Discrimination

February 4th, 2010

I have wealthy friends, I have friends who are doing worse than me. Most of my friends are university graduates, some of them barely finished primary school. I don’t pay too much attention of their attractiveness but some of them ought to be more beautiful than others. Some of my friends speak with a clear, good accent while others have strong eastern accents. You might find the way they speak funny. But I don’t. I make friends with people who are streetwise and resourceful. You might call some other friends of mine “mother’s darling”. Of all these things I have mentioned above; none of them factor in how close a friend is to me. I wouldn’t go so far as to say I don’t care. But they are definitely not determining factors for my friendships. I don’t discriminate people based on their social status, education, appearance, etc.

My relationship doesn’t go very far with somebody who is not a man of his word. My close friends are punctual, reliable and they keep their promises. I don’t hang out with snobs, envious, scornful people. But the most important qualities I look for in a friend is being cultivated and refined and open minded. I am attracted to people who has a spark in their eyes. You know, the opposite of being a zombie. I make very strong distinctions on these matters. Everybody wants to have nice friends. I’m a nazi about it. I’d rather not waste any time on shallow people and their silly affairs.

We live in a society where any and all kinds of discrimination is frowned upon. I call this “but he is a good guy” disorder. That but clearly indicates there is something not good about that guy, he is not all-good. Scrape any negative attribute off of people. Are we all equally honest? Are we all self-sacrificing and understanding? And then that guy is actually a very nice person. Very nice my ass.

Discrimination based on gender or color or whatever superficial feature is bad. Even kindergarden children know that. Not discriminating at all is idiotic at best and bad for the society at worst. You have to make a choice between people, you can’t be intimate with everybody. You better not base your choice on stupid criteria like coolness or popularity or, god forbid, assertiveness. This is the exact same mistake we make choosing our rulers… ehm, I mean political representatives.

This is not just about human beings. We need to pick out the good from bad when we are making spending decisions. I have seen Food, Inc. recently and it talks about this (near the end). Corporations hide behind many layers of obscurity and we don’t easily see how they are doing their business. To compound that we, as individuals, think ourselves as too small to matter against big companies. But the truth is the only thing we can do happens to be a very powerful way to send them a message. If a company is evil, don’t buy its product. You can’t do much else anyway, but this one act should be enough.

So, make a choice. Or not. Choice is yours.

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Developing Reusable Django Apps: App Settings

January 26th, 2010

Eventually your app will need some sort of configuration. Supplying many parameters to customise your views, template tags and filters to allow template authors easily harness the power of your app and implementing registration pattern are all sensible things to do. But at some point you will need configuration for your app at project level. Because app level configuration is stupidnot reusable. Consumers of your app should never have to change its source code. And what better place to put our configuration statements than settings.py? Remember; we want to make things easier for our app’s consumers, not harder. There is no need to add a new file to the project for a few lines of settings1.

We already know we shouldn’t import settings.py directly. Instead we import the settings object (much like a singleton) of django.conf module:

from django.conf import settings

Now you can access different configuration options as attributes on this object. But I suggest you to use getattr() in order to avoid getting AttributeErrors. Also, notice how we didn’t hardcode the name of the attribute in the second method below:

# This is too verbose
try:
    some_setting = settings.SOME_SETTING
except AttributeError:
    some_setting = DEFAULT_VALUE

# Plain and simple
some_setting = getattr(settings, 'SOME_SETTING', DEFAULT_VALUE)

Instead of requiring consumers to define all your app settings it is better to supply sensible defaults. Also I find it useful to prefix names of app settings within settings.py.

# in settings.py
MYAPP_FOO_CHOICES = [('bar', u'Bar'), ('baz', u'Baz')]


# in myapp/models.py
from django.db import models
from django.conf import settings


FOO_CHOICES = getattr(settings, 'MYAPP_FOO_CHOICES', [('quux', u'Quux')])


class FooRecord(models.Model):
    foo = models.CharField(max_length=10, choices=FOO_CHOICES)

This works fine for simple apps with fewer settings. But it can easily get out of hand when your app grows. An app_settings.py module would help keeping track of configuration by keeping all configuration options in one place:

# in myapp/app_settings.py
from django.conf import settings


FOO_CHOICES = getattr(settings, 'MYAPP_FOO_CHOICES', [('quux', u'Quux')])


# in myapp/models.py
from django.db import models
from app_settings import FOO_CHOICES


class FooRecord(models.Model):
    foo = models.CharField(max_length=10, choices=FOO_CHOICES)

To summarize the points above:

  • Import settings from django.conf
  • Use getattr()
  • Always supply a default value.
  • Prefix settings you made up in settings.py
  • Use app_settings.py if you have many

1: If your configuration is long, say more than 100 lines, you should step back and reconsider. Perhaps you should prefer a strategy similar to django.contrib.sitemaps or django.contrib.syndication.

