WWW::Mechanize::FAQ(3)User Contributed Perl DocumentatioWWW::Mechanize::FAQ(3)NAMEWWW::Mechanize::FAQ - Frequently Asked Questions about WWW::Mechanize
How to get help with WWW::Mechanize
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JavaScript
I have this web page that has JavaScript on it, and my Mech program doesn't
work.
That's because WWW::Mechanize doesn't operate on the JavaScript. It
only understands the HTML parts of the page.
I thought Mech was supposed to work like a web browser.
It does pretty much, but it doesn't support JavaScript.
I added some basic attempts at picking up URLs in "window.open()" calls
and return them in "$mech->links". They work sometimes. Beyond that,
there's no support for JavaScript.
Are you going to add JavaScript support?
I will if anyone sends me the code to do it. I'm not going to write a
JavaScript processor myself.
Wouldn't that be a great thing to have in WWW::Mechanize?
Yes.
Would it be hard to do?
Hard enough that I don't want to deal with it myself. Plus, I don't
use JavaScript myself, so I don't have an itch to scratch.
Is anyone working on it?
I've heard noises from people every so often over the past couple of
years, but nothing you'd pin your hopes on.
It would really help me with a project I'm working on.
I'm sure it would.
Do you know when it might get added?
I have no idea if or when such a thing will ever get done. I can
guarantee that as soon as there's anything close to JavaScript support
I will let everyone know.
Maybe I'll ask around and see if anyone else knows of a solution.
If you must, but I doubt that anyone's written JavaScript support for
Mechanize and neglected to tell me about it.
So what can I do?
Since Javascript is completely visible to the client, it cannot be used
to prevent a scraper from following links. But it can make life
difficult, and until someone writes a Javascript interpreter for Perl
or a Mechanize clone to control Firefox, there will be no general
solution. But if you want to scrape specific pages, then a solution is
always possible.
One typical use of Javascript is to perform argument checking before
posting to the server. The URL you want is probably just buried in the
Javascript function. Do a regular expression match on
"$mech->content()" to find the link that you want and "$mech->get" it
directly (this assumes that you know what you are looking for in
advance).
In more difficult cases, the Javascript is used for URL mangling to
satisfy the needs of some middleware. In this case you need to figure
out what the Javascript is doing (why are these URLs always really
long?). There is probably some function with one or more arguments
which calculates the new URL. Step one: using your favorite browser,
get the before and after URLs and save them to files. Edit each file,
converting the the argument separators ('?', '&' or ';') into newlines.
Now it is easy to use diff or comm to find out what Javascript did to
the URL. Step 2 - find the function call which created the URL - you
will need to parse and interpret its argument list. Using the
Javascript Debugger Extension for Firefox may help with the analysis.
At this point, it is fairly trivial to write your own function which
emulates the Javascript for the pages you want to process.
Here's annother approach that answers the question, "It works in
Firefox, but why not Mech?" Everything the web server knows about the
client is present in the HTTP request. If two requests are identical,
the results should be identical. So the real question is "What is
different between the mech request and the Firefox request?"
The Firefox extension "Tamper Data" is an effective tool for examining
the headers of the requests to the server. Compare that with what LWP
is sending. Once the two are identical, the action of the server should
be the same as well.
I say "should", because this is an oversimplification - some values are
naturally unique, e.g. a SessionID, but if a SessionID is present, that
is probably sufficient, even though the value will be different between
the LWP request and the Firefox request. The server could use the
session to store information which is troublesome, but that's not the
first place to look (and highly unlikely to be relevant when you are
requesting the login page of your site).
Generally the problem is to be found in missing or incorrect POSTDATA
arguments, Cookies, User-Agents, Accepts, etc. If you are using mech,
then redirects and cookies should not be a problem, but are listed here
for completeness. If you are missing headers, "$mech->add_header" can
be used to add the headers that you need.
How do I do X?
Can I do [such-and-such] with WWW::Mechanize?
If it's possible with LWP::UserAgent, then yes. WWW::Mechanize is a
subclass of LWP::UserAgent, so all the wondrous magic of that class is
inherited.
How do I use WWW::Mechanize through a proxy server?
See the docs in LWP::UserAgent on how to use the proxy. Short version:
$mech->proxy(['http', 'ftp'], 'http://proxy.example.com:8000/');
or get the specs from the environment:
$mech->env_proxy();
# Environment set like so:
gopher_proxy=http://proxy.my.place/
wais_proxy=http://proxy.my.place/
no_proxy="localhost,my.domain"
export gopher_proxy wais_proxy no_proxy
How can I see what fields are on the forms?
Use the mech-dump utility, optionaly installed with Mechanize.
