from mininet.topo import Topo

class MyTopo(Topo):

def __init__(self):    # initilaize topology    Topo.__init__(self)    # add hosts and switches    h1 = self.addHost('h1')    h2 = self.addHost('h2')    h3 = self.addHost('h3')    h4 = self.addHost('h4')    sw1 = self.addSwitch('s1')    sw2 = self.addSwitch('s2')sw3 = self.addSwitch('s3')sw4 = self.addSwitch('s4')    # add links    self.addLink(sw1,h1, 1, 1)    self.addLink(sw1,sw2, 2, 1)    self.addLink(sw1,sw3, 3, 1)    self.addLink(sw4,sw4, 4, 1)    self.addLink(sw2,sw4, 2, 2)    self.addLink(sw3,sw4, 2, 3)    self.addLink(sw4,h2, 4 ,1)    self.addLink(sw4,h3, 5, 1)    self.addLink(sw4,h4, 6, 1)

topos = {'mytopo': (lambda: MyTopo())}