Perl & LWP

When direct database access to the information you need isn't available, but web pages with the right data are, you might pursue "screen-scraping" -- fetching a web page and scanning its text for the appropriate pieces of text in order to do further processing. LWP (Library for WWW access in Perl) is a collection of module to help you do this. mir writes: " Perl & LWP is a solid, no-nonsense book that will teach you how to do screen-scraping using Perl. It describes how to automatically retrieve and use information from the web. An introduction to LWP and related modules from simple to advanced uses and various ways to extract information from the returned HTML." Perl & LWP author Sean M. Burke pages 264 publisher O'Reilly and Associates rating 9 reviewer mir ISBN 0596001789 summary Excellent introduction to extracting and processing information from web sites.

The good: The book has a nice style and good coverage of the subject, includes introduction to all the modules used, reference material and includes good, well-developed examples. I really liked the way the authors describe the basic methodology to develop screen-scraping code, from analyzing an HTML page to extracting and displaying only what you are interested in.

The bad: Not much is bad, really. Some chapters are a little dry, though, and sometimes the reference material could be better separated from the rest of the text. The book covers only simple access to web sites; I would have liked to see an example where the application engages in more dialogue with the server. In addition, the appendixes are not really useful. More Info:

If it had not been published by O'Reilly, Perl and LWP could have been titled Leveraging the Web: Object-Oriented techniques for information re-purposing, or Web Services, Generation 0. An even better title would have been Screen-scraping for fun and profit: one day we might all use Web Services and easily get the information we need from various providers using SOAP or REST, but in the meantime the common way to achieve this goal is just to write code to connect to a web server, retrieve a page and extract the information from the HTML. In short, "screen-scraping." This will teach you all about using Perl to get Web pages and extract their "substantifique moëlle" (the pith essence, the essentials) for your own usage. It showcases the power of Perl for that kind of job, from regular expressions to powerful CPAN modules.

At 200 pages, plus 40 pages of appendices and index, this one is part of that line of compact O'Reilly books which covers only a narrow topic in each volume but which covers those topics well. Just like Perl & XML , its target audience is Perl programmers who need to tackle a new domain. It gives them a toolbox and basic techniques that to provide a jump start and avoid many mistakes.

Perl & LWP starts from the basics: installing LWP, using LWP::Simple to retrieve a file from a URL, then goes on to a more complete description of the advanced LWP methods for dealing with forms and munging URLs. It continues with five chapters on how to process the HTML you get, using regular expressions, an HTML tokenizer and HTML::TreeBuilder, a powerful module that builds a tree from the HTML. It goes on with an explanation of how to allow your programs to access sites that require cookies, authentication or the use of a specific browser. The final chapter wraps it all up in a bigger example: a web-spider.

The book is well-written and to-the-point. It is structured in a way that mimics what a programmer new to the field would do: start from the docs for a module, play with it, write snippets of code that use the various functions of the module, then go on to coding real-life examples. I particularly liked the fact that the author often explains the whys, and not only the hows, of the various pieces of code he shows us.

It is interesting to note that going from regular expressions to ever more powerful modules is a path followed also by most Perl programmers, and even by the language itself: when Perl starts being applied to a new domain first there are no modules, then low-level ones start appearing, then, as the understanding of the problem grows, easier-to-use modules are written.

Finally I would like to thank the author for following his own advice by including interesting examples and above all for not including anything about retrieving stock-quotes.

Another recommended book on the subject is Network Programming with Perl by Lincoln D. Stein, which covers a wider subject but devotes 50 pages to this topic and is also very good.

Breakdown by chapter:

• 1. Introduction to Web Automation (15 pages): an overview of what this book will teach you, how to install Gisle Aas' LWP, some interesting words of caution about the brittleness of screen-scraping code, copyright issues and respect for the servers you are about to hammer, and finally a very simple example that shows the basic process of web automation.

  2. Web Basics (16p): describes how to use LWP::Simple, an easy way to do some simple processing.

  3. The LWP Class Model (17p): a slightly steeper read, closer to a reference than to a real introduction that lays out the ground work for the good stuff ahead.

