'Fail the test if :py:data:`first` is :py:data:`second`.  This is an
obect-identity-equality test, not an object equality
(i.e. :py:func:`__eq__`) test.
:param msg: if msg is None, then the failure message will be
\'%r is %r\' % (first, second)'

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

    if (first is second):
        raise self.failureException((msg or ('%r    is    %r' % (first, second))))
    return first
