Unable to call custom method

Answer 1

It's a bit hard to tell, but the problem could be with your indentation. The indentation in the code you've pasted is inconsistent, so it wouldn't work properly anyway - I suspect in your actual code, def retrieve_rankdata is a couple more spaces to the left.

Don't forget that Python uses indentation to tell whether or not an attribute or a method is part of a class. So if your def retrieve_rankdata line is not actually indented at the same level as the def __unicode__ one, it won't be considered part of the Problem class.