code: def add_handler(self, iface_name, handler):
          
          if self.contract.has_interface(iface_name):
              self.handlers[iface_name] = handler
          else:
              raise RpcException(ERR_INVALID_REQ, "Unknown interface: '%s'", iface_name)
docs: Associates the given handler with the interface name.  If the interface does not exist in
          the Contract, an RpcException is raised.
  
          :Parameters:
            iface_name
              Name of interface that this handler implements
            handler
              Instance of a class that implements all functions defined on the interface