The locking section of some caching code I wrote.

gistfile1.py

@classmethod
def resolve(cls, request, caching_client):
    """If this request isn't yet cached, cache it, otherwise return
    a file containing the cached contents"""
    hash = cls.hash(request)
    is_caching = False
    active_thread = None
    # If there is no cached file
    if not cls.hit(hash):
        # obtain a lock for the active caches
        with cls.active_lock:
            # if there is no active cache make one
            if hash not in cls.active_caches:
                print "Cache Miss:", urlunparse(request.url)
                active_thread = Cacher(cls.new_cache(hash), request, caching_client)
                is_caching = True
                active_thread.daemon = True
                active_thread.start()
                cls.active_caches[hash] = active_thread
            # otherwise get th already active cache
            else:
                print "Cache Loading:", urlunparse(request.url)
                active_thread = cls.active_caches[hash]
    else:
        print "Cache Hit:", urlunparse(request.url)

    # If our request if being actively cached, wait for it to complete
    if active_thread is not None:
        active_thread.join()

    # If our request was cached by the Cacher, return None, the file
    # was already sent to the server (this is a speed improvement
    if is_caching: 
        return None
    return cls.cache_lookup(hash)