You should return the answer, and not print it. If you tried returning it and it still did not work, you probably have some other problem. Maybe this paragraph, or the overall article, will be helpful.

None means that your function did not return anything. If your function would have, for example, returned the number 3 the output would be: "3 should equal [2]"
You can return an empty list with:

list = []
return list

Presumably you aren't testing the test that it fails for
Suggestion: print the input