How do I edit the value of a variable or append a list using a function?

Question

So I'm trying to build a blackjack game and I'm trying to build a system to record the player's score. I'd like to add the value of the card to the player's score, but I'm encountering some problems.

players_hand = []
player_score = 0

def play_the_round():

  card_list_select = cards_list[random.randint(0,len(cards_list)-1)] #this picks a card
  if card_list_select == "Ace": 
    ace_input = int(input("You have gotten an Ace! What value do you want your ace to be, 1 or 11: "))
    card_value = ace_input
  if not card_list_select == "Ace":
    card_value = cards_value_dict[card_list_select] #this extracts the card's value from a dictionary
    player_score += int(card_value)
    players_hand.append(card_list_select) 

Im currently getting this error: UnboundLocalError: local variable 'player_score' referenced before assignment

Answer 1

You can do something like this, declaring player_score as global:

import random
cards_list = ['Ace', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Jack', 'Queen', 'King']
cards_value_dict = dict(zip(cards_list, [1,2,3,4,5,6,7,8,9,10,10,10,10]))

players_hand = []
global player_score
player_score = 0
def play_the_round():
    global player_score
    card_list_select = cards_list[random.randint(0,len(cards_list)-1)] #this picks a card
    if card_list_select == "Ace": 
        ace_input = int(input("You have gotten an Ace! What value do you want your ace to be, 1 or 11: "))
        card_value = ace_input
    if not card_list_select == "Ace":
        card_value = cards_value_dict[card_list_select] #this extracts the card's value from a dictionary
    player_score += int(card_value)
    players_hand.append(card_list_select)

play_the_round()
print(players_hand, player_score)

... or you can do something like this which will allow you to avoid making player_score global since it is passed to the function as an argument and updated based on the function's return value:

players_hand = []
player_score = 0
def play_the_round(score):
    card_list_select = cards_list[random.randint(0,len(cards_list)-1)] #this picks a card
    if card_list_select == "Ace": 
        ace_input = int(input("You have gotten an Ace! What value do you want your ace to be, 1 or 11: "))
        card_value = ace_input
    if not card_list_select == "Ace":
        card_value = cards_value_dict[card_list_select] #this extracts the card's value from a dictionary
    score += int(card_value)
    players_hand.append(card_list_select)
    return score

player_score = play_the_round(player_score)
print(players_hand, player_score)

UPDATE: example of using a class where methods have access to class attributes, so that attributes can be used instead of global variables.

import random
class Blackjack:
    def __init__(self):
        self.cards_list = ['Ace', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Jack', 'Queen', 'King']
        self.cards_value_dict = dict(zip(self.cards_list, [1,2,3,4,5,6,7,8,9,10,10,10,10]))
        self.players_hand = []
        self.player_score = 0
    def play_the_round(self):
        card_list_select = self.cards_list[random.randint(0,len(self.cards_list)-1)] #this picks a card
        if card_list_select == "Ace": 
            ace_input = int(input("You have gotten an Ace! What value do you want your ace to be, 1 or 11: "))
            card_value = ace_input
        if not card_list_select == "Ace":
            card_value = self.cards_value_dict[card_list_select] #this extracts the card's value from a dictionary
        self.player_score += int(card_value)
        self.players_hand.append(card_list_select)

game = Blackjack()
game.play_the_round()
game.play_the_round()
while game.player_score < 21:
    more_cards = int(input(f"You have {len(game.players_hand)} cards with a score of {game.player_score}. Another card (1 for one more card, 0 for no more cards)?"))
    if more_cards > 0:
        game.play_the_round()
    else:
        break
print(game.players_hand, game.player_score, f"that's a busted hand" if game.player_score > 21 else f"you're still in the game")