I've actually only used Django, Flask, and CherryPy for Python projects, so I haven't researched that stuff.
In general, I strongly support the idea of tying together tiny libraries, rather than using a framework. It takes months to learn every line of code in an end-to-end framework, but if you're pulling in just the 2-3 libraries you need, you can understand every line of your code. That's really important for efficiency/bugfixing/maintenance.
You might want to check out Bobo: http://bobo.digicool.com/en/latest/index.html