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27Aug/111

Python 3 Web Development Review

Python 3 Web DevelopmentThe problem with Python 3 Web Development Beginner's Guide, by Michel Anders, is one of expectations (disclaimer: I received a free eBook from Packt for review). Let's start with the title... First we have Python 3 Web Development. This immediately sets the wrong expectations because:

  1. There's almost as much jQuery & Javascript as there is Python.
  2. Most of the Python code is not Python 3 specific, and the code that is could easily be translate to Python 2.
  3. Much of the Python code either uses CherryPy or is for generating HTML. This is not immediately obvious, but becomes apparent in Chapter 3 (which is available as a free PDF download: Chapter 3 - Tasklist I Persistence).

Second, this book is also supposed to be a Beginner's Guide, but that is definitely not the case. To really grasp what's going on, you need to already know the basics of HTML, jQuery interaction, and how HTTP works. Chapter 1 is an excellent introduction to HTTP and web application development, but the book as a whole is not beginner material. I think that anything that uses Python metaclasses automatically becomes at least intermediate level, if not expert, and the main thrust of Chapter 7 is refactoring all your straightforward database code to use complicated metaclasses.

However, if you mentally rewrite the title to be "Web Framework Development from scratch using CherryPy and jQuery", then you've got the right idea. The book steps you through web app development with CherryPy, database models with sqlite3, and plenty of HTML and jQuery for interface generation and interaction. While creating example applications, you slowly build up a re-usable framework. It's an interesting approach, but unfortunately it gets muddied up with inline HTML rendering. I never thought a language as simple and elegant as Python could be reduced to the ugliness of common PHP, but generating HTML with string interpolation inside the same functions that are accessing the database gets pretty close. I kept expecting the author to introduce template rendering, which is a major part of most modern web development frameworks, but it never happened, despite the plethora of excellent Python templating libraries.

While reading this book, I often had the recurring thought "I'm so glad I use Django". If your aim is rapid application development, this is not the book for you. However, if you're interested in creating your own web development framework, or would at least like to understand how a framework like Django could be created, then buy a copy Python 3 Web Development.

3Jan/1114

Django Application Conventions

A Django application is really just a python package with a few conventionally named modules. Most apps will not need all of the modules described below, but it's important to follow the naming conventions and code organization because it will make your application easier to use. Following these conventions gives you a common model for understanding and building the various pieces of a Django application. It also makes it possible for others who share the same common model to quickly understand your code, or at least have an idea of where certain parts of code are located and how everything fits together. This is especially important for reusable applications. For examples, I highly recommend browsing through the code of applications in django.contrib, as they all (mostly) follow the same conventional code organization.

models.py

models.py is the only module that's required by Django, even if you don't have any code in it. But chances are that you'll have at least 1 database model, signal handler, or perhaps an API connection object. models.py is the best place to put these because it is the one app module that is guarenteed to be imported early. This also makes it a good location for connection objects to NoSQL databases such as Redis or MongoDB. Generally, any code that deals with data access or storage should go in models.py, except for simple lookups and queries.

managers.py

Model managers are sometimes placed in a separate managers.py module. This is optional, and often overkill, as it usually makes more sense to define custom managers in models.py. However, if there's a lot going in your custom manager, or if you have a ton of models, it might make sense to separate the manager classes for clarity's sake.

admin.py

To make your models viewable within Django's Admin system, then create an admin.py module with ModelAdmin objects for each necessary model. These models can then be autodiscovered if you use the admin.autodiscover() call in your top level urls.py.

views.py

View functions (or classes) have 3 responsibilities:

  1. request handling
  2. form processing
  3. template rendering

If a view function is doing anything else, then you're doing it wrong. There are many things that fall under request handling, such as session management and authentication, but any code that does not directly use the request object, or that will not be used to render a template, does not belong here. One valid is exception is sending signals, but I'd argue that a form or models.py is a better location. View functions should be short & simple, and any data access should be primarily read-only. Code that updates data in a database should either be in models.py or the save() method of a form.

Keep your view functions short & simple - this will make it clear how a specific request will produce a corresponding response, and where potential bottlenecks are. Speed has business value, and the easiest way to speed up code is to make it simpler. Do less, and move the complexity elsewhere, such as forms.py.