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Quest For Ultimate Development/Deployment Toolset: Fabric, Pip & Virtualenv

January 21st, 2010

DISCLAIMER: This post is a summary of my short experience with the tools mentioned. There is no flash news or revolutionary recipes. If you are familiar with them, you will probably find it boring. If you are not, this post might make your introduction less painful.

I stumbled upon this post following my frustration with setuptools after virtualenvising telvee repository. I have played with buildout to create repeatable deployments before. Don’t get me wrong, buildout is superb. But I have a few minor issues with it:

  • Buildout can create an interpreter but the isolation from the host system is not a good as virtualenv. (Please correct me if I’m wrong)
  • Buildout is a dependency1 whereas virtualenv is a system-wide tool.
  • It is, naturally, more work to set it up than a couple of ad-hoc bash scripts.

None of these issues make buildout less awesome, I am just not comfortable enough with it. I am considering other alternatives and their strengths and weaknesses.

Virtualenv

I am quite comfortable with virtualenv. It creates an isolated Python environment for you. For instance the following command will create an environment inside test directory that doesn’t have access to packages installed system-wide:

~/$ virtualenv --no-site-packages test

That is of course when you activate the virtual environment with:

~/$ cd test
~/test/$ source bin/activate
(test)~/test/$

Note the (test) prefix to your prompt. Any packages you install within this environment will be installed only for itself and won’t be available system-wide. If you have omitted --no-site-packages argument you could have access to globally installed packages too. When you are done with this virtual environment you can issue deactivate command to return to your normal shell. That’s basically what virtualenv does.

You can script virtualenv to a certain extend:

#!/usr/bin/env bash

virtualenv $1
cd $1
source bin/activate

echo $PATH
echo $PYTHONPATH

Or you can create bootstrap scripts with virtualenv. Virtualenv doesn’t provide a clean and powerful enough API here, just a callback and two methods to modify commandline arguments.

Virtualenv is great at what it does. But I think setuptools as a package installer (and it’s just an installer, not a manager) cripples virtualenv. Good news is newer versions have a commandline argument --distribute that appereantly substitutes distribute for setuptools.

Pip

My experiments with pip went just fine. Except I bumped into this problem. Globally installed pip was seeing system-wide packages when used on a virtual environment created with --no-site-packages. Again, good news is it is fixed in the trunk.

I will play with pip more once figure out how to best integrate the these applications.

Fabric

I see fabric as the glue that binds everything together. I have known about it long before, but I never had the chance to experiment. I have read most of the documentation2 and played with it a little. Basically the following script is an attempt to create a virtual environment and install pip and django on it, much like the shell script above:

from __future__ import with_statement
import os
from fabric.api import *
from fabric import context_managers


def _get_virtualenv_location():
    location = prompt('New location: ', default='../test')
    env.envdir, env.envname = os.path.split(os.path.abspath(location))
    env.envpath = os.path.join(env.envdir, env.envname)
    print('using "%(envname)s" at "%(envdir)s" as virtualenv' % env)


def _virtualenv(command):
    with context_managers.cd(env.envpath):
        result = local('. bin/activate && ' + command)
    return result


def clone():
    _get_virtualenv_location()
    with context_managers.cd(env.envdir):
        local('virtualenv --no-site-packages --clear %(envname)s' % env)
    print _virtualenv('echo $PATH')
    _virtualenv('easy_install pip')
    print _virtualenv('pip install django==1.1.1')

It took me a while to figure out how I can issue commands within a virtual environment. Since fabric commands don’t share state sourcing activate has no effect on subsequent commands. This SO entry helped me to write _virtualenv() function. It is kind of ugly making all functions but fab commands private. I think if fab used __all__ or something similar it would be more explicit. Also a contrib module for virtualenv would be nice3.

