By default, urllib will download content with the Python-urllib/3.x user agent, where 3.x is the environment's current version of Python. It would be preferable to use an identifiable user agent in case problems occur with our web crawler. Also, some websites block this default user agent, perhaps after they have experienced a poorly made Python web crawler overloading their server. For example,  http://www.meetup.com/ currently returns a 403 Forbidden when requesting the page with urllib's default user agent.

To download sites reliably, we will need to have control over setting the user agent. Here is an updated version of our download function with the default user agent set to 'wswp' (which stands forWeb Scraping with Python):

def download(url, user_agent='wswp', num_retries=2): 
    print('Downloading:', url) 
    request = urllib.request.Request(url) 
    request.add_header('User-agent', user_agent)
    try: 
        html = urllib.request.urlopen(request).read() 
    except (URLError, HTTPError, ContentTooShortError) as e:
        print('Download error:', e.reason)
        html = None 
        if num_retries > 0: 
            if hasattr(e, 'code') and 500 <= e.code < 600: 
                # recursively retry 5xx HTTP errors 
                return download(url, num_retries - 1) 
    return html

If you now try meetup.com, you will see valid HTML. Our download function can now be reused in later code to catch errors, retry the site when possible, and set the user agent.