python爬虫高级功能
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，比方站点中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。及有些站点对爬虫的风控措施。设计的爬虫下载限速功能。
1、解析robots.txt
首先，我们须要解析robots.txt文件。以避免下载禁止爬取的URL。适用Python自带的robotparser模块，就能够轻松的完毕这项工作，如以下的代码。
robotparser模块首先载入robots.txt文件。然后通过can_fetch()函数确定指定的用户代理是否同意訪问网页。
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，我们须要在crawl循环中加入该检查。
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
有时我们须要使用代理訪问某个站点。比方，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easy（能够尝试使用更友好的Python HTTP 模块requests来实现该功能）以下是使用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)
以下是集成了该功能的新版本号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
假设我们爬取站点的速度过快，就会面临被封禁或是造成server过载的风险。为了减少这些风险，我们能够在两次下载之间加入延时，从而对爬虫限速。以下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类记录了每一个域名上次訪问的时间。假设当前时间距离上次訪问时间小于指定延时。则运行睡眠操作。我们能够在每次下载之前调用Throttle对爬虫进行限速。
4、避免爬虫陷阱
眼下，我们的爬虫会跟踪全部之前没有訪问过的链接。
可是。一些站点会动态生成页面内容，这样就会出现无限多的网页。
比方，站点有一个在线日历功能。提供了能够訪问下一个月或下一年的链接，那么下个月的页面中相同会有訪问再下个月的链接。这样页面就会无止境的链接下去。这样的情况成为爬虫陷阱。
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加入该网页中的链接了。
要实现这一功能，我们须要改动seen变量。该变量原先仅仅记录訪问过的网页链接，如今改动为一个字典，添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)
如今有了这一功能，我们就有信心爬虫的终于一定能够完毕。假设想要禁用该功能。仅仅须要将max_depth设为一个负数就可以，此时当前深度永远不会与之相等。
终于版本号
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
认为好。就打赏下小编吧~
