valberg.dk/content/django-view-decorator.md

Title: Bringing Locality of Behaviour to Django Views and URLs
Date: 2023-06-20
Status: hidden
Tags: django, views, locality-of-behaviour
Slug: bringing-locality-of-behaviour-to-django-views-and-urls
Authors: Víðir Valberg Guðmundsson
Summary: Introducing django-view-decorator, a Django package which brings Locality of Behaviour to your Views and URLs

---

It seems that "The Location of Behaviour principle" (shortened as LoB) is gaining traction these days. This has given me the urge to try to influence the direction of Django to bring more LoB to the connection between views and URLs.

But first, what is "LoB"? The principle is coined by the author of HTMX in [a short, but great, essay](https://htmx.org/essays/locality-of-behaviour/). The principle states:

> The behaviour of a unit of code should be as obvious as possible by looking only at that unit of code

Given a very simple view:

    :::python
    # views.py
    def foo(request: HttpRequest) -> HttpResponse:
        return "bar"

It is not apparent how to access the view with HTTP. To see what URL the view is tied to we have to look at urls.py:

    :::python
    # urls.py
    from django.urls import path
    from .views import foo
    urlspatterns = [
        path("foo/", foo, name="foo")
    ]

Carlton Gibson [mentions this](https://youtu.be/_3oGI4RC52s?t=315) in is talk at DjangoCon Europe 2023, and why this means that he often puts view code in the same file as his URLs.

But why this disconnect? Other frameworks, like Flask and FastAPI use a rather simple "pattern" using a decorator which puts the URL information where the view is defined:

    :::python
    # flask_example.py
    from flask import Flask

    app = Flask(__name__)

    @app.route("foo/")
    def foo():
        return "bar"

Just by looking at the decorator tied to the `foo` function we know that we can access it via `foo/`. 

Another pitfall due to this disconnect is that there is no guarantee that a view has a URL pointing at it.

Can we apply the same pattern to Django? Yes we can!

# Introducing django-view-decorator!

`django-view-decorator` is my attempt to implement a decorator which can be used to apply this pattern to Django views.

The project is very much under development, and it is currently in the "research and development" phase. But it does work!

## Basics

First we setup our project URLconf to include URLs from `django-view-decorator`:

    :::python

    # project/urls.py (this is what we point the ROOT_URLCONF setting at)
    from django.urls import path

    from django_view_decorator import include_view_urls

    urlpatterns = [
        path("", include_view_urls()),
    ]

Then we can use the `view` decorator like so (we dive deeper in what the decorator does later on):

    :::python
    # foos/views.py
    from django_view_decorator import view

    @view(paths="/foo/", name="foo")
    def foo(request: HttpRequest) -> HttpResponse:
        return "bar"

We now have information about how the view is to be accessed right there next to the view itself.

Even class-based views are supported:

    ::python
    @view(paths="/foo/", name="foo-list")
    class FooList(ListView):
        model = Foo

## More advanced usage

Multiple URLs can point at the same view, and you might have noticed that the argument is the plural `paths`. This is because we can pass a list of paths which point at the same view. Like so:

    :::python
    @view(
        paths=[
            "/foo/",
            "/foo/<int:id>/",
        ], 
        name="foo",
        namespace="foos",
    )
    def foo(request: HttpRequest, id: int | None = None) -> HttpResponse:
        foos = Foo.objects.all()

        if id:
            context = {"foo": get_object_or_404(foos, id=id)}
            template_name = "foo_detail.html"
        else:
            context = {"foos": foos}
            template_name = "foo_list.html"

        return render(
            request,
            template_name=template_name,
            context=context
        )

Looking at the view we can grok that it is exposed on two paths, under the `foos` namespace, one which lists all `Foo` objects and one which given an integer gives us the detail for a single `Foo`. That's pretty powerful if you ask me!

If we want different names for each path we can simply apply the decorator multiple times:

    :::python
    @view(
        paths="/foo/",
        name="foo_list",
        namespace="foos",
    )
    @view(
        paths="/foo/<int:id>/",
        name="foo_detail",
        namespace="foos",
    )
    def foo(request: HttpRequest, id: int | None = None) -> HttpResponse:
        ...

