looking for good examples of source from large Python projects

I was recently asked for some advice about Python source code that would serve as a good example of organization, style, and documentation (including comments in the code itself). I had a few suggestions for projects to look at, but I wonder what other people think about the question.

Can you point to a project with source that you would hold up as an example of how to set up a medium to large code base? What makes it a good example?