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Text File | 2009-06-15 | 25KB | 838 lines

=head1 NAME

lwptut -- An LWP Tutorial

=head1 DESCRIPTION

LWP (short for "Library for WWW in Perl") is a very popular group of
Perl modules for accessing data on the Web. Like most Perl
module-distributions, each of LWP's component modules comes with
documentation that is a complete reference to its interface. However,
there are so many modules in LWP that it's hard to know where to start
looking for information on how to do even the simplest most common
things.

Really introducing you to using LWP would require a whole book -- a book
that just happens to exist, called I<Perl & LWP>. But this article
should give you a taste of how you can go about some common tasks with
LWP.


=head2 Getting documents with LWP::Simple

If you just want to get what's at a particular URL, the simplest way
to do it is LWP::Simple's functions.

In a Perl program, you can call its C<get($url)> function.  It will try
getting that URL's content.  If it works, then it'll return the
content; but if there's some error, it'll return undef.

  my $url = 'http://freshair.npr.org/dayFA.cfm?todayDate=current';
    # Just an example: the URL for the most recent /Fresh Air/ show

  use LWP::Simple;
  my $content = get $url;
  die "Couldn't get $url" unless defined $content;

  # Then go do things with $content, like this:

  if($content =~ m/jazz/i) {
    print "They're talking about jazz today on Fresh Air!\n";
  }
  else {
    print "Fresh Air is apparently jazzless today.\n";
  }

The handiest variant on C<get> is C<getprint>, which is useful in Perl
one-liners.  If it can get the page whose URL you provide, it sends it
to STDOUT; otherwise it complains to STDERR.

  % perl -MLWP::Simple -e "getprint 'http://cpan.org/RECENT'"

That is the URL of a plain text file that lists new files in CPAN in
the past two weeks.  You can easily make it part of a tidy little
shell command, like this one that mails you the list of new
C<Acme::> modules:

  % perl -MLWP::Simple -e "getprint 'http://cpan.org/RECENT'"  \
     | grep "/by-module/Acme" | mail -s "New Acme modules! Joy!" $USER

There are other useful functions in LWP::Simple, including one function
for running a HEAD request on a URL (useful for checking links, or
getting the last-revised time of a URL), and two functions for
saving/mirroring a URL to a local file. See L<the LWP::Simple
documentation|LWP::Simple> for the full details, or chapter 2 of I<Perl
& LWP> for more examples.



=for comment
 ##########################################################################



=head2 The Basics of the LWP Class Model

LWP::Simple's functions are handy for simple cases, but its functions
don't support cookies or authorization, don't support setting header
lines in the HTTP request, generally don't support reading header lines
in the HTTP response (notably the full HTTP error message, in case of an
error). To get at all those features, you'll have to use the full LWP
class model.

While LWP consists of dozens of classes, the main two that you have to
understand are L<LWP::UserAgent> and L<HTTP::Response>. LWP::UserAgent
is a class for "virtual browsers" which you use for performing requests,
and L<HTTP::Response> is a class for the responses (or error messages)
that you get back from those requests.

The basic idiom is C<< $response = $browser->get($url) >>, or more fully
illustrated:

  # Early in your program:

  use LWP 5.64; # Loads all important LWP classes, and makes
                #  sure your version is reasonably recent.

  my $browser = LWP::UserAgent->new;

  ...

  # Then later, whenever you need to make a get request:
  my $url = 'http://freshair.npr.org/dayFA.cfm?todayDate=current';

  my $response = $browser->get( $url );
  die "Can't get $url -- ", $response->status_line
   unless $response->is_success;

  die "Hey, I was expecting HTML, not ", $response->content_type
   unless $response->content_type eq 'text/html';
     # or whatever content-type you're equipped to deal with

  # Otherwise, process the content somehow:

  if($response->decoded_content =~ m/jazz/i) {
    print "They're talking about jazz today on Fresh Air!\n";
  }
  else {
    print "Fresh Air is apparently jazzless today.\n";
  }

There are two objects involved: C<$browser>, which holds an object of
class LWP::UserAgent, and then the C<$response> object, which is of
class HTTP::Response. You really need only one browser object per
program; but every time you make a request, you get back a new
HTTP::Response object, which will have some interesting attributes:

=over

=item *

A status code indicating
success or failure
(which you can test with C<< $response->is_success >>).

