old_contents: #!/usr/bin/python
  
  "Create a 64-node tree network, and test connectivity using ping."
  
  from mininet.log import setLogLevel
  from mininet.net import init, Mininet
  from mininet.node import KernelSwitch, UserSwitch, OVSKernelSwitch
  from mininet.topolib import TreeNet
  
  def treePing64():
      "Run ping test on 64-node tree networks."
  
      results = {}
      switches = { 'reference kernel': KernelSwitch,
          'reference user': UserSwitch,
          'Open vSwitch kernel': OVSKernelSwitch }
  
      for name in switches.keys():
          print "*** Testing", name, "datapath"
          switch = switches[ name ]
          network = TreeNet( depth=2, fanout=8, switch=switch )
          result = network.run( network.pingAll )
          results[ name ] = result
  
      print
      print "*** Tree network ping results:"
      for name in switches.keys():
          print "%s: %d%% packet loss" % ( name, results[ name ] )
      print
  
  if __name__ == '__main__':
      setLogLevel( 'info' )
      treePing64()
  
new_contents: #!/usr/bin/python
  
  "Create a 64-node tree network, and test connectivity using ping."
  
  from mininet.log import setLogLevel
  from mininet.net import init, Mininet
  from mininet.node import KernelSwitch, UserSwitch, OVSKernelSwitch
  from mininet.topolib import TreeNet
  
  def treePing64():
      "Run ping test on 64-node tree networks."
  
      results = {}
      switches = { 'reference kernel': KernelSwitch,
          'reference user': UserSwitch,
          'Open vSwitch kernel': OVSKernelSwitch }
  
      for name in switches:
          print "*** Testing", name, "datapath"
          switch = switches[ name ]
          network = TreeNet( depth=2, fanout=8, switch=switch )
          result = network.run( network.pingAll )
          results[ name ] = result
  
      print
      print "*** Tree network ping results:"
      for name in switches:
          print "%s: %d%% packet loss" % ( name, results[ name ] )
      print
  
  if __name__ == '__main__':
      setLogLevel( 'info' )
      treePing64()
  
subject: Use switches rather than switches.keys().