Debugging tox python tests with pdb?

I've read much of the documentation around Openstack(python cloud framework) tests, tox, and testools. All I've found indicates debugging can be done, but only by running the entire test suite.

I'd like the ability to run a single test module with pdb.set_trace() breakpoints inserted, then step through the test. I've tried this but it causes test failures on a test that would otherewise succeed. The command I use to run the test is similar to this: tox -e py27

Is there some way to debug single tests that I haven't found? If not, how is everyone doing test development without the ability to debug?