I'm trying to get started with unit testing in Python and I was wondering if someone could explain the advantages and disadvantages of doctest and unittest.  
What conditions would you use each for?
I don't use doctest as a replacement for unittest. Although they overlap a bit, the two modules don't have the same function:

I use unittest as a unit testing framework, meaning it helps me determine quickly the impact of any modification on the rest of the code.
I use doctest as a guarantee that comments (namely docstrings) are still relevant to current version of the code. 

The widely documented benefits of test driven development I get from unittest. doctest solves the far more subtle danger of having outdated comments misleading the maintenance of the code.
Python - doctest vs. unittest