Python Hangman问题(第12行: UnboundLocalError:赋值前引用的局部变量'a‘)

import random

def hangman():
    words = ['apple', 'banana', 'orange', 'watermelon', 'strawberry']
    word = random.choice(words)
    guessed_letters = []
    max_wrong_guesses = 6
    wrong_guesses = 0
    game_over = False

    while not game_over:
        display_word = ''
        for letter in word:
            if letter in guessed_letters:
                display_word += letter
            else:
                display_word += '_'
        
        print('Word:', display_word)
        print('Guessed letters:', guessed_letters)
        print('Wrong guesses left:', max_wrong_guesses - wrong_guesses)

        if display_word == word:
            print('Congratulations! You guessed the word:', word)
            game_over = True
        elif wrong_guesses == max_wrong_guesses:
            print('Game over! You failed to guess the word:', word)
            game_over = True
        else:
            guess = input('Guess a letter: ').lower()
            if guess.isalpha() and len(guess) == 1:
                if guess in guessed_letters:
                    print('You already guessed that letter!')
                elif guess in word:
                    print('Correct guess!')
                    guessed_letters.append(guess)
                else:
                    print('Wrong guess!')
                    wrong_guesses += 1
                    guessed_letters.append(guess)
            else:
                print('Invalid guess! Please enter a single letter.')

hangman()