Check is string a is prefix of string b

def prefix (a, b):
        i=0
        while a[i] <> 0
                if b[i] == 0:
                        return false
                if a[i]<>b[i]:
                        return false
        return true

prefix ('' , 'f') = 1
prefix ('f', '') = 1
perfix ('f','fg') = 1