Error: Can't get the length of a hand

import random


class Die:
    def __init__(self, sides=2):
        if sides < 2:
            raise ValueError("Can't have fewer than two sides")
        self.sides = sides
        self.value = random.randint(1, sides)

    def __int__(self):
        return self.value

    def __add__(self, other):
        return int(self) + other

    def __radd__(self, other):
        return self + other

class D20(Die):
    def __init__(self):
        super().__init__(sides=20)

import dice

class Hand(list):    
    @property
    def total(self):
        return sum(self)

    def roll(self, num):
        for i in range(num):
            self.append(dice.D20())