You can try PyGTK; it's cross-platform, well documented and many apps have been written with it for you to refer and learn.
Although I've to mention that wxWidgets (wxPython is its python binding) is a native mode toolkit; it's a thin abstraction layer over the platform's native tools i.e. say a textbox control in wxWidgets will not be written from scratch unlike GTK+, Qt, etc. but it'll actually expose the original platform's text box in a consistent manner across all platforms like Windows, Linux, Mac, etc. In that way it's different than the others and better in some areas.
Edit: Came to know about Kivy which definitely deserves a mention here, since it has more to offer than the aforementioned toolkits; it can run not only on desktop platforms, but also on portable devices i.e. it runs on Windows, Linux, Mac AND iOS (iPad, iPod, iPhone), Android. It's also GPU accelerated (uses Open GL ES2).
Refer GUI Programming in Python for a comprehensive list of toolkits available.
I'd suggest you try out a small prototype kinda app in both and see for yourself; since these are only suggestions, it's always what the programmer finds best at home in the end.