《用python写网络爬虫》

def download3(url, num_retries=2):
    print(‘Downloading:‘, url)
    try:
        html = urllib2.urlopen(url).read()
    except urllib2.URLError as e:
        print(‘Download error:‘, e.reason)
        html = None
        if num_retries > 0:
            if hasattr(e, ‘code‘) and 500 <= e.code < 600:
                # retry 5XX HTTP errors
                html = download3(url, num_retries-1)
    return html

hasattr(object, name)
# 用于查看对象是否有对应的属性，返回布尔值