For test discovery to work where a dependent module is optional, you end up needing to do something like what is done in http://hg.python.org/cpython/rev/15ddd683c321:

-crypt = support.import_module('crypt')
+def setUpModule():
+    # this import will raise unittest.SkipTest if _crypt doesn't exist,
+    # so it has to be done in setUpModule for test discovery to work
+    global crypt
+    crypt = support.import_module('crypt')

That's kind of ugly. It would be better if unittest recognized SkipTest at import time during test discovery