KMP string matching algorithm in Python

kmp.py

from sets import Set

def gen_xfix_set(string, generator):
    ret,n = Set([]),len(string)-1
    while True:
        if n==0: return ret
        else: 
            ret.add(generator(string, n))
            n -= 1

def gen_prefix_set(string):
    return gen_xfix_set(string, lambda s,n: s[:n])

def gen_suffix_set(string):
    return gen_xfix_set(string, lambda s,n: s[n:])

# count partial matches
def cpm(string):
    if string=="": raise(Exception("it's null string!"))
    prefix,suffix=gen_prefix_set(string),gen_suffix_set(string)
    return len(prefix.intersection(suffix))

def kmp(string, what):
    lstr = len(string)
    lwhat = len(what)
    if lstr < lwhat:  return False
        
    i,j=0,0
    while True:
        if i > lstr: return False
        elif string[i] == what[j]: 
            if j == (lwhat-1): return i-j,i+1
            else: i,j=i+1,j+1
        elif j==0: i,j=i+1,j 
        else: i,j=i+j-cpm(what[:j]),0