Python has a facility to add "pending" calls, which get called as a part of
the main interpreter loop.

These functions are added with Py_AddPendingCall().  If no python code is
being executed, then Py_MakePendingCalls() must be called in order to flush
the pending calls.  This function returns negative on an exception.

Currently, the signal handling code is the only thing I know of that uses
this code, but there may be others.

The gobject or gtk modules should set up a main loop source that calls
python pending calls.  This would make the signal module work correctly in
pygtk programs.