Posted 01 Feb 2012 in efficiency and python

Python and other scripting languages are sometimes dismissed because of their inefficiency compared to compiled languages like C. For example here are implementations of the fibonacci sequence in C and Python:

int fib(int n){
   if (n < 2)
     return n;
   else
     return fib(n - 1) + fib(n - 2);
}

int main() {
    fib(40);
    return 0;
}
def fib(n):
  if n < 2:
     return n
  else:
     return fib(n - 1) + fib(n - 2)
fib(40)

And here are the execution times:

$ time ./fib
3.099s
$ time python fib.py
16.655s

As expected C has a much faster execution time - 5x faster in this case.

In the context of web scraping, executing instructions is less important because the bottleneck is I/O - downloading the webpages. But I use Python in other contexts too so let’s see if we can do better.

First install psyco. On Linux this is just:

sudo apt-get install python-psyco

Then modify the Python script to call psyco:

import psyco
psyco.full()

def fib(n):
  if n < 2:
     return n
  else:
     return fib(n - 1) + fib(n - 2)
fib(40)

And here is the updated execution time:

$ time python fib.py
3.190s

Just 3 seconds - with psyco the execution time is now equivalent to the C example! Psyco achieves this by compiling code on the fly to avoid interpreting each line.

I now add the below snippet to most of my Python scripts to take advantage of psyco when installed:

try:
    import psyco
    psyco.full()
except ImportError:
    pass # psyco not installed so continue as usual


Posted 04 Jan 2012 in big picture, opensource, and sitescraper

I have been interested in automatic approaches to web scraping for a few years now. During university I created the SiteScraper library, which used training cases to automatically scrape webpages. This approach was particularly useful for scraping a website periodically because the model could automatically adapt when the structure was updated but the content remained static.

However this approach is not helpful for me these days because most of my work involves scraping a website once-off. It is quicker to just specify the XPaths required than collect and test the training cases.

I would still like an automated approach to help me work more efficiently. Ideally I would have a solution that when given a website URL:

  • crawls the website
  • organize the webpages into groups that share the same template (a directory page will have a different HTML structure than a listing page)
  • the group with the most amount of webpages should be the listings
  • compare these listing webpages to find what is static (the template) and what changes
  • the parts that change represent dynamic data such as description, reviews, etc

Apparently this process of scraping data automatically is known as wrapper induction in academia. Unfortunately there do not seem to be any good open source solutions yet. The most commonly referenced one is Templatemaker, which is aimed at small text blocks and crashes in my test cases of real webpages. The author stopped development in 2007.

Some commercial groups have developed their own solutions so this certainly is technically possible:

If I do not find an open source solution I plan to attempt building my own later this year.


Posted 30 Dec 2011 in concurrent, efficiency, example, javascript, python, qt, and webkit

In a previous post I showed how to scrape a list of webpages. That is fine for small crawls but will take too long otherwise. Here is an updated example that downloads the content in multiple threads.

import sys
from collections import deque # threadsafe datatype
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtWebKit import *
NUM_THREADS = 3 # how many threads to use

class Render(QWebView):
    active = deque() # track how many threads are still active
    data = {} # store the data

    def __init__(self, urls):
        QWebView.__init__(self)
        self.loadFinished.connect(self._loadFinished)
        self.urls = urls
        self.crawl()

    def crawl(self):
        try:
            url = self.urls.pop()
            print 'downloading', url
            Render.active.append(1)
            self.load(QUrl(url))
        except IndexError:
            # no more urls to process
            if not Render.active:
                # no more threads downloading
                print 'finished'
                self.close()

    def _loadFinished(self, result):
        # process the downloaded html
        frame = self.page().mainFrame()
        url = str(frame.url().toString())
        Render.data[url] = frame.toHtml()
        Render.active.popleft()
        self.crawl() # crawl next URL in the list

app = QApplication(sys.argv) # can only instantiate this once so must move outside class 
urls = deque(['http://webscraping.com', 'http://webscraping.com/questions',
    'http://webscraping.com/blog', 'http://webscraping.com/projects'])
renders = [Render(urls) for i in range(NUM_THREADS)]
app.exec_() # will execute qt loop until class calls close event
print Render.data.keys()

Source code is available at my bitbucket account.


