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

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

    if (first is not second):
        raise self.failureException((msg or ('%r    is    not    %r' % (first, second))))
    return first
