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?
Using both is a valid and rather simple option. The doctest module provides the DoctTestSuite and DocFileSuite methods which create a unittest-compatible testsuite from a module or file, respectively.
So I use both and typically use doctest for simple tests with functions that require little or no setup (simple types for arguments). I actually think a few doctest tests help document the function, rather than detract from it.
But for more complicated cases, and for a more comprehensive set of test cases, I use unittest which provides more control and flexibility.
Python - doctest vs. unittest