It's not. This code is an old (wrong) one. It worked at that time because the different files involved in a python kata where "brutally" merged together before execution, so preloaded, solution and tests parts where actually in the same global scope. And since the inputs are all defined at the root of file of the tests, using the same name than the argument of the function, that worked while being effectiveley totally wrong (You can check in the fork panel, it throws a NameError).