Use decorators generously for validating requests. require_GET, require_POST, or require_http_methods should go first. Next, use login_required or permission_required as necessary. Finally, use ajax_request or render_to from django annoying so that your view can simply return a dict of data that will be translated into a JSON response or a RequestContext. It's not unheard of to have view functions with more decorators than lines of code, and that's ok because the process flow is still clear, since each decorator has a specific purpose. However, if you're distributing a pluggable app, then do not use render_to. Instead, use a template_name keyword argument, which will allow developers to override the default template name if they wish. This template name should be prefixed by an appropriate subdirectory. For example, django.contrib.auth.views uses the template subdirectory registration/ for all its templates. This encourages template organization to mirror application organization.

If you have lots of views that can be grouped into separate functionality, such as account management vs everything else, then you can create separate view modules. A good way to do this is to create a views subpackage with separate modules within it. The comments contrib app organizes its views this way, with the user facing comments views in views/comments.py, and the moderator facing moderation views in views/moderation.py.

decorators.py

Before you write your own decorators, checkout the http decorators, admin.views.decorators, auth.decorators, and annoying.decorators. What you want may already be implemented, but if not, you'll at least get to see a bunch of good examples for how to write useful decorators.

If you do decide to write your own decorators, put them in decorators.py. This module should contain functions that take a function as an argument and return a new function, making them higher order functions. This enables you to attach many decorators to a single view function, since each decorators wraps the function returned from the next decorator, until the final view function is reached.

You can also create functions that take arguments, then return a decorator. So instead of being a decorator itself, this kind of function generates and returns a decorator based on the arguments provided. render_to is such a higher order function: it takes a template name as an argument, then returns a decorator that renders that template.

middleware.py

Any custom request/response middleware should go in middleware.py. Two commonly used middleware classes are AuthenticationMiddleware and SessionMiddleware. You can think of middleware as global view decorators, in that a middleware class can pre-process every request or post-process every response, no matter what view is used.

urls.py

It's good practice to define urls for all your application's views in their own urls.py. This way, these urls can be included in the top level urls.py with a simple include call. Naming your urls is also a good idea - see django.contrib.comments.urls for an example.

forms.py

Custom forms should go in forms.py. These might be model forms, formsets, or any kind of data validation & transformation that needs to happen before storing or passing on request data. The incoming data will generally come from a request QueryDict, such as request.GET or request.POST, though it could also come from url parameters or view keyword arguments. The main job of forms.py is to transform that incoming data into a form suitable for storage, or for passing on to another API.

You could have this code in a view function, but then you'd be mixing data validation & transformation in with request processing & template rendering, which just makes your code confusing and more deeply nested. So the secondary job of forms.py is to contain complexity that would otherwise be in a view function. Since form validation is often naturally complicated, this is appropriate, and keeps the complexity confined to a well defined area. So if you have a view function that's accessing more than one variable in request.GET or request.POST, strongly consider using a form instead - that's what they're for!

Forms often save data, and the convention is to use a save method that can be called after validation. This is how model forms behave, but you can do the same thing in your own non-model forms. For example, let's say you want to update a list in Redis based on incoming request data. Instead of putting the code in a view function, create a Form with the necessary fields, and implement a save() method that updates the list in redis based on the cleaned form data. Now your view simply has to validate the form and call save() if the data is valid.

There should generally be no template rendering in forms.py, except for sending emails. All other template rendering belongs in views.py. Email template rendering & sending should also be implemented in a save() method. If you're creating a pluggable app, then the template name should be a keyword argument so that developers can override it if they want. The PasswordResetForm in django.contrib.auth.forms provides a good example of how to do this.

tests.py

Tests are always a good idea (even if you're not doing TDD), especially for reusable apps. There are 2 places that Django's test runner looks for tests:

  1. doctests in models.py
  2. unit tests or doctests in tests.py

You can put doctests elsewhere, but then you have to define your own test runner to run them. It's often easier to just put all non-model tests into tests.py, either in doctest or unittest form. If you're testing views, be sure to use Django's TestCase, as it provides easy access to the test client, making view testing quite simple. For a complete account of testing Django, see Django Testing and Debugging.

