Dreamwidth Development Training

DW object-oriented programming explained (Part 3) - using classes

Tue, 01 May 2012 22:16:31 GMT

Posted by: sophie

It's been a while since I posted anything in this series of entries, and I apologise for that! Today, though, I'll be explaining how to actually use classes in Perl and the Dreamwidth code and constructing new objects.

( How do you use classes? )

That's a lot to take in, so I'll leave off for now! I hope I've explained things well, but I'm sure that there are bits that are going to be incomprehensible. If you need me to explain anything further, please ask in the comments - I'll be happy to try to explain it.

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DW object-oriented programming explained (Part 2)

Fri, 17 Feb 2012 23:27:28 GMT

Posted by: sophie

Welcome to the second part of the series on object-oriented programming - or OO - as it applies to the Dreamwidth codebase. :)

If you haven't already read the first part, you'll want to do that before reading this part. I also realise that I never got around to explaining what 'methods' are in the first post, so I'm going to do that right now before delving into the main part of this post:

( What are methods? )

As with the last post, if you have any questions on this, feel free to let me know in the comments!

So, with that explanation of methods out of the way, it's time to move onto our next topic - how it applies to the DW codebase.

I'm going to do this as a few posts, each dealing with their own topic, because I've got a fair amount to say about them. I'm still not entirely sure how many there'll be, but I'm writing them one at a time so there may be some time (a few days to a week) between each one.

A couple of things to note before I begin:

This post may require some basic knowledge of Perl and/or programming in general. Not much, I promise! (Things such as what a 'string' is, etc.) But all the same, if anybody finds themselves confused by anything I write, feel free to ask for clarification in the comments. I won't bite!

Secondly, if you're used to OO from another language, you'll find some things about Perl's implementation of OO to be strange and baffling. That's because Perl wasn't actually designed with OO in mind; OO support came later, and to be honest, it shows. Still, it's what we use, so I hope I can at least help with understanding it.^(**)

^(**) There is a version of Perl in the works which does a much better job of not only OO but a lot of other things - Perl 6 - but at the cost of revamping a lot of the language such that you probably wouldn't be able to use it without spending some time making sure your code conformed to it. For this series, therefore, I'll be concentrating on Perl 5, which is what most Perl developers - including DW and LJ - use.

With all that said, let's move onto our first topic!

( What is an 'object' in Perl? )

That's about it for this post. There's a lot of stuff here so feel free to ask questions if there's anything you don't understand! My next post will probably talk about how you can create and use an object, as well as some example of existing classes in the codebase.

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Strip tab characters from multiple text files and replace them with spaces (or something else)

Fri, 03 Feb 2012 20:09:30 GMT

Posted by: delladea

I switch between Gedit, Notepad++, and vim fairly often depending on what I'm doing and whose computer I'm on. Sometimes I end up with tab characters where I really wanted four spaces, mainly when I'm using vim and I haven't figured out how to get vim to not do this. Gedit and Notepad++ have settings to use spaces instead of tabs, so there's no issue there.

Either I don't notice the tab characters until after I've put lots of them in the file I'm editing, or I'm editing a file from someone else whose editor uses tab characters for indentation. I know its not a big deal to some people, but tab indentation mixed with space indentation is a huge pet peeve of mine.

Thus, a perl script was born:

#!/usr/bin/perl -w

use File::Copy;

# This script replaces tab characters in a file with four spaces, or whatever else you want

$num_args = $#ARGV + 1;
$num_warnings = 0;
$tab_replace = "    ";  # Change this to whatever you want in place of tabs

if ( $num_args == 0 ) {
    print "usage: strip-tabs.pl file1 [file2, file3...]\n";
    exit;
}

