@property
def std(self):
    expected_sum_square = (self.sum_square / self.count)
    expected_sum = self.avg
    return math.sqrt((expected_sum_square - (expected_sum * expected_sum)))
@property
def std(self):
    expected_sum_square = (self.sum_square / self.count)
    expected_sum = self.avg
    return math.sqrt((expected_sum_square - (expected_sum * expected_sum)))