code: def _default_interface(self, route_output=None):
          
          if not route_output:
              out, __, __ = exec_cmd('/sbin/ip route')
              lines = out.splitlines()
          else:
              lines = route_output.split("\n")
  
          for line in lines:
              line = line.split()
              if 'default' in line:
                  iface = line[4]
                  return self.interfaces.get(iface, None)
docs: :param route_output: For mocking actual output