'Verify that Default dpid assignment raises an Exception if the
name of the switch does not contin a digit. Also verify the
exception message.'

def testDefaultDpidAssignmentFailure(self):

    with self.assertRaises(Exception) as raises_cm:
        Mininet(Topo(), self.switchClass, Host, Controller).addSwitch('A')
    self.assertEqual(raises_cm.exception.message, 'Unable    to    derive    default    datapath    ID    -    please    either    specify    a    dpid    or    use    a    canonical    switch    name    such    as    s23.')
