Python has a lot of GUI libraries: tkinter, wxWidgets, pyGTK etc. But all these GUIs need to be installed and quite heavyweight, so it's a bit complex to deploy end-user GUI python apps that relay on mentioned GUI libraries.
Recently, I have thought about python's built-in ctypes module. Theoretically, it's possible to create a pure python GUI library that will use ctypes on windows ( windll.user32.CreateWindowEx, etc ), native pyObjC on MacOS and pyGTK / pyQt on gnome / kde. Does such a library exist? If not, what do you think is wrong with this idea?
Not really sure what you mean by "heavyweight."
wx uses native controls on each platform, and is about as easy to use in Python as I can imagine; after all, GUI APIs are complex because GUIs can get complex.
I think wx, for the effort required to build a window and the quality of what shows up on screen, is great.  I don't think you're likely to roll something better on your own.
Pure python gui library?