python 爬虫-1：下载网页源代码

import urllib2
def download(url, num_retries = 5):
'''
function: 下载网页源代码，如果遇到 5xx 错误状态，则继续尝试下载，直到下载 num_retries 次为止。
'''
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:
return download(url, num_retries-1)

return html