lowercasing to strings before comparison

[SheeppOSU]

I have some code for logging in. Like most logins, I don't care about uppercase and lowercase for the username. Is there a way I can lowercase all the letters inside both usernames before comparison

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Sorry I should have just looked this up before posting

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Actually I have a new problem. I have this function for signing in. I can lowercase what the player types in for the username and I have a list of all the username lowercased. How do i identify the original username from the dictionary of Sign ins. Here's the code -

def SignIn():
    global username
    signInListlower = []
    for i in signInList:
        signInListlower.append(string.lower(i))
    username = input('(Type "new" for new account) Type in username: ')

    if username == 'new':
        newUsername = input('Type your Username: ')
        if string.lower(newUsername) in signInListlower:
            print('The username %s is already taken' %newUsername)
            SignIn()
        else:
            newPassword = input('Type your Password: ')
            username = newUsername
            print('Now return to Destination and confirm')
            signInList.update({newUsername : newPassword})
            UserDefense.update({newUsername : 1})
            UserLvl.update({newUsername : 1})
            UserExp.update({newUsername : 0})
            UserWeap.update({newUsername : WoodBat})
            UserArmor.update({newUsername : WoodenArmor})
            strUserWeap.update({newUsername : 'Wood Bat'})
            strUserArmor.update({newUsername : 'Wooden Armor'})
            UserPts.update({newUsername : 0})
            UserExtraDamage.update({newUsername : 1})
            UserAgility.update({newUsername : 1})
            UserGold.update({newUsername : 50})
            UserFloor.update({newUsername : 1})
            for x in UserLvlListings:
                x.update({newUsername : False})

            username = newUsername

    else:
        if string.lower(username) in signInListlower:
            usernameNum = signInListlower.index(lower(username))
            for k in singInList:
                if string.lower(k) == string.lower(username):
                    username = k
            password = input('Please type your password: ')
            if password == signInList[username]:
                print('Now return to Destination and confirm')
            else:
                print('Invalid Password')
                time.sleep(2)
                SignIn()
        else:
            print('Invalid Username')
            time.sleep(2)
            SignIn()

If you don't understand just tell me and I'll try to explain it more thoroughly.