Hangman with dashes in Python – can't figure out why the code is not working

I started to learn Python. I use Thonny and the Python version is 3.7.9

This Hangman code choses random word from the list successfully and it shows dashes instead of letters. However, when I guess a letter, dash doesn’t get replaced with that letter. So what is wrong with the following code?

import random
import string
from words import words #words.py file with list of words in it

def get_valid_word(words):
    word = random.choice(words)
    while "-" in word or " " in word:
        word = random.choice(words)
    
    return word

def hangman():
    word = get_valid_word(words)
    word_letters = set(word)
    alphabet = set(string.ascii_uppercase)
    used_letters = set() #what the user has guessed
    
    #get user input
    while len(word_letters) > 0:
        print("you have used these letters: ", " ".join(used_letters))
        
        #current word:
        word_list = [letter if letter in used_letters else "-" for letter in word]
        print("current word: ", " ".join(word_list))
        print("current word: ", word)
        
        user_letter = input("guess a letter: ").upper()
        if user_letter in alphabet - used_letters:
            used_letters.add(user_letter)
            if user_letter in word_letters:
                word_letters.remove(user_letter)
        elif user_letter in used_letters:
            print("you already used that letter")
        else:
            print("invalid character")

#get_valid_word(words)
hangman()

>Solution :

You are only storing upper letters in used_letters. So you should do:

word_list = [letter if letter.upper() in used_letters else "-" for letter in word]