> python -m pdb test.py

pdb commands:
h(elp):  getting help
w(here): print a stack trace, with most recent frame at bottom
d(own): move the current frame one level down in th estack trace (to a newer frame)
u(p): one level up
b(reak): to put a break point
s(tep):
n(ext):
c(ontinue):
a(rgs): print the argument list of the current function
q(uit):