example function in Python: counting words

I'm a bit rusty in Python and am just looking for help implementing an example function to count words (this is just a sample target for a scons script that doesn't do anything "real"):

def countWords(target, source, env):
  if (len(target) == 1 and len(source) == 1):
    fin = open(str(source[0]), 'r')
    # do something with "f.read()"
    fin.close()

    fout = open(str(target[0]), 'w')
    # fout.write(something)
    fout.close()
  return None

Could you help me fill in the details? The usual way to count words is to read each line, break up in开发者_运维知识库to words, and for each word in the line increment a counter in a dictionary; then for the output, sort the words by decreasing count.

edit: I'm using Python 2.6 (Python 2.6.5 to be exact)

from collections import defaultdict

def countWords(target, source, env):
    words = defaultdict(int)
    if (len(target) == 1 and len(source) == 1):
        with open(str(source[0]), 'r') as fin:
            for line in fin:
                for word in line.split():
                    words[word] += 1

        with open(str(target[0]), 'w') as fout:
            for word in sorted(words, key=words.__getitem__, reverse=True):
                fout.write('%s\n' % word)
    return None

Without knowing why env exists, I can only do the following:

def countWords(target, source, env):
    wordCount = {}
    if len(target) == 1 and len(source) == 1:
        with fin as open(source[0], 'r'):
            for line in f
                for word in line.split():
                    if word in wordCount.keys():
                        wordCount[word] += 1
                    else:
                        wordCount[word] = 0

        rev = {}
        for v in wordCount.values():
            rev[v] = []
        for w in wordCount.keys():
            rev[wordCOunt[w]].append(w)
        with open(target[0], 'w') as f:
            for v in rev.keys():
                f.write("%d: %s\n" %(v, " ".join(rev[v])))

There is a helpful example here. It works roughly as you describe and also counts sentences.

Not too efficient but it is concise!

with open(fname) as f:
   res = {}
   for word in f.read().split():
       res[word] = res.get(word, 0)+1
with open(dest, 'w') as f:
    f.write("\n".join(sorted(res, key=lambda w: -res[w])))

Here my version:

import string
import itertools as it
drop = string.punctuation+string.digits

def countWords(target, source, env=''):
    inputstring=open(source).read()
    words = sorted(word.strip(drop)
                   for word in inputstring.lower().replace('--',' ').split())
    wordlist = sorted([(word, len(list(occurances)))
                      for word, occurances in it.groupby(words, lambda x: x)],
                        key = lambda x: x[1],
                      reverse = True)
    with open(target,'w') as results:
        results.write('\n'.join('%16s : %s' % word for word in wordlist))