Summary of Python crawler skills

Python crawler: Summary of some commonly used crawler techniques

There are also many reuse processes in the development process of crawlers. Here is a summary, which can save some things in the future.

1. Basic crawling of web pages

get method

import urllib2
url "http://www.baidu.com"
respons = urllib2.urlopen(url)
print response.read()

post method

import urllib
import urllib2

url = "http://abcde.com"
form = {'name':'abc','password':'1234'}
form_data = urllib.urlencode(form)
request = urllib2.Request(url,form_data)
response = urllib2.urlopen(request)
print response.read()

2. Use proxy IP

In the process of developing crawlers, IPs are often blocked. In this case, you need to use the proxy IP;

There is a ProxyHandler class in the urllib2 package, through which you can set up a proxy to access the web page, as shown in the following code snippet:

import urllib2

proxy = urllib2.ProxyHandler({'http': '127.0.0.1:8087'})
opener = urllib2.build_opener(proxy)
urllib2.install_opener(opener)
response = urllib2.urlopen('http://www.baidu.com')
print response.read()

3. Cookies processing

Cookies are data (usually encrypted) stored on the user's local terminal by some websites in order to identify the user's identity and perform session tracking. , Python provides the cookielib module for processing cookies. The main function of the cookielib module is to provide objects that can store cookies, so that it can be used with the urllib2 module to access Internet resources.

Code snippet:

import urllib2, cookielib

cookie_support= urllib2.HTTPCookieProcessor(cookielib.CookieJar())
opener = urllib2.build_opener(cookie_support)
urllib2.install_opener(opener)
content = urllib2.urlopen('http://XXXX').read()

The key is CookieJar(), which is used to manage HTTP cookie values, store cookies generated by HTTP requests, and add cookie objects to outgoing HTTP requests. The entire cookie is stored in memory, and the cookie will be lost after garbage collection of the CookieJar instance. All processes do not need to be operated separately.

Add cookie manually

Copy code The code is as follows:

cookie = "PHPSESSID=91rurfqm2329bopnosfu4fvmu7; kmsign= 55d2c12c9b1e3; KMUID=b6Ejc1XSwPq9o756AxnBAg="
request.add_header("Cookie", cookie)

4. Disguise as a browser

Some websites are disgusted with crawlers Visit, so all requests to crawlers will be rejected. Therefore, HTTP Error 403: Forbidden often occurs when using urllib2 to directly access websites.

Pay special attention to some headers. The server will check these headers.

1).User-Agent Some Server or Proxy will check this value to determine whether it is a Request
2).Content-Type. When using the REST interface, Server will check this value to determine how the content in the HTTP Body should be parsed. .

This can be achieved by modifying the header in the http package. The code snippet is as follows:

import urllib2

headers = {
 'User-Agent':'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.6) Gecko/20091201 Firefox/3.5.6'
}
request = urllib2.Request(
 url = 'http://my.oschina.net/jhao104/blog?catalog=3463517',
 headers = headers
)
print urllib2.urlopen(request).read()

5. Page parsing

The most powerful tool for page parsing is of course regular expressions. This is different for different users of different websites. Without too much explanation, here are two better URLs:

Regular Expressions Online test: http://tool.oschina.net/regex/

The second is the parsing library. Two commonly used ones are lxml and BeautifulSoup. For the use of these two, we will introduce two better websites. ：

lxml：http://my.oschina.net/jhao104/blog/639448

BeautifulSoup：http://cuiqingcai.com/1319.html

for My evaluation of these two libraries is that they are both HTML/XML processing libraries. Beautifulsoup is implemented purely in python and is inefficient, but its functions are practical. For example, the source code of an HTML node can be obtained through search results; lxmlC language encoding is highly efficient. , Support Xpath

6, Verification code processing

For some simple verification codes, simple identification can be performed. I have only done some simple verification code recognition. However, some anti-human verification codes, such as 12306, can be manually coded through the coding platform. Of course, this requires a fee.

7. Gzip compression

Have you ever encountered some web pages that are garbled no matter how they are transcoded? Haha, that means you don’t know that many web services have the ability to send compressed data, which can reduce the large amount of data transmitted on the network line by more than 60%. This is especially true for XML web services because XML data can be compressed to a very high degree.

But generally the server will not send compressed data for you unless you tell the server that you can handle compressed data.

So you need to modify the code like this:

import urllib2, httplib
request = urllib2.Request('http://xxxx.com')
request.add_header('Accept-encoding', 'gzip') 1
opener = urllib2.build_opener()
f = opener.open(request)

This is the key: create a Request object and add an Accept-encoding header to tell the server that you can Accept gzip compressed data

Then decompress the data:

import StringIO
import gzip

compresseddata = f.read() 
compressedstream = StringIO.StringIO(compresseddata)
gzipper = gzip.GzipFile(fileobj=compressedstream) 
print gzipper.read()

8. Multi-threaded concurrent capture

If a single thread is too slow, multi-threading is needed. Here is a simple thread pool template. This program simply prints 1-10, but it can be seen that it is concurrent.

Although Python's multi-threading is useless, it can still improve efficiency to a certain extent for network-frequent crawlers.

from threading import Thread
from Queue import Queue
from time import sleep
# q是任务队列
#NUM是并发线程总数
#JOBS是有多少任务
q = Queue()
NUM = 2
JOBS = 10
#具体的处理函数，负责处理单个任务
def do_somthing_using(arguments):
 print arguments
#这个是工作进程，负责不断从队列取数据并处理
def working():
 while True:
 arguments = q.get()
 do_somthing_using(arguments)
 sleep(1)
 q.task_done()
#fork NUM个线程等待

 alert(“Hello CSDN”);
for i in range(NUM):
 t = Thread(target=working)
 t.setDaemon(True)
 t.start()
#把JOBS排入队列
for i in range(JOBS):
 q.put(i)
#等待所有JOBS完成
q.join()

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