# -*- coding: utf-8 -*-
‘‘‘
Created on 2019年5月7日

@author: 薛衛衛
‘‘‘
import itertools
import urllib.request
import re

def download(url, user_agent="wswp",num_retries=2):
    print("Downloading: " , url)
    headers = { ‘User-agent‘: user_agent}
    request = urllib.request.Request(url, headers=headers)
    try:
        html = urllib.request.urlopen(request).read()
    except urllib.request.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, user_agent, num_retries-1)
    return html

for page in itertools.count(1):
    url = ‘http://example.webscraping.com/view/-%d‘ % page
    html = download(url)
    if html is None:
        break
    else:
        # success - can scrape the result
        pass
    
#     
# # maximum number of consecutive download errors allowed
# max_error = 5
# # current number of consecutive download errors
# nun_errors = 0
# for page in itertools.count(1):
#     url = ‘http://example.webcraping.com/view/-%d‘ % page
#     html = download(url)
#     if html is None:
#         # received an error trying to download this webpage
#         num_errors +=1
#         if num_errors == max_errors:
#             # reached maxinum number of 
#             # consecutive errors so exit
#             break
#         else:
#             # success - can scrape the result
#             # ...
#             num_errors = 0

1.4.2python ID遍歷爬蟲（每天一更）