$ mech-dump --forms http://search.cpan.org
Dumping forms
GET http://search.cpan.org/search
query=
mode=all (option) [*all|module|dist|author]
<NONAME>=CPAN Search (submit)
How do I get Mech to handle authentication?
use MIME::Base64;
my $agent = WWW::Mechanize->new();
my @args = (
Authorization => "Basic " .
MIME::Base64::encode( USER . ':' . PASS )
);
$agent->credentials( ADDRESS, REALM, USER, PASS );
$agent->get( URL, @args );
If you want to use the credentials for all future requests, you can
also use the LWP::UserAgent "default_header()" method instead of the
extra arguments to "get()"
$mech->default_header(
Authorization => 'Basic ' . encode_base64( USER . ':' . PASSWORD ) );
How can I get WWW::Mechanize to execute this JavaScript?
You can't. JavaScript is entirely client-based, and WWW::Mechanize is
a client that doesn't understand JavaScript. See the top part of this
FAQ.
How do I check a checkbox that doesn't have a value defined?
Set it to to the value of "on".
$mech->field( my_checkbox => 'on' );
How do I handle frames?
You don't deal with them as frames, per se, but as links. Extract them
with
my @frame_links = $mech->find_link( tag => "frame" );
How do I get a list of HTTP headers and their values?
All HTTP::Headers methods work on a HTTP::Response object which is
returned by the get(), reload(), response()/res(), click(),
submit_form(), and request() methods.
my $mech = WWW::Mechanize->new( autocheck => 1 );
$mech->get( 'http://my.site.com' );
my $res = $mech->response();
for my $key ( $response->header_field_names() ) {
print $key, " : ", $response->header( $key ), "\n";
}
How do I enable keep-alive?
Since WWW::Mechanize is a subclass of LWP::UserAgent, you can use the
same mechanism to enable keep-alive:
use LWP::ConnCache;
...
$mech->conn_cache(LWP::ConnCache->new);
How can I change/specify the action parameter of an HTML form?
You can access the action of the form by utilizing the HTML::Form
object returned from one of the specifying form methods.
Using "$mech->form_number($number)":
my $mech = WWW::mechanize->new;
$mech->get('http://someurlhere.com');
# Access the form using its Zero-Based Index by DOM order
$mech->form_number(0)->action('http://newAction'); #ABS URL
Using "$mech->form_name($number)":
my $mech = WWW::mechanize->new;
$mech->get('http://someurlhere.com');
#Access the form using its Zero-Based Index by DOM order
$mech->form_name('trgForm')->action('http://newAction'); #ABS URL
How do I save an image? How do I save a large tarball?
An image is just content. You get the image and save it.
$mech->get( 'photo.jpg' );
$mech->save_content( '/path/to/my/directory/photo.jpg' );
You can also save any content directly to disk using the
":content_file" flag to "get()", which is part of LWP::UserAgent.
$mech->get( 'http://www.cpan.org/src/stable.tar.gz',
':content_file' => 'stable.tar.gz' );
How do I pick a specific value from a "<select>" list?
Find the "HTML::Form::ListInput" in the page.
my ($listbox) = $mech->find_all_inputs( name => 'listbox' );
Then create a hash for the lookup:
my %name_lookup;
@name_lookup{ $listbox->value_names } = $listbox->possible_values;
my $value = $name_lookup{ 'Name I want' };
If you have duplicate names, this method won't work, and you'll have to
loop over "$listbox->value_names" and "$listbox->possible_values" in
parallel until you find a matching name.
How do I get Mech to not follow redirects?
You use functionality in LWP::UserAgent, not Mech itself.
$mech->requests_redirectable( [] );
Or you can set "max_redirect":
$mech->max_redirect( 0 );
Both these options can also be set in the constructor. Mech doesn't
understand them, so will pass them through to the LWP::UserAgent
constructor.
Why doesn't this work: Debugging your Mechanize program
My Mech program doesn't work, but it works in the browser.
Mechanize acts like a browser, but apparently something you're doing is
not matching the browser's behavior. Maybe it's expecting a certain
web client, or maybe you've not handling a field properly. For some
reason, your Mech problem isn't doing exactly what the browser is
doing, and when you find that, you'll have the answer.
My Mech program gets these 500 errors.
A 500 error from the web server says that the program on the server
side died. Probably the web server program was expecting certain
inputs that you didn't supply, and instead of handling it nicely, the
program died.
Whatever the cause of the 500 error, if it works in the browser, but
not in your Mech program, you're not acting like the browser. See the
previous question.
Why doesn't my program handle this form correctly?
