code: def from_events(self, instance, ev_args, ctx):
          
          obj = yield from self._process(instance, ev_args, ctx)
          self.__set__(instance, obj)
          return obj
docs: Detect the object to instanciate from the arguments `ev_args` of the
          ``"start"`` event. The new object is stored at the corresponding
          descriptor attribute on `instance`.
  
          This method is suspendable.