It's based on the idea of formal proofs for programs. There is a really good paper about this by dijkstra, but I can't seem to find it now. Basically you defi

It's based on the idea of formal proofs for programs. There is a really good paper about this by dijkstra, but I can't seem to find it now. Basically you defined preconditions and postconditions for each function and it looks like this "contracts" flag will raise an exception if those conditions aren't met. This is probably a better way to test software, but you still need to know how to write the preconditions and postconditions concisely.