=item *

An HTTP status
line that is hopefully informative if there's failure (which you can
see with C<< $response->status_line >>,
returning something like "404 Not Found").

=item *

A MIME content-type like "text/html", "image/gif",
"application/xml", etc., which you can see with
C<< $response->content_type >>

=item *

The actual content of the response, in C<< $response->decoded_content >>.
If the response is HTML, that's where the HTML source will be; if
it's a GIF, then C<< $response->decoded_content >> will be the binary
GIF data.

=item *

And dozens of other convenient and more specific methods that are
documented in the docs for L<HTTP::Response>, and its superclasses
L<HTTP::Message> and L<HTTP::Headers>.

=back



=for comment
 ##########################################################################



=head2 Adding Other HTTP Request Headers

The most commonly used syntax for requests is C<< $response =
$browser->get($url) >>, but in truth, you can add extra HTTP header
lines to the request by adding a list of key-value pairs after the URL,
like so:

  $response = $browser->get( $url, $key1, $value1, $key2, $value2, ... );

For example, here's how to send some more Netscape-like headers, in case
you're dealing with a site that would otherwise reject your request:


  my @ns_headers = (
   'User-Agent' => 'Mozilla/4.76 [en] (Win98; U)',
   'Accept' => 'image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*',
   'Accept-Charset' => 'iso-8859-1,*,utf-8',
   'Accept-Language' => 'en-US',
  );

  ...

  $response = $browser->get($url, @ns_headers);

If you weren't reusing that array, you could just go ahead and do this:

  $response = $browser->get($url,
   'User-Agent' => 'Mozilla/4.76 [en] (Win98; U)',
   'Accept' => 'image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*',
   'Accept-Charset' => 'iso-8859-1,*,utf-8',
   'Accept-Language' => 'en-US',
  );

If you were only ever changing the 'User-Agent' line, you could just change
the C<$browser> object's default line from "libwww-perl/5.65" (or the like)
to whatever you like, using the LWP::UserAgent C<agent> method:

   $browser->agent('Mozilla/4.76 [en] (Win98; U)');



=for comment
 ##########################################################################



=head2 Enabling Cookies

A default LWP::UserAgent object acts like a browser with its cookies
support turned off. There are various ways of turning it on, by setting
its C<cookie_jar> attribute. A "cookie jar" is an object representing
a little database of all
the HTTP cookies that a browser can know about. It can correspond to a
file on disk (the way Netscape uses its F<cookies.txt> file), or it can
be just an in-memory object that starts out empty, and whose collection of
cookies will disappear once the program is finished running.

To give a browser an in-memory empty cookie jar, you set its C<cookie_jar>
attribute like so:

  $browser->cookie_jar({});

To give it a copy that will be read from a file on disk, and will be saved
to it when the program is finished running, set the C<cookie_jar> attribute
like this:

  use HTTP::Cookies;
  $browser->cookie_jar( HTTP::Cookies->new(
    'file' => '/some/where/cookies.lwp',
        # where to read/write cookies
    'autosave' => 1,
        # save it to disk when done
  ));

That file will be an LWP-specific format. If you want to be access the
cookies in your Netscape cookies file, you can use the
HTTP::Cookies::Netscape class:

  use HTTP::Cookies;
    # yes, loads HTTP::Cookies::Netscape too

  $browser->cookie_jar( HTTP::Cookies::Netscape->new(
    'file' => 'c:/Program Files/Netscape/Users/DIR-NAME-HERE/cookies.txt',
        # where to read cookies
  ));

You could add an C<< 'autosave' => 1 >> line as further above, but at
time of writing, it's uncertain whether Netscape might discard some of
the cookies you could be writing back to disk.



