Yeah if all you have is straight calls to functions, as soon as you do stuff like assign function to a variable, method calls you need to do sophisticated code analysis and that's not even touching the untraceable stuff like string/dynamic access, monkeypatching, etc. etc.

Refactoring in python is bad even if you restrict yourself to "sane" code (no metaprogramming and abusing dynamic stuff so the tools can follow what you're doing) if you need something that will work for everything out there it's just impossible to do.