上一篇文章中我们介绍了爬虫的实现，及爬虫爬取数据的功能，这里会遇到几个问题，比如网站中robots.txt文件，里面有禁止爬取的URL，还有爬虫是否支持代理功能，及有些网站对爬虫的风控措施，设计的爬虫下载限速功能。 1、解析robots.txt 首先，我们需要解析robots.txt文件，以避免下载禁止爬取的URL。适用Python自带的robotparser模块，就可以轻松的完成这项工作，如下面的代码。 robotparser模块首先加载robots.txt文件，然后通过can_fetch()函数确定指定的用户代理是否允许访问网页。
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为了将该功能集成到爬虫中，我们需要在crawl循环中添加该检查。
while crawl_queue: url = crawl_queue.pop() # check url passes robots.txt restrictions if rp.can_fetch(user_agent,url): ... else： print 'Blocked by robots.txt:',url2、支持代理 有时我们需要使用代理访问某个网站。比如，Netflix屏蔽了美国以外的大多数国家。使用urllib2支持代理并没有想象的那么容易（可以尝试使用更友好的Python HTTP 模块requests来实现该功能）下面是使用urllib2支持代理的代码。 proxy = … opener = urllib2.build_opener() proxy_params = {urlparse.urlparse(url).scheme:proxy} opener.add_header(urllib2.ProxyHandler(proxy_params)) response = opener.open(request) 下面是集成了该功能的新版本download函数。
def download(url, headers, proxy, num_retries, data=None): print 'Downloading:', url request = urllib2.Request(url, data, headers) opener = urllib2.build_opener() if proxy: proxy_params = {urlparse.urlparse(url).scheme: proxy} opener.add_handler(urllib2.ProxyHandler(proxy_params)) try: response = opener.open(request) html = response.read() code = response.code except urllib2.URLError as e: print 'Download error:', e.reason html = '' if hasattr(e, 'code'): code = e.code if num_retries > 0 and 500 <= code < 600: # retry 5XX HTTP errors html = download(url, headers, proxy, num_retries - 1, data) else: code = None return html3、下载限速 如果我们爬取网站的速度过快，就会面临被封禁或是造成服务器过载的风险。为了降低这些风险，我们可以在两次下载之间添加延时，从而对爬虫限速。下面是实现了该功能的类的代码。
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): """Delay if have accessed this domain recently """ domain = urlparse.urlsplit(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() Throttle类记录了每个域名上次访问的时间，如果当前时间距离上次访问时间小于指定延时，则执行睡眠操作。我们可以在每次下载之前调用Throttle对爬虫进行限速。4、避免爬虫陷阱 目前，我们的爬虫会跟踪所有之前没有访问过的链接。但是，一些网站会动态生成页面内容，这样就会出现无限多的网页。比如，网站有一个在线日历功能，提供了可以访问下一个月或下一年的链接，那么下个月的页面中同样会有访问再下个月的链接，这样页面就会无止境的链接下去。这种情况成为爬虫陷阱。 想要避免陷入爬虫陷阱，一个简单的方法是积累到达当前网页经过了多少个链接，也就是深度。当到达最大深度时，爬虫就不再向对列中添加该网页中的链接了。要实现这一功能，我们需要修改seen变量。该变量原先只记录访问过的网页链接，现在修改为一个字典，增加了页面深度的记录。 def link_crawler(…,max_length = 2): max_length = 2 seen = {} … depth = seen[url] if depth != max_depth: for link in links: if link not in seen: seen[link] = depth + 1 crawl_queue.qppend(link) 现在有了这一功能，我们就有信心爬虫的最终一定能够完成。如果想要禁用该功能，只需要将max_depth设为一个负数即可，此时当前深度永远不会与之相等。 最终版本
import re import urlparse import urllib2 import time from datetime import datetime import robotparser import Queue from scrape_callback3 import ScrapeCallback 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, scrape_callback=None): """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 = [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() depth = seen[url] # 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 = [] if scrape_callback: links.extend(scrape_callback(url, html) or []) if depth != max_depth: # can still crawl further if link_regex: # filter for links matching our regular expression 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): """Delay if have accessed this domain recently """ domain = urlparse.urlsplit(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=None): print 'Downloading:', url request = urllib2.Request(url, data, headers) opener = urllib2.build_opener() if proxy: proxy_params = {urlparse.urlparse(url).scheme: proxy} opener.add_handler(urllib2.ProxyHandler(proxy_params)) try: response = opener.open(request) html = response.read() code = response.code except urllib2.URLError as e: print 'Download error:', e.reason html = '' if hasattr(e, 'code'): code = e.code if num_retries > 0 and 500 <= code < 600: # retry 5XX HTTP errors html = download(url, headers, proxy, num_retries - 1, data) else: code = None return html def normalize(seed_url, link): """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=["\'](.*?)["\']', 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') link_crawler('http://fund.eastmoney.com',r'/fund.html#os_0;isall_0;ft_;pt_1',max_depth=-1,scrape_callback=ScrapeCallback觉得好，就打赏下小编吧~
