I'm about to embark upon extending and modifying PyUnit. For instance, I will add warnings to it, in addition to failures.
I'm interested in hearing words of advice on how to start, for instance, subclass every PyUnit class? What to avoid and misc caveats.
Looking for input from those that have extended PyUnit already.
I recommend studying the nose project, a popular and well designed extension of PyUnit. You can browse its sources online here or get a copy on your machine via Mercurial, aka hg, a nice distributed version control system in which nose keeps its sources on Google Code Hosting.
You may well disagree with some of nose's design decisions, but in general they have executed very well on those decisions, so the sources are worth studying anyway even if you decide that your extension will go in completely different directions.