PDB's quit command works by raising an exception (Bdb.BdbQuit). If that exception gets caught, I cannot figure out a way to kill the program short of killing the entire shell. CTRL+C works by raising a KeyboardInterrupt exception, which can also be caught.
You can recreate this problem with this simple script.
foo = 0
while True:
try:
import pdb; pdb.set_trace()
foo += 1
except:
pass
This script cannot be stopped from within PDB with the quit command or CTRL+C.
I'm aware this is bad programming and you should never use an except without an exception type. I ask because I ran into this issue while debugging and a third-party library trapped me in the loop.