Learning python by reading

[Sergi Pasoev]

Hello

Do you know some python programs that make good reading to learn the
language by studying and modifying the source code ? I often try to
understand things in the standard library, and apart from doing this
kind of search myself, I would be glad to hear some ideas from more
experienced programmers. 

Obviously, a good place to search is pypi and Python standard library,
but which packages/modules ?