How to solve "Page not found (404)" error in Django?

Solving 'Page not found (404)' Errors in Django Applications

A comprehensive guide to diagnosing and fixing 404 errors in your Django projects, covering common causes and solutions.

The 'Page not found (404)' error is one of the most common issues encountered when developing or deploying Django applications. It indicates that the server could not find a resource matching the requested URL. While frustrating, these errors are usually straightforward to diagnose and fix once you understand Django's URL routing mechanism. This article will walk you through the typical causes of 404s in Django and provide systematic solutions.

Understanding Django's URL Dispatcher

Django uses a URL dispatcher to match incoming HTTP requests to the appropriate view functions. This process involves urlpatterns defined in your project's urls.py and potentially in individual app's urls.py files. When a request comes in, Django iterates through these patterns in order until a match is found. If no pattern matches the requested URL, a 404 error is returned.

flowchart TD
    A[User Request URL] --> B{Django URL Dispatcher}
    B --> C{Iterate through urlpatterns}
    C --> D{URL Pattern Match?}
    D -- Yes --> E[Execute View Function]
    D -- No --> F{All Patterns Checked?}
    F -- Yes --> G[Return 404 Error]
    F -- No --> C

Django URL Dispatcher Flow

Common Causes and Solutions for 404 Errors

Most 404 errors stem from misconfigurations in your URL patterns or issues with how they are included. Let's explore the primary culprits and how to address them.

1. Incorrect URL Patterns or Typos

The most frequent cause of a 404 is a simple typo in the URL requested by the user or a mismatch between the URL defined in urlpatterns and the one being accessed. Ensure that the URL you are trying to reach exactly matches one of your defined patterns.

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

urlpatterns = [
    path('admin/', admin.site.urls),
    path('blog/', include('blog.urls')), # Correct: 'blog/'
    path('about-us/', views.about_page, name='about'),
]

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

urlpatterns = [
    path('', views.post_list, name='post_list'),
    path('<int:pk>/', views.post_detail, name='post_detail'),
]

Example of urlpatterns in project/urls.py and blog/urls.py

If you define path('blog/', include('blog.urls')) and try to access /blogs/ (with an 's'), you will get a 404. Similarly, if your blog/urls.py has path('posts/', views.post_list) and you try /blog/, it won't match. Pay close attention to trailing slashes as well; Django's APPEND_SLASH setting (which is True by default) attempts to redirect to a URL with a trailing slash if the non-slash version doesn't match, but this isn't always foolproof.

2. Missing include() for App URLs

When you create a new Django app, you typically define its URL patterns in an urls.py file within that app. For these patterns to be recognized by the project, you must include() them in your project's main urls.py file.

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

urlpatterns = [
    path('admin/', admin.site.urls),
    # Correct: Include your app's URLs
    path('myapp/', include('myapp.urls')), 
    # Incorrect: If 'myapp.urls' is not included, any URL starting with 'myapp/' will 404
]

Ensuring app URLs are included in the project's urls.py

3. Incorrect View Function Reference

Sometimes, the URL pattern itself is correct, but it points to a view function that doesn't exist or is incorrectly imported. This usually results in an ImportError or AttributeError rather than a 404, but it can manifest as a 404 if the server fails to start or reload correctly.

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

urlpatterns = [
    # Correct: views.my_view exists in myapp/views.py
    path('my-page/', views.my_view, name='my_page'),
    # Incorrect: If 'non_existent_view' is not defined in myapp/views.py
    # path('another-page/', views.non_existent_view, name='another_page'),
]

Verifying correct view function references

4. Regular Expression Issues (for re_path)

If you're using re_path for more complex URL patterns, incorrect regular expressions can lead to 404s. Regular expressions are powerful but can be tricky. Ensure your regex accurately captures the URLs you intend to match.

# myapp/urls.py
from django.urls import re_path
from . import views

urlpatterns = [
    # Matches /articles/2023/04/my-article-slug/
    re_path(r'^articles/(?P<year>[0-9]{4})/(?P<month>[0-9]{2})/(?P<slug>[\w-]+)/$', views.article_detail, name='article_detail'),
    # If you forget the '$' at the end, it might match /articles/2023/04/my-article-slug/extra-stuff
    # If you use 'd+' instead of '[0-9]{4}', it might match too many or too few digits
]

Careful use of regular expressions with re_path

5. Debugging 404s with DEBUG = True

When DEBUG = True in your settings.py, Django provides a helpful 404 debug page. This page lists all urlpatterns that Django tried to match, along with the requested URL. This is an invaluable tool for pinpointing exactly where the mismatch occurs.

# project/settings.py

DEBUG = True # Set to True during development

ALLOWED_HOSTS = ['127.0.0.1', 'localhost'] # Required when DEBUG = False, good practice even when True

Ensuring DEBUG = True for detailed 404 pages

6. Static and Media Files Not Served

If your 404s are specifically for static files (CSS, JS, images) or user-uploaded media files, the issue is likely with your static/media file configuration, not your URL patterns for views. In development, you need to configure Django to serve these files. In production, a web server like Nginx or Apache should handle them.

# project/settings.py

import os

STATIC_URL = '/static/'
STATIC_ROOT = os.path.join(BASE_DIR, 'staticfiles') # For production collection

MEDIA_URL = '/media/'
MEDIA_ROOT = os.path.join(BASE_DIR, 'media') # Where uploaded files are stored

# project/urls.py (for development only!)
from django.contrib import admin
from django.urls import path, include
from django.conf import settings
from django.conf.urls.static import static

urlpatterns = [
    path('admin/', admin.site.urls),
    # ... other URL patterns ...
]

if settings.DEBUG:
    urlpatterns += static(settings.STATIC_URL, document_root=settings.STATIC_ROOT)
    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)

Configuring static and media files for development

Remember to run python manage.py collectstatic to gather all static files into STATIC_ROOT when preparing for production. For production environments, never rely on Django to serve static or media files; configure your web server (e.g., Nginx) to do so directly.

7. Custom 404 Handlers

Django allows you to define a custom 404 view. If you've implemented one, ensure it's correctly configured in your urls.py and settings.py.

# project/settings.py

# ...

# Define your custom 404 handler view path
HANDLER404 = 'myproject.views.custom_404_view'

# myproject/views.py
from django.shortcuts import render

def custom_404_view(request, exception):
    return render(request, '404.html', {}, status=404)

Implementing a custom 404 handler

A misconfigured custom 404 handler can sometimes mask the underlying issue or even cause a 404 itself if the handler view or template is missing.

1. Step 1: Check Server Console Output

Always start by looking at your Django development server's console. It often prints detailed traceback information for 404s, including the URL requested and the patterns tried.

2. Step 2: Verify DEBUG = True (Development Only)

Ensure DEBUG = True in settings.py during development to get Django's detailed 404 page, which lists all urlpatterns.

3. Step 3: Inspect project/urls.py

Confirm that your app's urls.py is correctly include()d and that the base path for your app is accurate (e.g., path('myapp/', include('myapp.urls'))).

4. Step 4: Review app/urls.py

Check for typos in your URL patterns, ensure correct use of path converters (e.g., <int:pk>), and verify that view functions are correctly referenced and imported.

5. Step 5: Test URLs Manually

Try accessing the problematic URL directly in your browser. If it's a dynamic URL, try a few variations to see if any match.

6. Step 6: Check Static/Media Configuration

If 404s are for static or media files, review STATIC_URL, STATIC_ROOT, MEDIA_URL, and MEDIA_ROOT in settings.py, and ensure django.conf.urls.static.static is used in urls.py for development.