Solution
How did you do? Here's our final version for comparison (make sure you don't copy them over yours!): testlatlong.py; latlong.py.
In building the tests, we're aiming at the very highest debugging circle: debugging before the code has even been written! We have reached a transcendental state of perfection! For more on Unit Tests, including how to structure multiple tests more rigorously and build interlinked tests, see the unittest library documentation.