pdb.set_trace() causing frozen nosetests, does not drop into debugger

I'm running a suite of tests (.py files) using nosetests. Using a classic

import pdb; pdb.set_trace()

the nosetests run just never completes. It just hangs right where the breakpoint has been set, but never drops into the pdb debugger.

Any ideas why this would be? I've tried moving the breakpoint to a number of different positions (other test functions, other files) to no avail.

Run nose with the -s / --nocapture option and you'll be able to see the pdb prompt and interact with the debugger normally.

If using the commandline that means:-

python manage.py  test -s [other-opts-and-args]

Nose is capturing the output and redirecting it. So, the breakpoint is hit, but you just don't see it. You need to turn off the output redirection so that the debug output shows up on the screen.

Nose can do this for you, if you use:

from nose.tools import set_trace; set_trace()

instead of:

import pdb;pdb.set_trace()