word-freq2.py

import collections

counter = collections.Counter()
word = ""
with open('sherlock.txt') as f:
    while True:
        c = f.read(1)
        if not c:
            break
        if c.isalnum():
            word += c.lower()
        else:
            if word != "":
                counter[word] +=1
                word = ""

for w, c in counter.most_common(20):
    print(w, c)