Dice Roller Q - It works in the workspace but not on the tester?

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)

from dice import D20

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

    @classmethod
    def roll(cls, num_dice):
        new_hand = Hand()
        die_class = D20
        for _ in range(num_dice):
            new_hand.append(die_class)
        return cls(new_hand)