NAME¶
WWW::Mechanize::FAQ - Frequently Asked Questions about WWW::Mechanize
How to get help with WWW::Mechanize¶
If your question isn't answered here in the FAQ, please turn to the communities
at:
- •
- <http://perlmonks.org>
- •
- The libwww-perl mailing list at
<http://lists.perl.org>
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 "$mech->links". They work sometimes.
Since Javascript is completely visible to the client, it cannot be used to
prevent a scraper from following links. But it can make life difficult. If you
want to scrape specific pages, then a solution is always possible.
One typical use of Javascript is to perform argument checking before posting to
the server. The URL you want is probably just buried in the Javascript
function. Do a regular expression match on "$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. The
Javascript Debugger in the Firebug extension for Firefox helps 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.
Which modules work like Mechanize and have JavaScript
support?¶
In no particular order: Gtk2::WebKit::Mechanize, Win32::IE::Mechanize,
WWW::Mechanize::Firefox, WWW::Scripter, WWW::Selenium
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
Use the mech-dump utility, optionally 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;
...
$mech->conn_cache(LWP::ConnCache->new);
You can access the action of the form by utilizing the HTML::Form object
returned from one of the specifying form methods.
Using "$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.
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.
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 $mech->success;
There are many reasons why a "get()" can fail. The server can take you
to someplace you didn't expect. It can generate redirects which are not
properly handled. You can get time-outs. Servers are down more often than you
think! etc, etc, etc. A couple of places to start:
- 1 Check "$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' );"
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>"