Concise Way To Specify The Space Of All Python Programs Under A Certain Size And Running Time?
You can't determine the running time of a generic program automatically, this is an undecidable problem similar to the Halting problem. 

Maybe you can try to enumerate a very well defined subset of the possible codes up to certain size. But in order to be useful for an search/optimization algorithm (other than random search), this subset needs to be equipped with some sort of vicinity function. Moreover, this vicinity function should allow you to explore genuinely different programs. Preferably this function should form a connected space among all plausible programs in this subset, and have a short diameter so optimizer can succeed on the search. All that said, is pretty clear that the problem is far from being easy.

I read recently a very interesting post about applying genetic algorithm to produce brainfuck code to solve specific simple tasks. You can read it here. The post is from 2013, but there is no particular breakthrough in this area that I'm aware. This field is known as program-synthesis and there are a lot of teams working on it, most recently they are trying to use classic combinatorial algorithms boosted with deep learning to prune the search space.