My favorite resource for learning how to test your code is http://getpython3.com/diveintopython3/unit-testing.html

Once you start testing your code, it's addictive. Testing gives you confidence.

- By forcing you to detail your requirements, writing tests helps you make sure that you're building the right thing.

- Testing lets you refactor your code with confidence -- if your old tests still pass after you refactor, your new version works the same way as your old version.

- Testing makes your code much (MUCH!) more maintainable. As long as your old tests still pass, you can be confident that the new code you wrote didn't break any of your old code.