=for comment
 ##########################################################################



=head2 Posting Form Data

Many HTML forms send data to their server using an HTTP POST request, which
you can send with this syntax:

 $response = $browser->post( $url,
   [
     formkey1 => value1,
     formkey2 => value2,
     ...
   ],
 );

Or if you need to send HTTP headers:

 $response = $browser->post( $url,
   [
     formkey1 => value1,
     formkey2 => value2,
     ...
   ],
   headerkey1 => value1,
   headerkey2 => value2,
 );

For example, the following program makes a search request to AltaVista
(by sending some form data via an HTTP POST request), and extracts from
the HTML the report of the number of matches:

  use strict;
  use warnings;
  use LWP 5.64;
  my $browser = LWP::UserAgent->new;

  my $word = 'tarragon';

  my $url = 'http://www.altavista.com/sites/search/web';
  my $response = $browser->post( $url,
    [ 'q' => $word,  # the Altavista query string
      'pg' => 'q', 'avkw' => 'tgz', 'kl' => 'XX',
    ]
  );
  die "$url error: ", $response->status_line
   unless $response->is_success;
  die "Weird content type at $url -- ", $response->content_type
   unless $response->content_is_html;

  if( $response->decoded_content =~ m{AltaVista found ([0-9,]+) results} ) {
    # The substring will be like "AltaVista found 2,345 results"
    print "$word: $1\n";
  }
  else {
    print "Couldn't find the match-string in the response\n";
  }



=for comment
 ##########################################################################



=head2 Sending GET Form Data

Some HTML forms convey their form data not by sending the data
in an HTTP POST request, but by making a normal GET request with
the data stuck on the end of the URL.  For example, if you went to
C<imdb.com> and ran a search on "Blade Runner", the URL you'd see
in your browser window would be:

  http://us.imdb.com/Tsearch?title=Blade%20Runner&restrict=Movies+and+TV

To run the same search with LWP, you'd use this idiom, which involves
the URI class:

  use URI;
  my $url = URI->new( 'http://us.imdb.com/Tsearch' );
    # makes an object representing the URL

  $url->query_form(  # And here the form data pairs:
    'title'    => 'Blade Runner',
    'restrict' => 'Movies and TV',
  );

  my $response = $browser->get($url);

See chapter 5 of I<Perl & LWP> for a longer discussion of HTML forms
and of form data, and chapters 6 through 9 for a longer discussion of
extracting data from HTML.



=head2 Absolutizing URLs

The URI class that we just mentioned above provides all sorts of methods
for accessing and modifying parts of URLs (such as asking sort of URL it
is with C<< $url->scheme >>, and asking what host it refers to with C<<
$url->host >>, and so on, as described in L<the docs for the URI
class|URI>.  However, the methods of most immediate interest
are the C<query_form> method seen above, and now the C<new_abs> method
for taking a probably-relative URL string (like "../foo.html") and getting
back an absolute URL (like "http://www.perl.com/stuff/foo.html"), as
shown here:

  use URI;
  $abs = URI->new_abs($maybe_relative, $base);

For example, consider this program that matches URLs in the HTML
list of new modules in CPAN:

  use strict;
  use warnings;
  use LWP;
  my $browser = LWP::UserAgent->new;

  my $url = 'http://www.cpan.org/RECENT.html';
  my $response = $browser->get($url);
  die "Can't get $url -- ", $response->status_line
   unless $response->is_success;

  my $html = $response->decoded_content;
  while( $html =~ m/<A HREF=\"(.*?)\"/g ) {
    print "$1\n";
  }

When run, it emits output that starts out something like this:

  MIRRORING.FROM
  RECENT
  RECENT.html
  authors/00whois.html
  authors/01mailrc.txt.gz
  authors/id/A/AA/AASSAD/CHECKSUMS
  ...

