lwptut

NAME

DESCRIPTION

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

Getting documents with LWP::Simple

In a Perl program, you can call its "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 "get" is "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 "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 the LWP::Simple documentation for the full details, or chapter 2 of Perl & LWP for more examples.  

The Basics of the LWP Class Model

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

The basic idiom is "$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: $browser, which holds an object of class LWP::UserAgent, and then the $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:

*

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

*

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

*

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

*

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

*

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

Adding Other HTTP Request Headers

  $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 $browser object's default line from ``libwww-perl/5.65'' (or the like) to whatever you like, using the LWP::UserAgent "agent" method:

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

Enabling Cookies

To give a browser an in-memory empty cookie jar, you set its "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 "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 "'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.  

Posting Form Data

 $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";
  }

Sending GET Form Data

  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 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.  

Absolutizing URLs

  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 "new_abs" method, by changing the "while" loop to this:

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

(The "$response->base" method from 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 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 HTML::LinkExtor or HTML::TokeParser or even maybe HTML::TreeBuilder.  

Other Browser Attributes

*

"$browser->timeout(15);"

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

*

"$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''.

*

"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.

*

"$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/versionnumber``, like ''libwww-perl/5.65". You can change that to something more descriptive like this:

  $browser->agent( 'SomeName/3.14 (contact@robotplexus.int)' );

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

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

*

"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.

For more options and information, see the full documentation for LWP::UserAgent.  

Writing Polite Robots

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', 'you@yoursite.com');
    # Your bot's name and your email address

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

But HTTP::RobotUA adds these features:

*

If the robots.txt on $url's server forbids you from accessing $url, then the $browser object (assuming it's of class LWP::RobotUA) won't actually request it, but instead will give you back (in $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

*

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

For more options and information, see the full documentation for LWP::RobotUA.  

Using Proxies

To make a browser object use proxies that are defined in the usual environment variables ("HTTP_PROXY", etc.), just call the "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 the LWP::UserAgent documentation, specifically the "proxy", "env_proxy", and "no_proxy" methods.  

HTTP Authentication

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 "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 $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 "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 "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 "$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 "die" to be called.  

Accessing HTTPS URLs

  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 does have HTTPS support installed, then the response should be successful, and you should be able to consult $response just like with any normal HTTP response.

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

Getting Large Documents

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 ":content_file" option, the $response will have all the normal header lines, but "$response->content" will be empty.

Note that this ``:content_file'' option isn't supported under older versions of LWP, so you should consider adding "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 );

SEE ALSO

*

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

*

LWP --- overview of the libwww-perl modules

*

LWP::UserAgent --- the class for objects that represent ``virtual browsers''

*

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

*

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

*

URI --- class for objects that represent absolute or relative URLs

*

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

*

HTML::Entities --- functions for HTML-escaping and HTML-unescaping strings (like turning ``C. & E. Bronte.'' to and from ``C. & E. Brontë'')

*

HTML::TokeParser and HTML::TreeBuilder --- classes for parsing HTML

*

HTML::LinkExtor --- class for finding links in HTML documents

*

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

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