code: def stream_by_id(self, dst_id):
          
          return (
              self._stream_by_id.get(dst_id) or
              self._stream_by_id.get(mitogen.parent_id)
          )
docs: Return the :class:`Stream` that should be used to communicate with
          `dst_id`. If a specific route for `dst_id` is not known, a reference to
          the parent context's stream is returned.