Run mech-dump on your page and see what it says.
mech-dump is a marvelous diagnostic tool for figuring out what forms
and fields are on the page. Say you're scraping CNN.com, you'd get
this:
$ mech-dump http://www.cnn.com/
GET http://search.cnn.com/cnn/search
source=cnn (hidden readonly)
invocationType=search/top (hidden readonly)
sites=web (radio) [*web/The Web ??|cnn/CNN.com ??]
query= (text)
<NONAME>=Search (submit)
POST http://cgi.money.cnn.com/servlets/quote_redirect
query= (text)
<NONAME>=GET (submit)
POST http://polls.cnn.com/poll
poll_id=2112 (hidden readonly)
question_1=<UNDEF> (radio) [1/Simplistic option|2/VIEW RESULTS]
<NONAME>=VOTE (submit)
GET http://search.cnn.com/cnn/search
source=cnn (hidden readonly)
invocationType=search/bottom (hidden readonly)
sites=web (radio) [*web/??CNN.com|cnn/??]
query= (text)
<NONAME>=Search (submit)
Four forms, including the first one duplicated at the end. All the
fields, all their defaults, lovingly generated by HTML::Form's "dump"
method.
If you want to run mech-dump on something that doesn't lend itself to a
quick URL fetch, then use the "save_content()" method to write the HTML
to a file, and run mech-dump on the file.
Why don't https:// URLs work?
You need either IO::Socket::SSL or Crypt::SSLeay installed.
Why do I get "Input 'fieldname' is readonly"?
You're trying to change the value of a hidden field and you have
warnings on.
First, make sure that you actually mean to change the field that you're
changing, and that you don't have a typo. Usually, hidden variables
are set by the site you're working on for a reason. If you change the
value, you might be breaking some functionality by faking it out.
If you really do want to change a hidden value, make the changes in a
scope that has warnings turned off:
{
local $^W = 0;
$agent->field( name => $value );
}
I tried to [such-and-such] and I got this weird error.
Are you checking your errors?
Are you sure?
Are you checking that your action succeeded after every action?
Are you sure?
For example, if you try this:
$mech->get( "http://my.site.com" );
$mech->follow_link( "foo" );
and the "get" call fails for some reason, then the Mech internals will
be unusable for the "follow_link" and you'll get a weird error. You
must, after every action that GETs or POSTs a page, check that Mech
succeeded, or all bets are off.
$mech->get( "http://my.site.com" );
die "Can't even get the home page: ", $mech->response->status_line
unless $mech->success;
$mech->follow_link( "foo" );
die "Foo link failed: ", $mech->response->status_line
unless $mech->success;
How do I figure out why "$mech->get($url)" doesn't work?
There are many reasons why a "get()" can fail. The server can take you
to someplace you didn't expect. It can generate redirects which are not
properly handled. You can get time-outs. Servers are down more often
than you think! etc, etc, etc. A couple of places to start:
1 Check "$mech->status()" after each call
2 Check the URL with "$mech->uri()" to see where you ended up
3 Try debugging with "LWP::Debug".
If things are really strange, turn on debugging with "use LWP::Debug
qw(+);" Just put this in the main program. This causes LWP to print out
a trace of the HTTP traffic between client and server and can be used
to figure out what is happening at the protocol level.
It is also useful to set many traps to verify that processing is
proceeding as expected. A Mech program should always have an "I didn't
expect to get here" or "I don't recognize the page that I am
processing" case and bail out.
Since errors can be transient, by the time you notice that the error
has occurred, it might not be possible to reproduce it manually. So for
automated processing it is useful to email yourself the following
information:
· where processing is taking place
· An Error Message
· $mech->uri
· $mech->content
You can also save the content of the page with "$mech->save_content(
'filename.html' );"
I submitted a form, but the server ignored everything! I got an empty form
back!
The post is handled by application software. It is common for PHP
programmers to use the same file both to display a form and to process
the arguments returned. So the first task of the application programmer
is to decide whether there are arguments to processes. The program can
check whether a particular parameter has been set, whether a hidden
parameter has been set, or whether the submit button has been clicked.
(There are probably other ways that I haven't thought of).
In any case, if your form is not setting the parameter (e.g. the submit
button) which the web application is keying on (and as an outsider
there is no way to know what it is keying on), it will not notice that
the form has been submitted. Try using "$mech->click()" instead of
"$mech->submit()" or vice-versa.
I've logged in to the server, but I get 500 errors when I try to get to
protected content.
Some web sites use distributed databases for their processing. It can
take a few seconds for the login/session information to percolate
through to all the servers. For human users with their slow reaction
times, this is not a problem, but a Perl script can outrun the server.
So try adding a sleep(5) between logging in and actually doing anything
(the optimal delay must be determined experimentally).
Mech is a big memory pig! I'm running out of RAM!
Mech keeps a history of every page, and the state it was in. It
actually keeps a clone of the full Mech object at every step along the
way.
You can limit this stack size with the "stack_depth" parm in the
"new()" constructor. If you set stack_size to 0, Mech will not keep
any history.
AUTHOR
Copyright 2005-2009 Andy Lester "<andy at petdance.com>"
perl v5.10.1 2010-04-11 WWW::Mechanize::FAQ(3)