python: dice-roller/rpg-roller 2/2 | Error

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, sides=20):
        super().__init__(sides)

from dice import D20

class Hand(list):
    def __init__(self, size=0, die_class=None, *args, **kwargs):
        if not die_class:
            raise ValueError("You must define a dice class")
        super().__init__()

        for _ in range(size):
            self.append(die_class())
        self.sort()


    # Now update Hand in hands.py. 

    # I'm going to use code similar to Hand.roll(2) 
    # and I want to get back an instance of Hand with 
    # two D20s rolled in it. 
    @classmethod
    def roll(cls, dice):
        dr = []
        for _ in range(dice):
            dr.append(cls(size=2, die_class=D20))

        return dr  

    # I should then be able to call .total on the instance 
    # to get the total of the two dice.
    @property
    def total(self):
        data = self

        return sum(data[0])