python网络爬虫 - 设定重试次数内反复抓取

import urllib.request

def download(url, num_retries=2):
    print('Downloading:', url)
    try:
        html = urllib.request.urlopen(url).read()
    except urllib.URLError 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

url = 'http://www.google.com'
print(download(url, 3))