code: def cancelRequest(self, requestId, reason=None, _=None):
          
          if reason is None:
              reason = CancelledError()
          tReq = self.requests.pop(requestId)
          tReq.d.errback(reason)
docs: Cancel a request: remove it from requests, & errback the deferred.
  
          NOTE: Attempts to cancel a request which is no longer tracked
            (expectResponse == False and already sent, or response already
            received) will raise KeyError