'Fail unless the two objects are identical.
Fail the test unless ``first`` and ``second`` are
identical, as determined by the ``is`` operator.'

def failUnlessIs(self, first, second, msg=None):

    if (first is not second):
        if (msg is None):
            msg = ('%(first)r    is    not    %(second)r' % vars())
        raise self.failureException(msg)
