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

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

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

def link_crawler(seek_url, link_regex):
    """Crawl from the given seed URL following links matched by _link_regexes
    """
    crawl_queue = [seek_url]
    while crawl_queue:
        url = crawl_queue.pop()
        html = download(url)
        
        html = html.decode("UTF-8")
        # filter for links matching our regular expression
        for link in get_links(html):
            if re.match(link_regex, link):
                link = urlparse.urljoin(seek_url,link)
                crawl_queue.append(link)
                
def get_links(html):
    """Return a list of links from html
    """
    # a regular experssion to extract all links from the webpage
    webpage_regex = re.compile(‘<a[^>]+href=["\‘](.*?)["\‘]‘, re.IGNORECASE)
    # list of all links from the webpage
    return webpage_regex.findall(html)

# seek_url = ‘http://example.webscraping.com‘
# link_regex = ‘/(index|view)‘
# 
# link_crawler(seek_url, link_regex)

link_crawler("http://example.webscraping.com", ‘/(index|view)‘)

1.4.4鏈接爬蟲（每天一更）