code: def _call(self, msg, **metadata):
          
          if msg.msg_type:
              for callback in self._get_callbacks(msg.msg_type):
                  try:
                      callback(msg, **metadata)
                  except Handler._DeadCallbackException:
                      # The callback was an upstream iterator that has been garbage
                      # collected.  Remove it from our internal structures.
                      self.remove_callback(callback)
                      self._gc_dead_sinks()
                  except SystemExit:
                      raise
                  except:
                      import traceback
                      traceback.print_exc()
docs: Process message with all callbacks (global and per message type).