I want to hire a new programmer into my group. I know, that standard interview questions like "find where the program will crash" and "make a fast implementation of some widespread algorithm" won't be enough. And sometimes even not relevant at all.

Because even a superstar of hackathones has to work in a team. And his code must be usable and understandable by his workmates who have to use and mantain it.

Cannot you show me some good tasks for job interviews, that will highlight candidate's skills in writing not only working and not even fast, but also understandable and easy-to-use code?

I think there should be something like that:

1) Design and implement a class with given functionality in most human-readable and user-friendly way.

2) Code refactoring: given a piece of rotten code full of odd an obscure places, need to refactor it to good usability.

P.S. Our group specializes on DSP and numeric calculations and it will be the best if the tasks would be from this domain. Our main programming languages are Python and, a bit less, C++.