cached: Python contextmanager for cached URL fetches

cached.py

from contextlib import closing, contextmanager
from urllib2 import urlopen
import hashlib
import os
import time

@contextmanager
def cached(url, expires=0, root='/tmp'):
    '''
    Returns the contents of the URL through a file-based cache. If "expires" is
    greater than zero, the contents will be re-fetched from the URL after that
    many seconds.
    '''
    checksum = hashlib.md5(url).hexdigest()
    cache_file = os.path.join(root, checksum)
    if os.path.exists(cache_file) and time.time() - os.path.getctime(cache_file) < expires:
        with open(cache_file, 'r') as cache_fh:
            yield cache_fh.read()
    else:
        with closing(urlopen(url)) as url_fh:
            data = url_fh.read()
            with open(cache_file, 'w') as cache_fh:
                cache_fh.write(data)
            yield data