KeyErrors and how to raise a KeyError

Question

For my homework assignment, I am told to raise a key error if the key(text) the user enters contains any non alphabetic characters and reprompt. So far I have this which seems to work but obviously doesn't use the expected try/except structure

key=input("Please enter the key word you want to use: ")
ok=key.isalpha()
while (ok==False):
    print("The key you entered is invalid. Please try again")
    key=input("Please enter the key word you want to use")

Answer 1

This is not appropriate usage of KeyError (it's supposed to be used for dict lookups, or similar situations), but if it is what you have been asked to do then try something like this :

def prompt_thing():
    s = raw_input("Please enter the key word you want to use: ")
    if s == '' or not s.isalnum():
        print("The key you entered is invalid. Please try again")
        raise KeyError('non-alphanumeric character in input')
    return s

s = None
while s is None:
    try:
        s = prompt_thing()
    except KeyError:
        pass