'Describe node placement'

def do_placement(self, _line):

    mn = self.mn
    nodes = ((mn.hosts + mn.switches) + mn.controllers)
    for server in mn.servers:
        names = [n.name for n in nodes if (hasattr(n, 'server') and (n.server == server))]
        output(('%s:    %s\n' % (server, '    '.join(names))))
