Python libraries have (or should have) built-in documentation through docstrings. Furthermore, python code is (mostly) very readable, and reading the source (.py or even .c) is actually the preferred way for many developers to get the information they are looking for, especially since some corner cases may not even be documented.
I found myself looking at the source from time to time, as if it was a natural step in finding functionality, either because I was curious how they solve the problem, or because I find it faster than problems with obscure search engines and reading SO questions.