Hanker Rank: order of word

Given a string, find the lexical sequence:

import math
def num(word):
    length=len(word)
    ans=math.factorial(length)
    wordset=set(word)
    for letter in wordset:
        fre=word.count(letter)
        ans=ans/fre
    return ans
def  get_ranks( words):
    if len(words)==1:
        return 0
    setwords=set(words)
    listwords=list(setwords)
    listwords.sort()
    prenumber=0
    head=words[0]
    seq=listwords.index(head)
    for i in range(seq):
        absent=listwords[i]
        k=words.index(absent)
        prenumber+=num(words[0:k]+words[k+1:])

    return prenumber+get_ranks(words[1:])

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