code: def std(self) -> Optional[float]:  #, ddof=0):
          
          # TODO: Add DOF
          if self._stats:
              return np.sqrt(self.variance())
          else:
              return None
docs: Standard deviation of all values entered into histogram.
  
          This number is precise, because we keep the necessary data
          separate from bin contents.
  
          Returns
          -------
          float