  4. URLs (10p): another reference chapter, this one will teach you all you can do with URLs using the URI module. Although the chapter is clear and complete it includes little explanation as to why you will need to process URLs and it is not even mentioned in the introduction roadmap.

  5. Forms (28p): a complete and easy to read chapter. It includes a long description of HTML form fields that can be used as a reference, 2 fun examples (how to get the number of people living in any city in the US from the Census web site and how to check that your dream vanity plate is available in California) and how to use LWP to upload files to a server. It also describes the limits of the technique. I appreciated a very educative section showing how to go from a list of fields in a form to more and more useful code that queries that form.

  6. Simple HTML processing with Regular Expressions (15p): how to extract info from an HTML page using regexps. The chapter starts with short sections about various useful regexp features, then presents excellent advice on troubleshooting them, the limits of the technique and a series of examples. An interesting chapter, but read on for more powerful ways to process HTML. On the down side, I found the discussion of the s and m regexp modifiers a little confusing.

  7. HTML processing with Tokens (19p): using a real HTML parser is a better (safer) way to process HTML than regexps. This chapter uses HTML::TokeParser. It starts with a short, reference-type intro, then a detailed example. Another reference section describes the methods an alternate way of using the module, with short examples. This is the kind of reference I find the most useful, it is the simplest way to understand how to use a module.

  8. Tokenizing walkthrough (13p) a long Example showing step-by-step how to write a program that extracts data from a web site, using HTML::TokeParser. The explanations are very good, showing _why_ the code is built this way and including alternatives (both good and bad ones). This chapter describes really well the method readers can use to build their code.

  9. HTML processing with Trees (16p): even more powerful than an HTML tokenizer: HTML::TreeBuilder (written by the author of the book) builds a tree from the HTML. This chapter starts with a short reference section, then revisits 2 previous examples of extracting information from HTML using HTML::TreeBuilder.

  10. Modifying HTML with Trees (17p): More on the power of HTML::TreeBuilder: a reference/howto on the modification functions of HTML::TreeBuilder, with snippets of code for each function I really like HTML::TreeBuilder BTW, it is simple yet powerful.

  11. Cookies, Authentication and Advanced Requests (13p): Back to that LWP business... this chapter is simple and to-the-point: how to use cookies, authentication and referer to access even more web-sites. I just found that it lacked a description on how to code a complete session with cookies.

  12. Spiders (20p): a long example describing how to build a link-checking spider. It uses most of the techniques previously described in the book, plus some additional ones to deal with redirection and robots.txt files.

• Appendices

  I think the Appendices are actually the weakest part of the book, most of them are not really useful, apart from the ASCII table (every computer book should have an ASCII table IMHO ;--).

  • A. LWP modules (4p): the list and one line description of all modules in the LWP library, long and impressive! But not very useful,
  • B. HTTP status (2p): available elsewhere but still pretty useful,
  • C. Common MIME types (2p): lists both the usual extension and the MIME type,
  • D. Language Tags (2p): the author is a linguist ;--)
  • E. Common Content Encodings (2p): character set codes,
  • F. ASCII Table (13p): a very complete table, includes the ascii/unicode code, the corresponding HTML entity, description and glyph,
  • G. User's View of Object-Oriented Modules (11p): this is a very good idea. A lot of Perl programmers are not very familiar with OO, and in truth they don't need to be. They just need the basics of how to create an object in an existing class and call methods on it. I found the text too be sightly confusing though, in fact I believe it is a little too detailed and might confuse the reader.
  • Index (8p): I did not think the index was great (code is listed with references to 5 seemingly random pieces of code, type=file, HTML input element is listed twice, with and without the comma...), but this is not the kind of book where the index is the primary way to access the information. The Table of Content is complete and the chapters are focused enough that I have never needed to use the index.

You can purchase Perl & LWP from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.

Comments

Screen scraping cold war

[Anonymous Coward]

How long until web designers begin making small randomizations to their page layout to break any screen scrapers code?

In turn, screen scrapers will have to counter with further intelligence and the information cold war begins!

Just a thort.

Re:spammers thank you.

[killthiskid]

I'd combat it by making the infomation you DO want available very easy to get to, and the everything else hard.