# Have files to parse...
for my $f ( 0 .. ($num_args - 1) ) {
    if ( open(INPUTFILE, "<$ARGV[$f]") ) {
       if ( open(OUTPUTFILE, ">$ARGV[$f]~") ) {
           # Start parsing the file
           while ( my $line = <INPUTFILE> ) {
               $line =~ s/\t/$tab_replace/g;
               print OUTPUTFILE $line;
           }
           # Copy over original file here
           close(OUTPUTFILE);
           close(INPUTFILE);
           if ( !move("$ARGV[$f]~", $ARGV[$f]) ) {
               print "Could not write output to file $ARGV[$f]: $!\n";
               $num_warnings += 1;
           }
       }
       else {
           close(INPUTFILE);
           print "Could not create output file for $ARGV[$f]: $!\n";
           $num_warnings += 1;
       }
    }
    else {
        print "Could not open $ARGV[$f] for reading: $!\n";
        $num_warnings += 1;
    }
}
die "$num_warnings warnings encountered during file operation." unless $num_warnings == 0;

View Gist (strip-tabs.pl)

Feel free to gank away if you find it useful!

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"I wonder what this perl code does..."

Thu, 09 Jun 2011 16:29:32 GMT

Posted by: pauamma

When I get one of those moments, I reach for "perl -e" (because it's always available with perl), but someone mentioned the following 2 tools recently on a mailing list I read, so I thought I'd pass them on:

- perlconsole
- Devel::Repl

(Crossposted to perl.)

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Nifty perl trick I learned about today

Tue, 31 Aug 2010 16:43:54 GMT

Posted by: pauamma

Context was someone asking why

print "Hello, world!\n" && die;

doesn't output anything. Someone replied with a nifty trick to help figure this out:

perl -MO=Deparse -e 'print "Hello, world!\n" && die;'

(go ahead and try it in your Dreamhack).

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Writing to the database through a Perl script in Dreamwidth, placeholders

Sun, 25 Jul 2010 09:08:27 GMT

Posted by: anarres

In Dreamwidth you can interact with the database using Perl's DBI module, but with a few Dreamwidth-specific methods layered on top. I've been trying to figure out how to write new rows to a database table. After looking at some other Dreamwidth scripts to see how it's done, I wrote the following script (which doesn't work):

# -------------------- ~/dw/bin/dev/test.pl ----------------------------
#!/usr/bin/perl

use lib "$ENV{LJHOME}/cgi-bin";
require 'ljlib.pl';
use strict;
use warnings;

my $cart = {
authcode => 'blahblahblahblah',
userid => 12,
cartid => 33,
};

my $dbh = LJ::get_db_writer() or return undef;

$dbh->do(
q{INSERT INTO payments (userid, cartid) VALUES ( ?, ?)},
$cart->{userid}, $cart->{cartid}
);

die "Database error: " . $dbh->errstr . "\n" if $dbh->err;
# ---------------------------------------------------------------------

The question marks are SQL placeholders, which prevent SQL injection attacks.

When I ran this I got the error: 'DBI::db=HASH(0x426eff0)->do(...): attribute parameter '12' is not a hash ref at test.pl line 22.'

It seemed to be complaining that $cart->{userid} is not a hash ref. I had no idea why it would want a hash ref there, but I bemusedly decided to give it what it wanted, and replaced

$cart->{userid}, $cart->{cartid}

with:

$cart, $cart

Running the script again gave an SQL error instead of a Perl error: 'Database error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 2'.

So it seems like giving it a hash ref is the right thing to do, but I'm giving it the wrong kind of hash ref. I'm completely confused: what's the right syntax for inserting a row into the database?

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Dynamic images in Dreamwidth

Sat, 26 Jun 2010 07:47:15 GMT

Posted by: anarres

This seems like it should be easy but I'm really stuck. I'm trying to dynamically display images - i.e. generate the image with a perl script and display it without saving it to the server.

This simple example works on my Heliohost server, but not on Dreamwidth (the example is from http://www.perlmonks.org/?node_id=18565):

--------test.shtml------------
<img src="test.cgi?size=100" alt="" />
------------------------------

--------test.cgi------------
#!/usr/bin/perl -w

use strict;
use CGI;
use GD;

my $cgi=new CGI;
my $cgi_size=$cgi->param('size') || '50';

print "Content-type: image/gif\n\n";
my $gd=new GD::Image($cgi_size,$cgi_size);
my $blue=$gd->colorAllocate(0,0,255);
$gd->fill(0,0,$blue);
binmode STDOUT; #just in case we're on NT
print $gd->gif;
------------------------------

I'm stumped - has anyone got any ideas?

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