Posted 06 Dec 2011 in example, javascript, python, qt, and webkit

I made an earlier post about using webkit to process the JavaScript in a webpage so you can access the resulting HTML. A few people asked how to apply this to multiple webpages, so here it is:

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtWebKit import *

class Render(QWebPage):  
  def __init__(self, urls, cb):
    self.app = QApplication(sys.argv)  
    QWebPage.__init__(self)  
    self.loadFinished.connect(self._loadFinished)  
    self.urls = urls  
    self.cb = cb
    self.crawl()  
    self.app.exec_()  
      
  def crawl(self):  
    if self.urls:  
      url = self.urls.pop(0)  
      print 'Downloading', url  
      self.mainFrame().load(QUrl(url))  
    else:  
      self.app.quit()  
        
  def _loadFinished(self, result):  
    frame = self.mainFrame()  
    url = str(frame.url().toString())  
    html = frame.toHtml()  
    self.cb(url, html)
    self.crawl()  


def scrape(url, html):
    pass # add scraping code here


urls = ['http://webscraping.com', 'http://webscraping.com/blog']  
r = Render(urls, cb=scrape)

This is a simple solution that will keep all HTML in memory, which is not practical for large crawls. For large crawls you should save the results to disk. I use the pdict module for this. Updated script to take a callback for processing the download immediately and avoid storing in memory.

Source code is available at my bitbucket account.


Posted 03 Dec 2011 in big picture, learn, and python

I often get asked how to learn about web scraping. Here is my advice.

First learn a popular high level scripting language. A higher level language will allow you to work and test ideas faster. You don’t need a more efficient compiled language like C because the bottleneck when web scraping is bandwidth rather than code execution. And learn a popular one so that there is already a community of other people working at similar problems so you can reuse their work. I use Python, but Ruby or Perl would also be a good choice.

The following advice will assume you want to use Python for web scraping.
If you have some programming experience then I recommend working through the Dive Into Python book:

Make sure you learn all the details of the urllib2 module. Here are some additional good resources:

Learn about the HTTP protocol, which is how you will interact with websites.

Learn about regular expressions:

Learn about XPath:

If necessary learn about JavaScript:

These FireFox extensions can make web scraping easier:

Some libraries that can make web scraping easier:

Some other resources:


Posted 29 Nov 2011 in example and proxies

Proxies can be necessary when web scraping because some websites restrict the number of page downloads from each user. With proxies it looks like your requests come from multiple users so the chance of being blocked is reduced.

Most people seem to first try collecting their proxies from the various free lists such as this one and then get frustrated because the proxies stop working. If this is more than a hobby then it would be a better use of your time to rent your proxies from a provider like packetflip, USA proxies, or proxybonanza. These free lists are not reliable because so many people use them.

Each proxy will have the format login:password@IP:port
The login details and port are optional. Here are some examples:

  • bob:eakej34@66.12.121.140:8000
  • 219.66.12.12
  • 219.66.12.14:8080

With the webscraping library you can then use the proxies like this:

from webscraping import download  
D = download.Download(proxies=proxies, user_agent=user_agent)  
html = D.get(url)

The above script will download content through a random proxy from the given list. Here is a standalone version:

import urllib2
import gzip
import random  
import StringIO  
  
def fetch(url, data=None, proxies=None, user_agent='Mozilla/5.0'):  
    """Download the content at this url and return the content  
"""  
    opener = urllib2.build_opener()  
    if proxies:  
        # download through a random proxy from the list  
        proxy = random.choice(proxies)  
        if url.lower().startswith('https://'):  
            opener.add_handler(urllib2.ProxyHandler({'https' : proxy}))  
        else:  
            opener.add_handler(urllib2.ProxyHandler({'http' : proxy}))  
      
    # submit these headers with the request  
    headers =  {'User-agent': user_agent, 'Accept-encoding': 'gzip', 'Referer': url}  
      
    if isinstance(data, dict):  
        # need to post this data  
        data = urllib.urlencode(data)  
    try:  
        response = opener.open(urllib2.Request(url, data, headers))  
        content = response.read()  
        if response.headers.get('content-encoding') == 'gzip':  
            # data came back gzip-compressed so decompress it            
            content = gzip.GzipFile(fileobj=StringIO.StringIO(content)).read()  
    except Exception, e:  
        # so many kinds of errors are possible here so just catch them all  
        print 'Error: %s %s' % (url, e)  
        content = None  
    return content