a-web-crawler-with-asyncio-coroutines

The one with the callback pattern:

import socketfrom selectors import DefaultSelector,EVENT_WRITE,EVENT_READdef loop():    while not Stopped:        events = selector.select()        for event_key,event_mask in events:            callback = event_key.data            callback()class Fetcher:    def __init__(self,url):        self.response = b''        self.url = url        self.sock = None    #conncet a socket,    def fetch(self):        self.sock = socket.socket()        self.sock.setblocking(False)        try:            self.sock.connect(('xkcd.com',80))        except BlockingIOError:            pass        #register the nexr callback from connect to send request status        selector.register(self.sock.fileno(),EVENT_WRITE,self.connected)    def connected(self,key,mask):        print('connceted!')        selector.unregister(key.fd)        request = 'GET {} HTTP/1.0\r\nHost:xkcd.com\r\n\r\n'.format(self.url)        self.sock.send(request.encode('ascii'))        #register the next callback from send request to wait response status        selector.register(key.fd,EVENT_READ,self.read_response)    def read_response(self,key,mask):        global stopped        chunk = self.sock.recv(4096)        if chunk:            self.response += chunk        else:            selector.unregister(key.fd)            #parse the response data to a set of urls            links = self.parse_links()            #Python set-logic            for link in links.difference(seen_urls):                urls_todo.add(link)                Fetcher(link).fetch()            seen_urls.update(links)            urls_todo.remove(self.url)            if not urls_todo:                stopped = Trueselector = DefaultSelector()#global sets of the URLs we have yet to fetchurls_todo = set(['/'])seen_urls = set(['/'])

but callbacks are stubborn to debug due to the stack ripping