What would be a good default reason? How about the function name?

    if isinstance(reason, types.FunctionType):
        reason = reason.__name__

For example,

    from unittest import TestCase, skip

    class Test(TestCase):

        @skip
        def test_bad_skip(self):
            self.assertFalse(1.0)

        @skip('always skipped')
        def test_good_skip(self):
            self.assertFalse(1.0)

$ python -m unittest -v test.py 
test_bad_skip (test.Test) ... skipped 'test_bad_skip'
test_good_skip (test.Test) ... skipped 'always skipped'

----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK (skipped=2)


But this wouldn't be very helpful if test method is a lambda (just saying...)