Unit Test draft

I prepared the function I wrote for a peer code review presentation I will be doing tomorrow. I added documentation to the function elements, and tidied up the function so it looks like "clean" python code. I also wrote of a draft of a unit test for the function. I need clarification on "writing a test that fails" from the main author. I'm a little turned around on how a successful test is one that fails the function. I think I followed the pattern correctly from the Python docs, but choosing the right "assert" function was confusing. Why are coding docs so hard to read? So many words and so little communication of information.