UnboundLocalError: local variable 'a' referenced before assignment

[nilamo]

def not_bad(s):
    t = s.split()
    for x in t:
        if x == "not":
            a = t.index(x)
        if x == "bad":
            b = t.index(x)
        if a < b:
            newlist = [range (a, b)]
            del t[newlist]
    print (t)

a only exists if x=="not". You can't reference something that doesn't exist.
As was previously mentioned, you can fix this by simply defining your variables before hand with default values.