Fabric has cool features such as failure handling and code/config editing. It is a great tool to create repeatable deployments. It feels great to be coding in python (well, to some extend). Perhaps the resulting code is a little too complex for local operations. I wish I could write a fab command that handles both local and remote deployments. I think it is not unusual to deploy on the same machine in another location. But having to SSH into localhost is weird, don’t you think?

I am sure there are better ways to accomplish the goal of the script above. Maybe there is an entirely different way to integrage fabric, virtualenv and pip. So, comments and suggestions are welcome as usual.

Maybe I’ll revisit buildout again as well.

1: You need to bootstrap (install) buildout with each development/deployment site.

2: About one third of it I think.

3: I would happily attempt one, once I learn fabric a little better.

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Developing Reusable Django Apps

January 13th, 2010

Django app structure is an implementation of seperation of concerns. It is slightly different than what you can find in other MVC frameworks. The stack is split vertically, not horizontally. And then the app is split horizontally within, i.e. models, views, templates etc are in their seperate modules/packages/directories. This vertical splitting allows you to collect all ingredients of one functionality in your project in one place.

Framework Structure

I think apps1 are one of the strong points of Django. A selling point if you like. There is a great ecosystem of apps, you can find an app for almost anything posssible with Python. And Python is kick-ass when it comes to library wealth. But there is another major advantage of apps when they’re done right; a sane code base. Here is a slide from the Django in the Real World presentation by Jacob Kaplan-Moss:

The fivefold path

  • Do one thing, and do it well.
  • Don’t be afraid of multiple apps.
  • Write for flexibility.
  • Build to distribute.
  • Extend carefully.

I will focus on flexibility and interoperability of apps in this post. But before we proceed I would like to emphasize the first bullet point in the slide. Because the scope of your app plays a big role in its flexibility and interoperability. Apps should be small enough to easily understand and integrate (into a project). Many times I have moved away from an otherwise good app because of its many dependencies and/or excessive features. On the other hand apps should be big enough to allow for different configurations and allow extension without modifying their code. Do one thing, and do it well.

Scope of an app

Take django-tagging for example; it’s 1.3 KLOC but it does tagging and nothing else. There are no dependencies other than Django, you can add tags to any model without modifying the model source, a tag can be associated with any type of model and tagging hides the gory details from you… In short; finding the right size is important. This is why tagging is the tagging app for Django.

Building For Reuse

General advice is “even project specific apps should be reusable“. Slapping the same app onto another project is not the only advantage. In fact it may not be possible if you are not in the habit of upgrading your whole project to recent versions of Django. The main advantage as I have said before is sanity. I prefer Django to other web frameworks/environments because it provides a civilized way of development. Let’s accept it; web programming is not a particularly interesting, exciting or intellectually rewarding field. You write the same piece of code over and over. And worst of all the challanges you face are actually a result of either the underlying system was designed by morons or you are trying to use it for something it’s not intended to be used. So it is only natural that web programmers feel they’re rusting. Django eases the pain. If you stick to certain conventions serenity will follow as well.

Naturally the framework does most of the work regarding app flexibility and interoperability. Take URLs for instance include('myapp.urls') and you are good to go. You don’t have to bind views one by one. Is it inflexible? Who said urls.py can only contain a hardcoded list of URLs. You can do anything that is possible with Python. You can generate different urlpatterns based on a setting for instance.

It is relatively easy and straightforward to reuse and extend forms and views (both function based and class based). Models are a little harder to get right though. You should always think of the most difficult situation which is you can’t touch either app’s code. Registration pattern of admin app provides a good solution here. You can register a third party model to another third party app in just a few lines.

You don’t need to write lots of code to get the flexibility and interoperability. Well designed apps make good use of settings.py for example. Why should the project developer wrap a view when a single line assignment would do the job? Supplying good templatetags and template snipplets (includes) is another way to make things easy for app consumers.

Signals provide a great way to propagate the events generated from your app. Even though they are one way2, signals are extremely powerful. Any number of observers can connect to a signal and you can send a signal anywhere in your code. Literally. It is even possible your app suppying a signal and then another app sending it3.

There are many more ways to tame your app to be reusable. It all starts with your determination and discipline. Just like documentation, testing and maintaining a software generally. I will write more about reusable apps.