backends.py

If you need custom authentication backends, such as using an email address instead of a username, put these in backends.py. Then include them in the AUTHENTICATION_BACKENDS setting.

signals.py

If your app is defining signals that others can connect to, signals.py is where they should go. If you look at django.contrib.comments.signals, you'll see it's just a few lines of code with many more lines of comments explaining when each signal is sent. This is about right, as signals are essentially just global objects, and what's important is how they are used, and in what context they are sent.

management.py

The post_syncdb signal is a management signal that can only be connected to within a module named management.py. So if you need to connect to the post_syncdb signal, management.py is the only place to do it.

feeds.py

To define your own syndication feeds, put the subclasses in feeds.py, then import them in urls.py.

sitemaps.py

Custom Sitemap classes should go in sitemaps.py. Much like the classes in admin.py, Sitemap subclasses are often fairly simple. Ideally, you can just use GenericSitemap and bypass custom Sitemap objects altogether.

context_processors.py

If you need to write custom template context processors, put them in context_processors.py. A good case for a custom context processor is to expose a setting to every template. Context processors are generally very simple, as they only return a dict with no more than a few key-values. And don't forget to add them to the TEMPLATE_CONTEXT_PROCESSORS setting.

templatetags

The templatetags subpackage is necessary when you want to provide custom template tags or filters. If you're only creating one templatetag module, give it the same name as your app. This is what django.contrib.humanize does, among others. If you have more than one templatetag module, then you can namespace them by prefixing each module with the name of your app name followed by an underscore. And be sure to create __init__.py in templatetags/, so python knows it's a proper subpackage.

management/commands

If you want to provide custom management commands that can be used through manage.py or django-admin.py, these must be modules with the commands/ subdirectory of a management/ subdirectory. Both of these subdirectories must have __init__.py to make them python subpackages. Each command should be a separate module whose name will be the name of the command. This module should contain a single class named Command, which must inherit from BaseCommand or a BaseCommand subclass. For example, django.contrib.auth provides 2 custom management commands: changepassword and createsuperuser. Both of these commands are modules of the same name within django.contrib.auth.management.commands. For more details, see creating Django management commands.

8Mar/1016

jQuery Validation with Django Forms

Django has everything you need to do server-side validation, but it's also a good idea to do client-side validation. Here's how you can integrate the jQuery Validation plugin with your Django Forms.

jQuery Validation Rules

jQuery validation works by assigning validation rules to each element in your form. These rules can be assigned a couple different ways:

  1. Class Rules
  2. Metadata Rules
  3. Rules Object

Django Form Class Rules

The simplest validation rules, such as required, can be assigned as classes on your form elements. To do this in Django, you can specify custom widget attributes.

from django import forms
from django.forms import widgets

class MyForm(forms.Form):
    title = forms.CharField(required=True, widget=widgets.TextInput(attrs={
        'class': 'required'
    }))

In Django 1.2, there's support for a required css class, but you can still use the technique above to specify other validation rules.

Django Form Metadata Rules

For validation methods that require arguments, such minlength and maxlength, you can create metadata in the class attribute. You'll have to include the jQuery metadata plugin for this style of rules.

from django import forms
from django.forms import widgets

class MyForm(forms.Form):
    title = forms.CharField(required=True, minlength=2, maxlength=100, widget=widgets.TextInput(attrs={
        'class': '{required:true, minlength:2, maxlength:100}'
    }))

jQuery Validate Rules Object

If your validation requirements are more complex, or you don't want to use the metadata plugin or class based rules, you can create a rules object to pass as an option to the validate method. This object can be generated in your template like so:

<script type="text/javascript">
FORM_RULES = {
    '{{ form.title.name }}': 'required'
};

$(document).ready(function() {
    $('form').validate({
        rules: FORM_RULES
    });
});
</script>

The reason I suggest generating the rules object in your template is to avoid hardcoding the field name in your javascript. A rules object can also be used in conjunction with class and metadata rules, so you could have some rules assigned in individual element classes or metadata, and other rules in your rules object.

