code: def watch(self, selector, callback):
          
  
          if selector not in self._monitors:
              self._monitors[selector] = set()
  
          self._monitors[selector].add(callback)
docs: Call a function whenever a stream changes.
  
          Args:
              selector (DataStreamSelector): The selector to watch.
                  If this is None, it is treated as a wildcard selector
                  that matches every stream.
              callback (callable): The function to call when a new
                  reading is pushed.  Callback is called as:
                  callback(stream, value)