However, if you actually want to have those be absolute URLs, you
can use the URI module's C<new_abs> method, by changing the C<while>
loop to this:

  while( $html =~ m/<A HREF=\"(.*?)\"/g ) {
    print URI->new_abs( $1, $response->base ) ,"\n";
  }

(The C<< $response->base >> method from L<HTTP::Message|HTTP::Message>
is for returning what URL
should be used for resolving relative URLs -- it's usually just
the same as the URL that you requested.)

That program then emits nicely absolute URLs:

  http://www.cpan.org/MIRRORING.FROM
  http://www.cpan.org/RECENT
  http://www.cpan.org/RECENT.html
  http://www.cpan.org/authors/00whois.html
  http://www.cpan.org/authors/01mailrc.txt.gz
  http://www.cpan.org/authors/id/A/AA/AASSAD/CHECKSUMS
  ...

See chapter 4 of I<Perl & LWP> for a longer discussion of URI objects.

Of course, using a regexp to match hrefs is a bit simplistic, and for
more robust programs, you'll probably want to use an HTML-parsing module
like L<HTML::LinkExtor> or L<HTML::TokeParser> or even maybe
L<HTML::TreeBuilder>.




=for comment
 ##########################################################################

=head2 Other Browser Attributes

LWP::UserAgent objects have many attributes for controlling how they
work.  Here are a few notable ones:

=over

=item *

C<< $browser->timeout(15); >>

This sets this browser object to give up on requests that don't answer
within 15 seconds.


=item *

C<< $browser->protocols_allowed( [ 'http', 'gopher'] ); >>

This sets this browser object to not speak any protocols other than HTTP
and gopher. If it tries accessing any other kind of URL (like an "ftp:"
or "mailto:" or "news:" URL), then it won't actually try connecting, but
instead will immediately return an error code 500, with a message like
"Access to 'ftp' URIs has been disabled".


=item *

C<< use LWP::ConnCache; $browser->conn_cache(LWP::ConnCache->new()); >>

This tells the browser object to try using the HTTP/1.1 "Keep-Alive"
feature, which speeds up requests by reusing the same socket connection
for multiple requests to the same server.


=item *

C<< $browser->agent( 'SomeName/1.23 (more info here maybe)' ) >>

This changes how the browser object will identify itself in
the default "User-Agent" line is its HTTP requests.  By default,
it'll send "libwww-perl/I<versionnumber>", like
"libwww-perl/5.65".  You can change that to something more descriptive
like this:

  $browser->agent( 'SomeName/3.14 ([e-mail])' );

Or if need be, you can go in disguise, like this:

  $browser->agent( 'Mozilla/4.0 (compatible; MSIE 5.12; Mac_PowerPC)' );


=item *

C<< push @{ $ua->requests_redirectable }, 'POST'; >>

This tells this browser to obey redirection responses to POST requests
(like most modern interactive browsers), even though the HTTP RFC says
that should not normally be done.


=back


For more options and information, see L<the full documentation for
LWP::UserAgent|LWP::UserAgent>.



=for comment
 ##########################################################################



=head2 Writing Polite Robots

If you want to make sure that your LWP-based program respects F<robots.txt>
files and doesn't make too many requests too fast, you can use the LWP::RobotUA
class instead of the LWP::UserAgent class.

LWP::RobotUA class is just like LWP::UserAgent, and you can use it like so:

  use LWP::RobotUA;
  my $browser = LWP::RobotUA->new('YourSuperBot/1.34', '[e-mail]');
    # Your bot's name and your email address

  my $response = $browser->get($url);

But HTTP::RobotUA adds these features:


=over

=item *

If the F<robots.txt> on C<$url>'s server forbids you from accessing
C<$url>, then the C<$browser> object (assuming it's of class LWP::RobotUA)
won't actually request it, but instead will give you back (in C<$response>) a 403 error
with a message "Forbidden by robots.txt".  That is, if you have this line:

  die "$url -- ", $response->status_line, "\nAborted"
   unless $response->is_success;

then the program would die with an error message like this:

  http://whatever.site.int/pith/x.html -- 403 Forbidden by robots.txt
  Aborted at whateverprogram.pl line 1234

=item *

If this C<$browser> object sees that the last time it talked to
C<$url>'s server was too recently, then it will pause (via C<sleep>) to
avoid making too many requests too often. How long it will pause for, is
by default one minute -- but you can control it with the C<<
$browser->delay( I<minutes> ) >> attribute.

