def _handlePriorityUpdate(self, event):
        """
        Internal handler for when a stream priority is updated.

        @param event: The Hyper-h2 event that encodes information about the
            stream reprioritization.
        @type event: L{h2.events.PriorityUpdated}
        """
        try:
            self.priority.reprioritize(
                stream_id=event.stream_id,
                depends_on=event.depends_on or None,
                weight=event.weight,
                exclusive=event.exclusive,
            )
        except priority.MissingStreamError:
            # A PRIORITY frame arrived before the HEADERS frame that would
            # trigger us to insert the stream into the tree. That's fine: we
            # can create the stream here and mark it as blocked.
            self.priority.insert_stream(
                stream_id=event.stream_id,
                depends_on=event.depends_on or None,
                weight=event.weight,
                exclusive=event.exclusive,
            )
            self.priority.block(event.stream_id)

        Internal handler for when a stream priority is updated.

        @param event: The Hyper-h2 event that encodes information about the
            stream reprioritization.
        @type event: L{h2.events.PriorityUpdated}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/_http2.py