I need help calling a user defined function in a while loop in python

I defined this function:

def formatted_name(firstName, lastName, middleName=''):
    if middleName:
        full_name = f"{firstName} {lastName} {middleName}"
    else:
        full_name = f"{firstName} {lastName}"
    return full_name.title()

and then tried to use it like so:

prompt = 'Please enter your first and last name below'
prompt += '\nEnter stop to quit'
quit = 'stop'
while not quit:
    print(prompt)
    firstname = input('Enter your first name: ')
    lastname = input('Enter your last name: ')
    if firstname == quit:
        break
fullName = formatted_name(firstname,lastname)
print(fullName)

When I try this, I get a NameError. What is wrong with the code, and how do I fix it?

>Solution :

The problem is that firstname and lastname are not defined. It never executes the code inside your while loop because when you convert a string that is not empty to a bool which in this case is "stop" you get True, since you have "not quit" in your while loop then it Will never execute because not quit is False. A while loop with False in it never executes the code inside

>>> bool("stop")
True
>>> not bool("stop")
False