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:‘,url

2、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=http://www.mamicode.com/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

3、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=http://www.mamicode.com/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=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‘)
    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~