code: def std(self, values, axis=0, weights=None, dtype=None):
          
          unique, var = self.var(values, axis, weights, dtype)
          return unique, np.sqrt(var)
docs: standard deviation over each group
  
          Parameters
          ----------
          values : array_like, [keys, ...]
              values to take standard deviation of per group
          axis : int, optional
              alternative reduction axis for values
  
          Returns
          -------
          unique: ndarray, [groups]
              unique keys
          reduced : ndarray, [groups, ...]
              value array, reduced over groups