Error Messages

If you want to keep the client-side validation error messages consistent with Django's validation error messages, you'll need to copy Django's error messages and specify them in the metadata or in a messages object.

Metadata Messages

Messages must be specified per-field, and per-rule. Here's an example where I specify the minlength message for the title field.

from django import forms
from django.forms import widgets

class MyForm(forms.Form):
    title = forms.CharField(minlength=2, widget=widgets.TextInput(attrs={
        'class': '{minlength:2, messages:{minlength:"Ensure this value has at least 2 characters"}}'
    }))

Messages Object

Messages can also be specified in javascript object, like so:

<script type="text/javascript">
FORM_RULES = {
    '{{ form.title.name }}': 'required'
};

FORM_MESSAGES = {
    '{{ form.title.name }}': 'This field is required'
};

$(document).ready(function() {
    $('form').validate({
        rules: FORM_RULES,
        messages: FORM_MESSAGES
    });
});
</script>

Just like with validation rules, messages in element metadata can be used in conjunction with a global messages object. Note: if an element has a title attribute, then the title will be used as the default error message, unless you specify ignoreTitle: false in the jQuery validate options.

Error Labels vs Errorlist

Django's default error output is an error list, while the default for jQuery Validation errors is a label with class="error". So in order to unify your validation errors, there's 2 options:

  1. make jQuery Validation output an error list
  2. output error labels instead of an error list in the template

Personally, I prefer the simple error labels produced by jQuery validation. To make Django generate those instead of an error list, you can do the following in your templates:

{{ field }}
{% if field.errors %}
{# NOTE: must use id_NAME for jquery.validation to overwrite error label #}
<label class='error' for='id_{{ field.name }}' generated="true">{{ field.errors|join:". " }}</label>
{% endif %}

You could also create your own error_class for outputting the error labels, but then you'd lose the ability to specify the for attribute.

If you want to try to make jQuery validation produce an error list, that's a bit harder. You can specify a combination of jQuery validation options and get a list, but there's not an obvious way to get the errorlist class on the ul.

$('form').validate({
    errorElement: 'li',
    wrapper: 'ul'
});

Other options you can look into are errorLabelContainer, errorContainer, and a highlight function.

Final Recommendations

I find it's easiest to specify class and metadata rules in custom widget attributes 90% of the time, and use a rules object only when absolutely necessary. For example, if I want to require only the first elements in a formset, but not the rest, then I may use a rules object in addition to class and metadata rules. For error messages, I generally use a field template like the above example that I include for each field:

{% with form.title as field %}{% include "field.html" %}{% endwith %}

Or if the form is really simple, I do

{% for field in form %}{% include "field.html" %}{% endfor %}
13Apr/090

Django Datetime Snippets

I've started posting over at Django snippets, which is a great resource for finding useful bits of functionality. My first set of snippets is focused on datetime conversions.

The Snippets

FuzzyDateTimeField is a drop in replacement for the standard DateTimeField that uses dateutil.parser with fuzzy=True to clean the value, allowing the parser to be more liberal with the input formats it accepts.

The isoutc template filter produces an ISO format UTC datetime string from a timezone aware datetime object.

The timeto template filter is a more compact version of django's timeuntil filter that only shows hours & minutes, such as "1hr 30min".

JSON encode ISO UTC datetime is a way to encode datetime objects as ISO strings just like the isoutc template filter.

JSON decode datetime is a simplejson object hook for converting the datetime attribute of a JSON object to a python datetime object. This is especially useful if you have a list of objects that all have datetime attributes that need to be decoded.

Use Case

Imagine you're making a time based search engine for movies and/or events. Because your data will span many timezones, you decide that all dates & times should be stored on the server as UTC. This pushes local timezone conversion to the client side, where it belongs, simplifying the server side data structures and search operations. You want your search engine to be AJAX enabled, but you don't like XML because it's so verbose, so you go with JSON for serialization. You also want users to be able to input their own range based queries without being forced to use specific datetime formats. Leaving out all the hard stuff, the above snippets can be used for communication between a django webapp and a time based search engine.

   
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