I am getting the an error when running the code

class YatzyScoresheet:

  def score_ones(self, hand): 
    return sum(hand.ones)

  def score_twos(self, hand): 
    return sum(hand.twos)

  def score_threes(self, hand): 
    return sum(hand.threes)

  def score_fours(self, hand): 
    return sum(hand.fours)

  def score_fives(self, hand): 
    return sum(hand.fives)

  def score_sixes(self, hand): 
    return sum(hand.sixes)

  def _score_set(self, hand, set_size):
    scores = [0]
    for worth, count in hand._sets.items():
        if count == set_size:
          scores.append(worth * set_size)
        return max(scores)

  def score_one_pair(self, hand): 
    return self._score_set(hand, 2)

from dice import D6
class Hand(list): 
  def __init__(self, size=0, die_class=None, *args, **kwargs):
    if not die_class:
      raise ValueError("you must provide a die class")
    super().__init__()

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

  def _by_value(self, value):
    dice = []
    for die in self: 
      if die == value:
        dice.append(die)
      return dice

class YatzyHand(Hand):
  def __init__(self, *args, **kwargs):
    super().__init__(size=5, die_class=D6, *args, **kwargs)

    @property
    def ones(self):
      return self._by_value(1)

    @property
    def twos(self):
      return self._by_value(2)

    @property
    def threes(self):
      return self._by_value(3)

    @property
    def four(self):
      return self._by_value(4)

    @property
    def fives(self):
      return self._by_value(5)

    @property
    def sixes(self):
      return self._by_value(6)

    @property
    def _sets(self):
      return {
        1: len(self.ones),
        2: len(self.twos),
        3: len(self.threes),
        4: len(self.fours),
        5: len(self.fives),
        6: len(self.sixes)
      }

import random

class Die():

  def __init__(self, sides=2, value=0):
    if not sides >=2:
      raise ValueError("Must have atleast 2 sides")
    if not isinstance(sides, int):
      raise ValueError("Sides must be a whole number")
    self.value = value or random.randint(1, sides)

  def __int__(self): 
    return self.value

  def __eq__(self, other):
    return int(self) == other

  def __ne__(self, other): 
    return int(self) != other

  def __gt__(self, other):
    return int(self) > other

  def __lt__(self, other):
    return int(self) < other

  def __ge__(self, other):
    return int(self) > other or int(self)==other

  def __le__(self, other): 
    return int(self) < other or int(self) == other

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

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

  def __repr__(self):
    return str(self.value)

class D6(Die):
  def __init__(self, value=0):
    super().__init__(sides=6, value=value)