This changeset:

http://hg.python.org/cpython/rev/c3fb79d1c036

breaks nose, as it changes the behavior of unittest.TestProgram in a
non-backwards-compatible way. Previously, when called like:

TestProgram(testRunner=None)

self.testRunner would be None when runTests() was called. Now, it is 
populated with the default TextTestRunner class in TestProgram.__init__. 
nose expects self.testRunner to be None or a runner instance in 
runTests(), and thus fails immediately with 2.6.3.

If initialization of self.testRunner could be deferred to runTests() as 
in unitttest for 2.5 and earlier, that would be ideal for nose.

This change was made to fix a regression in 2.6.
In 2.5 the default was None and someone (else) made the also backwards
incompatible change to make the default the TextTestRunner class. This
broke test suites (like setuptools for example) which expected to be
able to pass None in and get sensible default behaviour.

The change can't be reverted as it was a bugfix, but a different change
could be applied.

Committed in revision 6418. Still needs fixing in trunk (note the patch
itself can't be applied as it is missing a couple of 'self's).

Having problems testing this fix without causing spurious output on stderr.

Oops - I meant revision 73997 of course.

Committed to trunk in revision 74007.