I know this is just a simple example but sum() doesn’t teach you about the concept of sums. It would have to be something like:

def sum_up(my_list):
    result = 0
    for item in my_list:
        result = result + item
    return result

Then you could run that through a debugger and see how the variables change at every step. That way you can develop an understanding of what’s going on there.