How do I convert a list to Uppercase : I am new to coding and I need help with this coding problem .
my problem is that I need to convert the "letter_guessed" input to lower case ,if its upper case and if the uppercase letter is already exists inside the list as a lower case  it will return false but I cant get to do it .

I have tried using isupper(),upper ,islower(), lower() in many ways , I am pretty sure that I am doing something wrong with "if" but cant get it right been stuck on it for two days .
Thanks in advance !!


def check_valid_input(letter_guessed, old_letters_guessed):

    while True:
        """ will work only if you enter one letter and do not contain special letters other then the abc 
        and if its all ready been entered it will show false """

        if len(letter_guessed) == 1  and letter_guessed not in old_letters_guessed :
            """if the letter is one letter  and not already inside old_letter_guessed only then  continue """

            old_letters_guessed.append(letter_guessed)
            print("True")
            letter_guessed = input(" : ")

        else:
            """ if its wrong input will print False Try again and if the input is correct it will go back to " if " """

            #old_letters_guessed.append(letter_guessed)
            print(False, 'Try again')
            old_letters_guessed.sort()
            print('->'.join(old_letters_guessed))
            letter_guessed = input(" : ")
        #if letter_guessed is letter_guessed.isupper()

new = input()
old = []
check_valid_input(new,old)

