Creating Your First Django App

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Django Applications(DOCUMENTATION)

In Django, a project is a collection of configurations and apps. An app is a web application that does something—a blog, a poll system, a user authentication system. A project can contain multiple apps.

Projects vs Apps

mysite/                 # PROJECT - configuration container
├── manage.py
├── mysite/             # Project settings
│   ├── settings.py
│   └── urls.py
├── blog/               # APP - does something specific
│   ├── models.py
│   ├── views.py
│   └── ...
└── polls/              # Another APP
    ├── models.py
    └── ...

Creating an App

Let's create a simple polls application:

bash
python manage.py startapp polls

This creates:

polls/
├── __init__.py
├── admin.py          # Admin interface configuration
├── apps.py           # App configuration
├── migrations/       # Database migrations
│   └── __init__.py
├── models.py         # Data models
├── tests.py          # Tests
└── views.py          # View functions

Register the App

Tell Django about your new app in settings.py:

python
# mysite/settings.py

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'polls',  # Add your app here
]

Creating Your First View

Views handle HTTP requests and return responses. Let's create a simple view:

python
# polls/views.py
from django.http import HttpResponse


def index(request):
    return HttpResponse("Hello, world! This is the polls index.")

Mapping URLs to Views

Create a urls.py file in your app:

python
# polls/urls.py
from django.urls import path

from . import views

urlpatterns = [
    path('', views.index, name='index'),
]

Include the app URLs in the project's main urls.py:

python
# mysite/urls.py
from django.contrib import admin
from django.urls import include, path

urlpatterns = [
    path('polls/', include('polls.urls')),
    path('admin/', admin.site.urls),
]

Test Your App

Start the server and visit http://127.0.0.1:8000/polls/:

bash
python manage.py runserver

You should see "Hello, world! This is the polls index."

Understanding the Request Flow

  1. User visits /polls/
  2. Django looks at mysite/urls.py
  3. Matches polls/ → includes polls/urls.py
  4. Matches empty string '' → calls views.index
  5. index() receives the request and returns a response
  6. Browser displays the response
http://127.0.0.1:8000/polls/
          │
          ▼
    mysite/urls.py
    path('polls/', include('polls.urls'))
          │
          ▼
    polls/urls.py
    path('', views.index)
          │
          ▼
    polls/views.py
    def index(request): return HttpResponse(...)

Common Pitfalls

  • Forgetting to register the app in INSTALLED_APPS: Your app's models, templates, and static files won't be discovered until you add it to settings.py.
  • Not creating urls.py in the app: Django does not auto-create this file. You must create it manually and include it in the project's urls.py.
  • Missing the trailing slash in URL patterns: Django expects path('polls/', include('polls.urls')) with a trailing slash by default.

Best Practices

  • Keep apps focused: Each app should handle one specific piece of functionality (e.g., polls, blog, accounts).
  • Use include() for app URLs: Always use include('app.urls') in the project's urls.py rather than defining all URLs in one file.
  • Name your URL patterns: Always add name='...' to path() calls so you can reference URLs by name in templates and views.

Summary

  • A Django project contains configuration; an app contains functionality
  • Create an app with python manage.py startapp appname
  • Register the app in INSTALLED_APPS in settings.py
  • Each app has its own models.py, views.py, admin.py, and tests.py
  • Use include() to wire app URLs into the project's URL configuration

Code Examples

python
# polls/views.py
from django.http import HttpResponse

def index(request):
    return HttpResponse("Hello, world! This is the polls index.")

# polls/urls.py
from django.urls import path
from . import views

urlpatterns = [
    path('', views.index, name='index'),
]
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