I got this message : Can't get the length of a `Hand` eventhough I'm not asked to provide the __len__ meth for hand cld

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)

    def roll(self, number_of_dice) :
        slots = []
        for _ in range(number_of_dice) : 
            slots = []
            slots.append(D20())
        return slots