Error: Can't import D20

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):

    def __init__(self, size=0, die_class=D20):
        self.size = size
        super().__init__()
        for _ in range(size):
            self.append(die_class())

    @classmethod
    def roll(cls, size=2):
        return cls(size)

    @property
    def total(self):
        return sum(self)