I was following the steps at http://docs.python.org/library/profile.html
to try to profile a pygtk program, but it wasn't working -- I was
getting no output at all from either cProfile or profile.

The problem turned out to be that I was exiting by calling sys.exit(0),
because that's what most pygtk programs I'd seen were doing. Changing
that to gtk.main_quit() made profiling work (but it took a long time to
track that down).

Apparently the same thing happens in pyqt if you call sys.exit(0).