code: def _try_disconnect(self, ref):
          
          with self.lock:
              weak = [s[0] for s in self.weak_subscribers]
              try:
                  index = weak.index(ref)
              except ValueError:
                  # subscriber was already removed by a call to disconnect()
                  pass
              else:
                  self.weak_subscribers.pop(index)
docs: Called by the weak reference when its target dies.
          In other words, we can assert that self.weak_subscribers is not
          None at this time.