code: def _update_key(self, mask, key):
          
          mask = np.asanyarray(mask)
          if key in self._data:
              self._data[key] = self._data[key][mask]
docs: Mask the value contained in the DataStore at a specified key.
  
          Parameters
          -----------
          mask: (n,) int
                (n,) bool
          key: hashable object, in self._data