Why does my variable return None, Even though being defined

I'm currently doing a python project that requires me to make a game; I have defined a class called Game that when initialized takes P1and P2 (Player class instances).

I then defined a list called deck, which then I shuffled and redefined as the new shuffled deck. The issue is when I print deck it prints None.

import random
class Game():
    deck = []
    def __init__(self, p1, p2):
        self.p1 = p1
        self.p2 = p2
        self.__generateCards()
    def __generateCards(self):
        colors = ['red', 'black', 'yellow']
        for x in range(0,3):
            for y in range(1,11):
                li = [colors[x], y]
                self.deck.append(li)
        self.deck = random.shuffle(self.deck)
        print(self.deck)