage tool: https://pypi.python.org/pypi/coverage/4.5.1
and others: https://pypi.python.org/pypi/coverage/4.5.1