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.

  • Issac Kelly

    Don’t forget admin.py, and possibly search.py (Haystack)

  • http://twitter.com/sidmitra Siddharth Mitra

    I also have two extra ones on most production apps called session.py and cache.py containing CacheManager, SessionManager classes (inheriting from a common base ofcourse) to isolate all session handling, cache logic etc.

    This way i don’t have request.session[‘TEST’] = 1 etc all over the code. I usually do session = SessionManager(request)
    session.save_something(‘test’)

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  • Anonymous

    The conventions for views.py should probably change with upcoming django 1.3: authors of django apps shouldn’t use `template_name` keyword argument, they should use TemplateResponse instead (http://docs.djangoproject.com/en/dev/ref/template-response/#using-templateresponse-and-simpletemplateresponse) or write class-based views (http://docs.djangoproject.com/en/dev/topics/class-based-views/).

  • Scott

    I’ve always struggled with the decision regarding where to put your business logic. I think that I feel pretty comfortable now with including it in models.py, but I have variably used a separate logic.py module also.

  • http://streamhacker.com/ Jacob Perkins

    Thanks for pointing out TemplateResponse objects. If I understand correctly, when a 3rd party app uses a TemplateResponse, then you’d have to wrap the view with your own function to modify the template name before rendering. Seems like a template_name keyword argument would still be useful and require less extra code.

    As for class based views, I’m not yet convinced they’re a good idea. I personally prefer a more functional style, which is one reason I love decorators.

  • http://streamhacker.com/ Jacob Perkins

    Business logic is such a vague term – could you be more specific about what kind of code you put in logic.py?

  • http://streamhacker.com/ Jacob Perkins

    I’ve got admin.py as 3rd from the top, but no search.py. From the haystack docs, it looks like it should actually be search_indexes.py. In either case, I haven’t used it yet and I think it’s less common than the official contrib apps.

  • http://streamhacker.com/ Jacob Perkins

    Hmm, never heard of doing that before. How is having custom manager classes better than the standard dictionary methods?

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  • Issac Kelly

    d’oh! My fault. for some reason I was expecting alphabetical order (even though they clearly aren’t)

  • http://twitter.com/sidmitra Siddharth Mitra

    I do it, so everything session related stays in one place. So for example i want to save the vote for an unauthenticated user. My session manageer class would have session.save_vote(‘blah blah’) and it wouldn’t know anything about which key i’m using, and also include any other related logic, keeping my views concise. For example in some cases i needed to save full user submitted forms to session until he logged in. Then instead of storing the fields separetely i just wrote session.save_form(form=some_form_instance). Hope that made sense?

    The other reason i that the session keys for 1 app are all in one place. They’re constants of the manager class.
    So i can just call session.clear() or reset and only clear the keys for that app not everything. etc. Same with cache. Maybe i’ll do a blog post to explain it better.

  • http://streamhacker.com/ Jacob Perkins

    Ok, thanks for explaining. It definitely makes sense if you’re doing a lot with the session/cache, and keeps keys defined in a single place, instead of having them in multiple views.

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  • http://ryankaskel.com/ Ryan Kaskel

    Also common is putting all callbacks connected to signals in listeners.py

    (http://www.google.com/codesearch?q=file:listeners.py)

  • http://streamhacker.com/ Jacob Perkins

    Thanks, I hadn’t seen that pattern before, and assumed most people (including myself) put the callbacks in models.py.

  • Anonymous

    Also, TemplateResponses in Django 1.3 eliminate the need for using django-annoying’s render_to decorator.