Python programme to find out 20 most common words in text

python.py

#!/usr/bin/env python3.2

import re
from operator import itemgetter

word2num = {}
num2word = {}

with open('potop.txt', mode='r', encoding='utf-8') as potop:
  for line in potop:
    line = re.sub("[],.\\-\\?:;\"()/'=$[`!]", ' ', line)
    for word in line.split():
      word2num[word] = 1 + word2num.get(word, 0)

s = sorted(word2num.items(), key=itemgetter(1))
s.reverse()
for w,c in s[:20]:
  print("{0} --> {1}".format(c,w))