For example, this code:

  $browser->delay( 7/60 );

...means that this browser will pause when it needs to avoid talking to
any given server more than once every 7 seconds.

=back

For more options and information, see L<the full documentation for
LWP::RobotUA|LWP::RobotUA>.





=for comment
 ##########################################################################

=head2 Using Proxies

In some cases, you will want to (or will have to) use proxies for
accessing certain sites and/or using certain protocols. This is most
commonly the case when your LWP program is running (or could be running)
on a machine that is behind a firewall.

To make a browser object use proxies that are defined in the usual
environment variables (C<HTTP_PROXY>, etc.), just call the C<env_proxy>
on a user-agent object before you go making any requests on it.
Specifically:

  use LWP::UserAgent;
  my $browser = LWP::UserAgent->new;

  # And before you go making any requests:
  $browser->env_proxy;

For more information on proxy parameters, see L<the LWP::UserAgent
documentation|LWP::UserAgent>, specifically the C<proxy>, C<env_proxy>,
and C<no_proxy> methods.



=for comment
 ##########################################################################

=head2 HTTP Authentication

Many web sites restrict access to documents by using "HTTP
Authentication". This isn't just any form of "enter your password"
restriction, but is a specific mechanism where the HTTP server sends the
browser an HTTP code that says "That document is part of a protected
'realm', and you can access it only if you re-request it and add some
special authorization headers to your request".

For example, the Unicode.org admins stop email-harvesting bots from
harvesting the contents of their mailing list archives, by protecting
them with HTTP Authentication, and then publicly stating the username
and password (at C<http://www.unicode.org/mail-arch/>) -- namely
username "unicode-ml" and password "unicode".

For example, consider this URL, which is part of the protected
area of the web site:

  http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html

If you access that with a browser, you'll get a prompt
like
"Enter username and password for 'Unicode-MailList-Archives' at server
'www.unicode.org'".

In LWP, if you just request that URL, like this:

  use LWP;
  my $browser = LWP::UserAgent->new;

  my $url =
   'http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html';
  my $response = $browser->get($url);

  die "Error: ", $response->header('WWW-Authenticate') || 'Error accessing',
    #  ('WWW-Authenticate' is the realm-name)
    "\n ", $response->status_line, "\n at $url\n Aborting"
   unless $response->is_success;

Then you'll get this error:

  Error: Basic realm="Unicode-MailList-Archives"
   401 Authorization Required
   at http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html
   Aborting at auth1.pl line 9.  [or wherever]

...because the C<$browser> doesn't know any the username and password
for that realm ("Unicode-MailList-Archives") at that host
("www.unicode.org").  The simplest way to let the browser know about this
is to use the C<credentials> method to let it know about a username and
password that it can try using for that realm at that host.  The syntax is:

  $browser->credentials(
    'servername:portnumber',
    'realm-name',
   'username' => 'password'
  );

In most cases, the port number is 80, the default TCP/IP port for HTTP; and
you usually call the C<credentials> method before you make any requests.
For example:

  $browser->credentials(
    'reports.mybazouki.com:80',
    'web_server_usage_reports',
    'plinky' => 'banjo123'
  );

So if we add the following to the program above, right after the C<<
$browser = LWP::UserAgent->new; >> line...

  $browser->credentials(  # add this to our $browser 's "key ring"
    'www.unicode.org:80',
    'Unicode-MailList-Archives',
    'unicode-ml' => 'unicode'
  );

...then when we run it, the request succeeds, instead of causing the
C<die> to be called.