## Behind the scenes

`django-view-decorator` works by having a registry in which all views and their URL information is stored. In fact the mechanism to do most of this work is the same as is used by `django.contrib.admin`. The `@view` decorator is quite similar to the well-known `@admin.register` decorator.

Here is a step-by-step for what is going on:

1. When Django starts and the app registry is ready, the `ready` method of `django_view_decorator.apps.ViewDecoratorAppConf` gets run.
2. The `ready` method calls `autodiscover_modules` from `django.utils.module_loading`. This imports `views.py` files from all apps in `INSTALLED_APPS`. The admin does the same thing, it just imports `admin.py` files.
3. By importing a `views.py` file we run all `@view()` invocations
4. In `view` decorator we gather information provided as arguments to the decorator and store this in a registry which is located at `django_view_decorator.apps.ViewRegistry`.
5. We can now use `ViewRegistry.urlpatterns()` to get the `urlpatterns` for all registrered views.

There are of course some small "buts and ifs" sprinkled around, but by and large this is how the whole thing works.

## Namespaces and the power of factories

In our more advanced example you might have noticed the `namespace="foos"`, which probably is going to quite tedious to repeat over and over again. One of the nice things about Django URLconfs is that we get namespacing by using the `include` function.

This is where the aptly named `namespaced_decorator_factory` comes into the picture. Let us look at an example:

    :::python
    # foo/views.py
    from django_view_decorator import namespaced_decorator_factory

    foo_view = namespaced_decorator_factory(
        namespace="foos", 
        base_path="foos/",
    )

    @foo_view(paths="", name="list")
    def foo_list(request: HttpRequest) -> HttpResponse:
        return "foo list"

    @foo_view(paths="<int:id>", name="detail")
    def foo_detail(request: HttpRequest, id: int) -> HttpResponse:
        return "foo detail"


By calling `namespaced_decorator_factory` we get a specialised decorator for our namespace and we can even provide it with a path which will be prepended to all URLs registered using it.

This opens up a quite nifty possibility of injecting URLs into a namespace from anywhere. For example:

    :::python
    # app_1/views.py
    app_1_view = namespaced_decorator_factory(
        namespace="app_1", 
        base_path="app_1/",
    )

    # app_2/views.py
    from app_1.views import app_1_view

    @app_1_view(
        paths="my-custom-view/",
        name="custom-view"
    )
    def custom_view(request: HttpRequest) -> HttpResponse:
        return "I'm in another namespace"


Now we can treat `custom_view` as if it was a part of the `app_1` namespace. Ie. `reverse("app_1:custom-view")` would give us `app_1/my-custom-view/`. Neat!

### Namespaces for AppConfigs

In the process of writing this blog post and trying to figure out how all this could be implemented into Django, I came up with the idea to leverage the applications framework in Django to get a namespaced decorator for a given Django app.

    :::python
    # foos/apps.py

    from django_view_decorator import AppConfig

    class FoosAppConfig(AppConfig):
        default_auto_field = "django.db.models.BigAutoField"
        name = "foos"
        namespace = "foos"  # defaults to name from above
        base_path = "foos"

    view = FoosAppConfig.get_view_decorator()

    # foos/views.py

    from .apps import view

    @view(paths="foo/", name="foo")
    def foo(request: HttpRequest) -> HttpResponse:
        return HttpResponse("foo")


I'm quite keen on this idea and I feel this might be an entry into introducing this pattern into Django.

Newcomers would learn that to hook up views to URLs they register their views into the app which the view belongs to - very much like how Flask does it with `@app`.

## The path to Django core

So, as I wrote initially, I have a mission to try to get this pattern into Django core. This is not going to be an easy feat. 

To cite Carlton Gibson from the previously mentioned talk:

> Put it in a third party package. See if there is community interest. Then maybe it gets merged to the core if there is. 

So this is what I'm doing. 

1. Write the third party package - check!
2. Write a blog post - check!
3. Gather community interest - ongoing
4. Merge!

## What do you think?

So now I'm throwing the ball to the Django community!

What do you think? Should we just keep views and URLs separate or am I on to something? Does my solution have any major downsides which I have been blind to? Is there any missed opportunities in my implementation that would make it sing even more?

Come discuss on the Django forum in this dedicated thread:

<https://forum.djangoproject.com/>