1. Crawling Part 2

2. Expected schedule tonight Quiz (~30 minutes) – In Blackboard. A bit more than a quiz. Review of project plans (~15 minutes) Lab on crawling (30 - 45 minutes) – Visualizing crawl – Using a basic crawler tool Explore the department pages Explore some other sites Highlight, extract and save elements, etc. Python for fetching web pages (~30 minutes) – Spot check activity Exception handling (~30 minutes) – Examples – Spot check activity 2

3. Next week – 9/20 First time I am gone Nakul (our class assistant) will be here Lab exercises – Create database – Deal with messy HTML (Beautiful Soup) Introduction of Nutch – Full-featured open source crawler that is widely used for real applications As time allows, begin work on your crawler in class with help as needed from Nakul – Complete as homework and submit through blackboard 3

4. 9/27 I will be in Berlin for a conference on digital libraries We will have an online class – Timed quiz to begin at the normal class time. – Presentation: What to do with a collection of documents after you have gathered them – An introduction to Indexing and Information Retrieval There will be exercises to do, embedded in the presentation. There will be places to put the exercise results in Blackboard. 4

5. Visualizing Crawl We looked at the architecture of a crawl. Now we will see what a crawl looks like as it is happening We will use a tool to direct a crawl – WebSphinx – Created at Carnegie Mellon – Now available at SourceForge – This is not a very robust application and it may be flaky, but we can see some interesting things by using it. 5

6. WebSphinx See http://www.cs.cmu.edu/~rcm/websphin x/ http://www.cs.cmu.edu/~rcm/websphin x/ and http://sourceforge.net/projects/websphi nx/ http://sourceforge.net/projects/websphi nx/ 6

7. WebSphinx lab Download the software Do a simple crawl: – Crawl: the subtree – Starting URLs: Pick a favorite spot. Don’t all use the same one (Politeness) – Action: none – Press Start – Watch the pattern of links emerging – When crawl stops, click on the statistics tab. How many threads? How many links tested?, links in queue? How many pages visited? Pages/second? Note memory use 7

8. Advanced WebSphinx Default is depth-first crawl Now do an advanced crawl: – Advanced Change Depth First to Breadth First Compare statistics Why is Breadth First memory intensive – Still in Advanced, choose Pages tab Action: Highlight, choose color URL *new* 8

9. Just in case … 9 Crawl site: http://www.acm.org

10. From acm crawl of “new” 10

11. Using WebSphinx to capture Action: extract HTML tag expression: as HTML to – give the file name the extension html as this does not happen automatically – click the button with … to show where to save the file on Pages: All Pages Start Example results: acm-images.htmlacm-images.html 11

12. Python module for web access urllib2 – Note – this is for Python 2.x, not Python 3 Python 3 splits the urllib2 materials over several modules – import urllib2 – urllib2.urlopen(url [,data][, timeout]) Establish a link with the server identified in the url and send either a GET or POST request to retrieve the page. The optional data field provides data to send to the server as part of the request. If the data field is present, the HTTP request used is POST instead of GET – Use to fetch content that is behind a form, perhaps a login page – If used, the data must be encoded properly for including in an HTTP request. See http://www.w3.org/TR/html4/interact/forms.html#h-17.13.4.1 http://www.w3.org/TR/html4/interact/forms.html#h-17.13.4.1 timeout defines time in seconds to be used for blocking operations such as the connection attempt. If it is not provided, the system wide default value is used. 12 http://docs.python.org/library/urllib2.html

13. URL fetch and use urlopen returns a file-like object with methods: – Same as for files: read(), readline(),fileno(), close() – New for this class: info() – returns meta information about the document at the URL getcode() – returns the HTTP status code sent with the response (ex: 200, 404) geturl() – returns the URL of the page, which may be different from the URL requested if the server redirected the request 13

14. URL info info() provides the header information that http returns when the HEAD request is used. ex: >>> print mypage.info() Date: Mon, 12 Sep 2011 14:23:44 GMT Server: Apache/1.3.27 (Unix) Last-Modified: Tue, 02 Sep 2008 21:12:03 GMT ETag: "2f0d4-215f-48bdac23" Accept-Ranges: bytes Content-Length: 8543 Connection: close Content-Type: text/html 14

15. URL status and code >>> print mypage.getcode() 200 >>> print mypage.geturl() http://www.csc.villanova.edu/~cassel/ 15

16. Spot check Work with a partner – Create a file with at least 3 good urls – For each url, fetch the page Create a file to hold the content of the page for each line on the page – strip off the html code – write the remaining content to the file – if there is a link on the page, add it to a list of links – Print out the list of links – Print out the number of lines in each file, with the file name 16 Most of this was done for homework.

17. Messy HTML So far, we have assumed that the HTML of a page is good. – Browsers may be forgiving. – Human and computerized html generators make mistakes. Tools for dealing with imperfect html include Beautiful Soup and NekoHTML. – Beautiful Soup is Python, NekoHTML is Java – Beautiful Soup – next week 17

18. 18 Exceptions: How to Deal with Error Situations number = 0 while not 1 <= number <= 10: try: number= int(raw_input('Enter number from 1 to 10: ')) if not 1 <= number <= 10: print 'Your number must be from 1 to 10:' except ValueError: print 'That is not a valid integer.' Here: recognize an error condition and deal with it If the named error occurs, the “except” clause is executed and the loop is terminated. book slide

19. 19 Exceptions (continued) What if a negative is entered for square root? Can raise an exception when something unusual occurs. def sqrE(number): if number < 0: raise ValueError('number must be positive') #do square root code as before Note: ValueError is an existing, defined error class book slide

20. 20 Exceptions (continued) #What if value entered is not a number? def sqrtF(number): if not isinstance(number, (int, float)): raise TypeError('number must be numeric') if number < 0: raise ValueError('number must be positive') #do square root code as before book slide

21. 21 How Much Type Checking is Enough? A function with little type checking of its parameters may be difficult to diagnose errors. A function with much type checking of its parameters will be harder to write and consume more time executing. If the function is one that only you will use you may need less type checking. A function written for others should have more type checking. book slide

22. Checking for failed url fetch 22 import urllib2 url = raw_input("Enter the URL of the page to fetch: ") try: linecount=0 page=urllib2.urlopen(url) result = page.getcode() if result == 200: for line in page: print line linecount+=1 print page.info() print page.getcode() print "Page contains ",linecount," lines." except: print "\nBad URL: ", url The except clause is triggered by any error in the try

23. Spot check on Exceptions In pairs, write python code to do the following: – Accept input from the keyboard Prompt with instructions to enter a number, and to enter some character you choose to end – Verify that the value read is numeric – Calculate the minimum, maximum and average of the values read – Terminate reading when a non numeric character is entered – Print out the values calculated 23

24. For October 4 Your basic crawler (Individual work) – Read a seed url – Fetch the page – Extract links from the page Put the links on a queue of pages to visit – Extract the text from the page, stripping off the html code Deal with possibly bad html Put the extracted documents in a database for later analysis – Take the next url from the queue and repeat – How will you deal with robot exclusions? – What will you do about rapid access to a server? – We will do individual presentations of working crawlers then. 24