=for comment
 ##########################################################################

=head2 Accessing HTTPS URLs

When you access an HTTPS URL, it'll work for you just like an HTTP URL
would -- if your LWP installation has HTTPS support (via an appropriate
Secure Sockets Layer library).  For example:

  use LWP;
  my $url = 'https://www.paypal.com/';   # Yes, HTTPS!
  my $browser = LWP::UserAgent->new;
  my $response = $browser->get($url);
  die "Error at $url\n ", $response->status_line, "\n Aborting"
   unless $response->is_success;
  print "Whee, it worked!  I got that ",
   $response->content_type, " document!\n";

If your LWP installation doesn't have HTTPS support set up, then the
response will be unsuccessful, and you'll get this error message:

  Error at https://www.paypal.com/
   501 Protocol scheme 'https' is not supported
   Aborting at paypal.pl line 7.   [or whatever program and line]

If your LWP installation I<does> have HTTPS support installed, then the
response should be successful, and you should be able to consult
C<$response> just like with any normal HTTP response.

For information about installing HTTPS support for your LWP
installation, see the helpful F<README.SSL> file that comes in the
libwww-perl distribution.


=for comment
 ##########################################################################



=head2 Getting Large Documents

When you're requesting a large (or at least potentially large) document,
a problem with the normal way of using the request methods (like C<<
$response = $browser->get($url) >>) is that the response object in
memory will have to hold the whole document -- I<in memory>. If the
response is a thirty megabyte file, this is likely to be quite an
imposition on this process's memory usage.

A notable alternative is to have LWP save the content to a file on disk,
instead of saving it up in memory.  This is the syntax to use:

  $response = $ua->get($url,
                         ':content_file' => $filespec,
                      );

For example,

  $response = $ua->get('http://search.cpan.org/',
                         ':content_file' => '/tmp/sco.html'
                      );

When you use this C<:content_file> option, the C<$response> will have
all the normal header lines, but C<< $response->content >> will be
empty.

Note that this ":content_file" option isn't supported under older
versions of LWP, so you should consider adding C<use LWP 5.66;> to check
the LWP version, if you think your program might run on systems with
older versions.

If you need to be compatible with older LWP versions, then use
this syntax, which does the same thing:

  use HTTP::Request::Common;
  $response = $ua->request( GET($url), $filespec );


=for comment
 ##########################################################################


=head1 SEE ALSO

Remember, this article is just the most rudimentary introduction to
LWP -- to learn more about LWP and LWP-related tasks, you really
must read from the following:

=over

=item *

L<LWP::Simple> -- simple functions for getting/heading/mirroring URLs

=item *

L<LWP> -- overview of the libwww-perl modules

=item *

L<LWP::UserAgent> -- the class for objects that represent "virtual browsers"

=item *

L<HTTP::Response> -- the class for objects that represent the response to
a LWP response, as in C<< $response = $browser->get(...) >>

=item *

L<HTTP::Message> and L<HTTP::Headers> -- classes that provide more methods
to HTTP::Response.

=item *

L<URI> -- class for objects that represent absolute or relative URLs

=item *

L<URI::Escape> -- functions for URL-escaping and URL-unescaping strings
(like turning "this & that" to and from "this%20%26%20that").

=item *

L<HTML::Entities> -- functions for HTML-escaping and HTML-unescaping strings
(like turning "C. & E. BrontE<euml>" to and from "C. & E. Brontë")

=item *

L<HTML::TokeParser> and L<HTML::TreeBuilder> -- classes for parsing HTML

=item *

L<HTML::LinkExtor> -- class for finding links in HTML documents

=item *

The book I<Perl & LWP> by Sean M. Burke.  O'Reilly & Associates, 2002.
ISBN: 0-596-00178-9.  C<http://www.oreilly.com/catalog/perllwp/>

=back


=head1 COPYRIGHT

Copyright 2002, Sean M. Burke.  You can redistribute this document and/or
modify it, but only under the same terms as Perl itself.

=head1 AUTHOR

Sean M. Burke C<[e-mail]>

=for comment
 ##########################################################################

=cut

# End of Pod