That's why it's easy to take a penny from the penny jar and hard to get to the safe at a store.

Re: Doesn't seem to discuss the legalities

[Antity]

How can this be less legal than surfing the pages with a browser regularly?

Additional question for 5 bonus points: Who the hack can sue me if I program my own browser and call it "Perl" or "LWP" and let it pre-fetch some news sites every morning at 8am?

VCRs can be programmed to record my favorite daily soap 5 days a week at 4pm as long as I'm on vacation. Some TV stations here in Europe even use VPS so my VCR starts and stops recording exactly when the show begins and ends, so I don't get commercials before/after. Illegal to automate this?

Disclaimer: I don't watch soaps. :)

Re:perldoc LWP

[Masem]

Books like this one, the Perl & XML, and other "compact" books certainly can be argued as repetition of the perldocs, but there is room for such books under ORA's wing. First off, it gives someone with a hankering to author a computer handbook the opportunity to do so: based on a search at Amazon, this is Seth Burke's first book, and so a quality job, even as something short as the LWP module with already extensive documentation, will help him get good inroads into writing other books and other possibilities (No, I don't know Seth personally, just using him as an example author). The second advantage is that most perldocs are written from an efficient manner: tell the developer exactly what they need to know when they need to know it. While there are examples, they are usually not fleshed out very well. A good book that covers the modules inside and out, with a philosophy of use, concrete examples, and useful reference material can be very helpful in understand the module further and using it more efficiently, and for the programmer unexperienced in the modules, it provides a solid background for them to get started quickly.

Books like these, that focus very narrowly but try to cover the topic well, is what ORA is well known for and why they are still the major distributor of books related to OSS development and usage. Other large publishers would seem to balk at these types of books and instead opt for the 1000+ pg books that try to cover everything, typically failing to cover topics adequetely or making mistakes, since the size of a book can be an influencing factor to some book purchasers. In fact, one could argue that a lot of what ORA offers is simply rehashs of free documentation, but if that were the case, I'd have expected to see ORA out of business years ago. Therefor, there is a demand for ORA's quality retakes of the manpages and free documentation, and books like these continue to extend their catalog in good ways.

Re:Yea!

[LunaticLeo]

I imagine you were just joking, but in the case that your jokes are a cry for help ...:)

First, perl has native threads in the current perl 5.8.0.

Second, if you are interested in threads (or more generally multiple concurrent processing), check out POE from CPAN. POE *is* the best thing to happen to perl since LWP. It is an event driven application framework, which allows cooperatively multi-tasking sessions to do work in parallel. It is the bees knees, and the cat's meow.

Re: Doesn't seem to discuss the legalities

[Spackler]

Who the hack can sue me if I program my own browser and call it "Perl" or "LWP" and let it pre-fetch some news sites every morning at 8am?


Many sites (Yes, our beloved Slashdot included) use detection methods. If the detector thinks you are using a script, BANG!, your IP is in the deny list until you can explain your actions. A nice profile that says "for the last 18 days, x.x.x.x IP address logged in each day at exactly 7:53 am and did blah..." will get you slapped from MSNBC pretty fast. I would advise you to get some type of permission from the owner of the site before running around with scripts to grab stuff all over the web. Someone might mistake you for a script kiddie.

SSL coverage omitted

[cazwax]

This book has no coverage of configuring your LWP module to support SSL connections. Perhaps it is trivial, but an overview would be useful to newbies.

Don't be unfair to the author......

[i_want_you_to_throw_]

Yeah yeah spammers can use it. So what? Spam/email harvesting is only one of thousands of uses for LWP and focusing on that fact alone is VERY unfair to the author. You want to address the spamming issue? Don't use mailto tags in your HTML. Use form submission instead. If you use mailto: tags you DESERVE to be spammed.

There. Now shut the fsck up about the issue.

I manage a few government web sites and this book has been tremendous help in writing the spiders that I use to crawl the sites and record HTTP responses that then generate reports about out of date pages, 404s and so on. That alone has made it worth the money.

Sean did a great job on this. His book doesn't deserve to be slammed for what the technology MAY be used for.