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5.6. POST Example: ABEBooks.com

ABEBooks.com is a web site that allows users to search the database of the books for sale at hundreds of used bookstores mostly in the U.S. and Canada. An eagle-eyed user can find anything from a $2 used copy of Swahili for Travellers, to an Figure 5.611,000 complete set of the 1777 edition of Diderot's Encyclopédie. The trick, as with any kind of bargain hunting, is to always keep looking, because one never knows when something new and interesting will arrive. The manual way of doing this is to fastidiously keep a list of titles, authors, and subjects for which you're keeping an eye out, and to routinely visit the ABEBooks site, key in each of your searches into the HTML search form, and look for anything new. However, this is precisely the kind of drudgery that computers were meant to do for us; so we'll now consider how to automate that task.

As with the license plate form in the previous section, the first step in automating form submission is to understand the form in question. ABEBooks's "Advanced Search" system consists of one form, which is shown in Figure 5-3.

Figure 5-3

Figure 5-3. ABEBooks query form

The process of searching with this form is just a matter of filling in the applicable fields and hitting "Start Search"; the web site then returns a web page listing the results. For example, entering "Codex Seraphinianus" in the "Title" field returns the web page shown in Figure 5-4.

Figure 5-4

Figure 5-4. ABEBooks results page

5.6.3. Adding Features

A little more experimentation with the form would show that a search on an author's name, instead of the title name, shows up in the an=author_name form pair, instead of the tn=title_name form pair. That is what we see if we go sifting through the HTML source to the search form:


We could alter our program to set the form pairs with something like this:

"an" => $author || "",
"tn" => $title  || "",

Moreover, if we wanted to allow the search to specify that only first editions should be shown, some experimentation with formpairs.pl and our local copy of the form shows that checking the "First Edition" checkbox produces a new form pair fe=on, between the bi= and prl= pairs, where previously there was nothing. This jibes with the HTML source code:


This could be modeled in our program with a variable $first_edition, which, if set to a true value, produces that form pair; otherwise, it produces nothing:

 "bi" => "",
 $first_edition ? ("fe" => "on") : ( ),
 "prl" => "",

This can all be bundled up in a single routine that runs a search based on three given parameters: author, title, and whether only first editions should be shown:

sub run_search {
  my($author, $title, $first_edition) = @_;
  my $response = $browser->post(
      "ph" => "2",
      "an" => $author || "",
      "tn" => $title  || "",
      "pn" => "",
      "sn" => "",
      "gpnm" => "All Book Stores",
      "cty" => "All Countries",
      "bi" => "Any Binding",
      $first_edition ? ("fe" => "on") : ( ),
      "prl" => "",
      "prh" => "",
      "sortby" => "0",
      "ds" => "100",
      "bu" => "Start Search",
  return $response;

That run_search( ) routine takes all we know about how any new-books query to ABEBooks needs to be performed and puts it all in a single place. From here, we need only apply initialization code and code to call the run_search routine, and do whatever needs doing with it:

use strict;
use LWP;
my $browser = LWP::UserAgent->new;
do_stuff( );

sub do_stuff {
  my $response = run_search( # author, title, first edition
    '', 'Codex Seraphinianus', ''
  process_search($response, 'result_seraph.html');

sub process_search {
  my($response, $out_file) = @_;
  die "Error: ", $response->status_line, "\n"
   unless $response->is_success;
  open(OUT, ">$out_file") || die "Can't write-open $out_file: $!";
  print OUT $response->content;
  print "Bytes saved: ", -s $out_file, " in $out_file\n";

5.6.4. Generalizing the Program

This program still just runs an ABEBooks search for books with the title "Codex Seraphinianus", and saves the results to result_seraph.html. But the benefit of reshuffling the code as we did is that now, by just changing do_stuff slightly, we change our program from being dedicated to running one search, to being a generic tool for running any number of searches:

my @searches = (  # outfile, author, title, first_edition
  ['result_seraph.html',       '', 'Codex Seraphinianus', ''],
  ['result_vidal_1green.html', 'Gore Vidal', 'Dark Green Bright Red', 1],
  ['result_marchand.html',     'Hans Marchand', 'Categories', ''],
  ['result_origins.html',      'Eric Partridge', 'Origins', ''],
  ['result_navajo.html',       '', 'Navajo',   ''],
  ['result_navaho.html',       '', 'Navaho',   ''],
  ['result_iroq.html',         '', 'Iroquois', ''],
  ['result_tibetan.html',      '', 'Tibetan',  ''],
do_stuff( );

sub do_stuff {
  foreach my $search (@searches) {
    my $out_file = shift @$search;
    my $resp = run_search(@$search);
    sleep 3; # Don't rudely query the ABEbooks server too fast!
    process_search($resp, $out_file);

Running this program saves each of those searches in turn:

% perl -w abesearch03.pl
Bytes saved: 15452 in result_seraph.html
Bytes saved: 57693 in result_vidal_1green.html
Bytes saved: 8009 in result_marchand.html
Bytes saved: 25322 in result_origins.html
Bytes saved: 125337 in result_navajo.html
Bytes saved: 128665 in result_navaho.html
Bytes saved: 127475 in result_iroq.html
Bytes saved: 130941 in result_tibetan.html

The user can then open each file and skim it for interesting new titles. Each book listed there comes with a working absolute URL to a book detail page on the ABEBooks server, which can be used for buying the book. For some of the queries that generate large numbers of results, it would be particularly convenient to have do_stuff( ) actually track which books it has seen before (using the book-detail URL of each) and report only on new ones:

my $is_first_time;
my (%seen_last_time, %seen_this_time, @new_urls);
sub do_stuff {
  if (-e 'seen_last_time.dat') {
    # Get URLs seen last time.
    open(LAST_TIME, "<seen_last_time.dat") || die $!;
    while (<LAST_TIME>) { chomp; $seen_last_time{$_} = 1 };
  } else {
    $is_first_time = 1;

  foreach my $search (@searches) {
    my $out_file = shift @$search;
    my $resp = run_search(@$search);
    process_search($resp, $out_file);

    foreach my $url ($resp->content =~
      # Extract URLs of book-detail pages:
      push @new_urls, $url unless $seen_last_time{$url}
       or $seen_this_time{$url};
      $seen_this_time{$url} = 1;

  # Save URLs for comparison next time.
  open(LAST_TIME, ">seen_last_time.dat") || die $!;
  for (keys %seen_this_time) { print LAST_TIME $_, "\n" }

  if($is_first_time) {
    print "(This was the first time this program was run.)\n";
  } elsif (@new_urls) {
    print "\nURLs of new books:\n";
    for (@new_urls) { print $_, "\n" }
  } else {
    print "No new books to report.\n";

A typical run of this will produce output as above, but with this addendum:

URLs of new books:

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