1: The word application is used both for a web application and a Django application. To avoid confusion I always use app to indicate the latter.

2: Signals don’t have return values. But you can use a callback AFAIK.

3: I can’t think of an example this would be useful, but still…

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Dynamic Translation Apps for Django

January 6th, 2010

When I needed multi-language flatpages and flatblocks for telvee I searched for available Django apps that do dynamic translation. By dynamic translation, I mean translations are entered and stored in the database. As I said I needed to be able to translate both full pages and chunks of text that I can include into another page. I ended up rolling my own, which is without a doubt lesser to some apps below. I will try to imrove it with the good ideas from existing projects and then open source. Meanwhile I would like to share my review of 6 dynamic translation apps with you. I hope someone out there finds it useful.

Django-multilingual

One of the few active projects I have reviewed is django-multilingual. To enable a model for translations you need to create a Translation inner-class and move translatable fields inside. Then a seperate model for those fields is created behind the scenes. Django-multilingual has admin integration and a multi-language flatpages app. Another nice thing about this app is that it’s using signals internally. The API is based on getter and setter methods. For instance you call get_<fieldname>(language_id=None) on your model to get the field value for the active translation. This app has no releases and less than satisfactory API documentation and has tests only on the example project.

Django-pluggable-model-i18n

Django-pluggable-model-i18n uses registration pattern of admin app. It creates an extra model for translated fields and stores translations of non-default languages in it. This app has no releases, no tests, no API documentation and it is clearly stated to be experimental. I would also like to note the last commit date is May 25, 2009.

Django-modeltranslation

Another app that implements registration pattern is django-modeltranslation. The advantage of registration pattern is you don’t have to modify the code of the 3rd party apps you want to translate dynamically. But the database schema still needs to be modified if you are using django-modeltranslation. Also there is one translations.py for the project. I think one translation definition file per app would be better (just like admin does). Django-modeltranslation has intuitive underscore-language_code API but it is somewhat inconsistent; when you read the unsuffixed field you read the currently active language, when you write you write the default language specified in settings.py. Also the modified model stores some redundant data. For instance if your DEFAULT_LANGUAGE is "ES" both <fieldname> and <fieldname>_es columns will store the same value. Django-modeltranslation has admin integration and a management command to update database schema. Most importantly this app is the only one which has both tests and a release. (Unfortunately tests require specific settings to run)

Transdb

Transdb takes a completely different approach to dynamic translation problem. It provides two new field types; TransCharField and TransTextField. Then it serializes all your translations within a single column for each field of those. This means no JOINs and no extra queries. Unfortunately transdb doesn’t implement underscore-language_code API, you need to use get_in_language() and set_in_language() methods. Transdb has default widgets that render one form field for each language. Last commit date is Nov 07, 2008 and there is a release.

Django-multilingual-model

This one is not actually an app but just one module with 33 42 lines of code. You need to define the model that holds translations manually. This introduces some code redundancy, since you also define which fields get translated in the original model. Django-multilingual-model doesn’t implement any translation API, so it’s rather verbose to do anything with it. There are no tests and no releases. I simply don’t recommend django-multilingual-model for anything serious.

Django-transmeta

Django-transmeta stores translations in extra columns it creates in the original field’s table similar to django-modeltranslation. But you enable translation assigning a metaclass for your model and then add a Meta attribute; this means you can’t make models in existing apps translatable without modifying their code. Django-transmeta implements underscore-language_code API, has admin integration and a management command to sync database when you add new languages or translatable fields. There are documentation and code examples but no tests or releases. Last commit date it Nov 24, 2009.

Comparison of Dynamic Translation Apps

Comparison of Dynamic Translation Apps

Software development is making choices. Would you rather have a clean and stable schema with an extra translations model or avoid extra JOINs and denormalize translations onto your original model’s table? Both have advantages and disadvantages. But some choices are not based on trade-offs. Documentation, examples, tests and releases for instance. Also in my opinion underscore-language_code API is way better than any of the alternatives.

Django platform is a very powerful and intuitive one. Many people have moved in last year. This popularity affected app ecosystem as well. But unfortunately a significant number of those apps are half baked fire-and-forget type. I wish 2010 to be the year of a significant increase in software quality of Django apps. I’ll try to do my part.

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