What subtleties does pdb / ipdb introudce to PySide?

Question

I have two PySide programs that use a UI with many of the same elements. I've constructed a UI that imports several shared QGroupBox and one that differs. One program works, the other locks up before the GUI appears, and no longer responds to keyboard interrupts.

But. I want to know why introducing set_trace() fixes things. The following:

 ui = UI(initValues)
 vcs.show()

hangs. The code successfully makes it through the instantiation, but then the show() never shows anything. However:

 ui = UI(initValues)
 ipdb.set_trace()
 vcs.show()

followed by typing "c" to the ipdb prompt and everything works as I expect it to. The GUI shows up and the program works fine. I've tried replacing the set_trace() with input() and with sleep(), both of which exhibit the original problem.