Implementation of python pubsub library - publisher subscriber

pubsub.py

# -*- coding: utf-8 -*-
#==============================================================================
# Name:         pubsub
#
# Purpose:      Simple publish & subscribe in pure python
#
# Author:       Zhen Wang
#
# Created:      23 Oct 2012
# Copyright:    
# Licence:      MIT License
#==============================================================================

from Queue import Queue as queue
from threading import Lock as lock
from functools import partial

#==============================================================================
# Global
#==============================================================================

MAX_QUEUE = 100
MAX_ID = 2 ** 31
PUBLISH_ID = True

channels = {}
count = {}

channels_lock = lock()
count_lock = lock()

#==============================================================================
# Exceptions
#==============================================================================
class UnsubscribeException(Exception):
    pass

#==============================================================================
# Functions
#==============================================================================

def subscribe(channel):
    if not channel:
        raise ValueError('channel')
    
    if channel not in channels:
        channels_lock.acquire()
        # Need to check again
        if channel not in channels:
            channels[channel] = []
        channels_lock.release()
        
    msg_q = queue()
    channels[channel].append(msg_q)
    
    msg_q.listen = partial(listen, msg_q)
    msg_q.unsubscribe = partial(unsubscribe, channel, msg_q)
    msg_q.name = channel
    return msg_q

def unsubscribe(channel, msg_q):
    if not channel:
        raise ValueError('channel')
    if not msg_q:
        raise ValueError('msg_q')
    try:
        channels[channel].remove(msg_q)
    except:
        pass
    
def listen(msg_q, block=True, timeout=None):
    while True:
        try:
            data = msg_q.get(block=block, timeout=timeout)
        except:
            return            
        if data is None:
            raise UnsubscribeException()
        yield data
    
def publish(channel, data):
    if not channel:
        raise ValueError('channel')
    if not data:
        raise ValueError('data')
        
    if channel not in channels:
        channels_lock.acquire()
        # Need to check again
        if channel not in channels:
            channels[channel] = []
        channels_lock.release()
    
    # Update message counts
    if PUBLISH_ID:
        count_lock.acquire()
        if channel not in count:
            count[channel] = 0L
        else:
            count[channel] = (count[channel] + 1) % MAX_ID
        count_lock.release()
    else:
        count[channel] = 0L
    
    # ID of current message
    _id = count[channel]
    
    # Push to all subscribers in channel
    for q in channels[channel]:
        # Garbage collect queues
        if q.qsize() > MAX_QUEUE:
            # Send termination msg and unsub
            q.put(None, block=False)
            unsubscribe(channel, q)
            continue
        
        q.put({'data': data, 'id': _id}, block=False)