Hypothesis is a useful one to use in Python. Particularly with its state-based testing that lets you describe actions, preconditions, and invariants. It then exercises the program by calling a random sequence of actions and seeing if everything holds as expected. Oh, trough some sequence the "save file" option ends up not saving the file? Good to know, record that seed and track down the problem using the supplied steps which you can now repeat automatically and manually to see where the problem occurred. Maybe add another invariant after doing some research and find it is breaking earlier in the sequence.

It's like having your QA/V&V/Test team doing exploratory testing but automated.