def cancel(self):
        """
        Cancel this L{DeferredList}.

        If the L{DeferredList} hasn't fired yet, cancel every L{Deferred} in
        the list.

        If the L{DeferredList} has fired, including the case where the
        C{fireOnOneCallback}/C{fireOnOneErrback} flag is set and the
        L{DeferredList} fires because one L{Deferred} in the list fires with a
        non-failure/failure result, do nothing in the C{cancel} method.
        """
        if not self.called:
            for deferred in self._deferredList:
                try:
                    deferred.cancel()
                except:
                    log.failure(
                        "Exception raised from user supplied canceller"
                    )

        Cancel this L{DeferredList}.

        If the L{DeferredList} hasn't fired yet, cancel every L{Deferred} in
        the list.

        If the L{DeferredList} has fired, including the case where the
        C{fireOnOneCallback}/C{fireOnOneErrback} flag is set and the
        L{DeferredList} fires because one L{Deferred} in the list fires with a
        non-failure/failure result, do nothing in the C{cancel} method.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/defer.py