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Crawler——链接爬虫
对数据的提取和收集也是数据分析中一大重点,所以,学习爬虫是非常有用的。完成数据采集,对后面的数据分析做下基础。
今天,要介绍的是来自《Web Scraping With Python》中的一个示例——链接爬虫。对于此类进行了简单的总结,便于相互学习。
#! /usr/bin/env python # -*- coding:utf-8 -*- import re import urlparse import urllib2 import time from datetime import datetime import robotparser import Queue # 链接爬虫 ‘‘‘ 一个链接爬虫需要考虑以下几个问题: 1.下载网页时,我们可能会遇到不可控制的错误,比如请求的网页可能不存在。就要用到try和except语句,捕获异常。 2.下载网页时,我们也可能会遇上临时性的错误,比如服务器过载返回的503 Service Unavailable错误。就要多尝试几次下载。 3.一些网站可能会封杀默认的用户代理,所以,我们应该重新设置一个用户代理user_agent=‘wswp‘。 4.下载网站链接时,应当考虑符合自己目标的链接,筛选出自己感谢的东西。通常用正则表达式来匹配这些链接。‘<a[^>]+href=http://www.mamicode.com/["\‘](.*?)["\‘]‘. 5.应当考虑网页中的链接是什么链接,如果是绝对链接就没事,如果是相对链接就应该创建绝对链接。urlparse.urljoin() 6.爬取网页的时候,经常会出现将要爬取的网页中也有爬取过的链接,这样会造成不断循环。所以要建立一个URL管理器,管理爬取过的和未爬取的 7.所有爬虫都应当遵守爬虫协议(robots.txt),所以要引入robotparser模块,以避免下载禁止爬取的URL 8.有时我们需要使用代理访问某个网站。 9.如果我们爬取网站的速度过快,就会面临被封禁或者服务器过载的风险。所以应当在两次下载之间添加延时。delay 10.有些网站中含有动态内容,如果爬取该网页就会出现无限制的网页,所以为了避免爬虫陷阱,最好设置一个爬取深度(max_depth)——记录到达当前网页经过了多少链接。 ‘‘‘ def link_crawler(seed_url, link_regex=None, delay=5, max_depth=-1, max_urls=-1, headers=None, user_agent=‘wswp‘, proxy=None, num_retries=1): """Crawl from the given seed URL following links matched by link_regex """ # the queue of URL‘s that still need to be crawled crawl_queue = Queue.deque([seed_url]) # the URL‘s that have been seen and at what depth seen = {seed_url: 0} # track how many URL‘s have been downloaded num_urls = 0 rp = get_robots(seed_url) throttle = Throttle(delay) headers = headers or {} if user_agent: headers[‘User-agent‘] = user_agent while crawl_queue: url = crawl_queue.pop() # check url passes robots.txt restrictions if rp.can_fetch(user_agent, url): throttle.wait(url) html = download(url, headers, proxy=proxy, num_retries=num_retries) links = [] depth = seen[url] if depth != max_depth: # can still crawl further if link_regex: # filter for links matching our regular expression for link in get_links(html): if re.match(link_regex, link): links.extend(link) # links.extend(link for link in get_links(html) if re.match(link_regex, link)) for link in links: link = normalize(seed_url, link) # check whether already crawled this link if link not in seen: seen[link] = depth + 1 # check link is within same domain if same_domain(seed_url, link): # success! add this new link to queue crawl_queue.append(link) # check whether have reached downloaded maximum num_urls += 1 if num_urls == max_urls: break else: print ‘Blocked by robots.txt:‘, url class Throttle: """Throttle downloading by sleeping between requests to same domain """ def __init__(self, delay): # amount of delay between downloads for each domain self.delay = delay # timestamp of when a domain was last accessed self.domains = {} def wait(self, url): domain = urlparse.urlparse(url).netloc last_accessed = self.domains.get(domain) if self.delay > 0 and last_accessed is not None: sleep_secs = self.delay - (datetime.now() - last_accessed).seconds if sleep_secs > 0: time.sleep(sleep_secs) self.domains[domain] = datetime.now() def download(url, headers, proxy, num_retries, data=http://www.mamicode.com/None):"""Normalize this URL by removing hash and adding domain """ link, _ = urlparse.urldefrag(link) # remove hash to avoid duplicates return urlparse.urljoin(seed_url, link) def same_domain(url1, url2): """Return True if both URL‘s belong to same domain """ return urlparse.urlparse(url1).netloc == urlparse.urlparse(url2).netloc def get_robots(url): """Initialize robots parser for this domain """ rp = robotparser.RobotFileParser() rp.set_url(urlparse.urljoin(url, ‘/robots.txt‘)) rp.read() return rp def get_links(html): """Return a list of links from html """ # a regular expression to extract all links from the webpage webpage_regex = re.compile(‘<a[^>]+href=http://www.mamicode.com/["\‘](.*?)["\‘]‘, re.IGNORECASE) # list of all links from the webpage return webpage_regex.findall(html) if __name__ == ‘__main__‘: link_crawler(‘http://example.webscraping.com‘, ‘/(index|view)‘, delay=0, num_retries=1, user_agent=‘BadCrawler‘) link_crawler(‘http://example.webscraping.com‘, ‘/(index|view)‘, delay=0, num_retries=1, max_depth=-1, user_agent=‘GoodCrawler‘)
Crawler——链接爬虫
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