1. Introduction

Goal of the Tutorial

This tutorial will guide you on how to effectively manage middleware execution order in Django. Middleware is a critical component in Django as it allows the developer to process request and response globally before reaching the view and after leaving the view respectively.

Learning Outcomes

By the end of this tutorial, you will be able to:
- Understand what middleware is and why the order of execution is important
- Know how to manage the order of middleware in Django
- Write middleware and add it to your Django project

Prerequisites

Before starting this tutorial, you should have a basic understanding of Python and Django. Familiarity with Django's request/response process will be beneficial.

2. Step-by-Step Guide

Concept of Middleware

In Django, middleware is a series of hooks into Django’s request/response processing. It's a way to pre-process requests and post-process responses. Middleware classes are executed in a specific order defined in the MIDDLEWARE setting in your Django project's settings module.

Execution Order Importance

The order in which middleware are processed is significant. Middleware classes specified first are run last when the response returns from the view. However, they're run first upon request before the view. In other words, the order is top-down for requests and bottom-up for responses.

3. Code Examples

Example 1: Basic Middleware

class SimpleMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        # Code to be executed for each request before
        # the view (and later middleware) are called.

        response = self.get_response(request)

        # Code to be executed for each request/response after
        # the view has been called.

        return response

This code defines a basic middleware. Before the view is called, code above self.get_response(request) is executed. After the view is called, code below is executed.

Example 2: Middleware Order

In your settings file, you might have:

MIDDLEWARE = [
    'django.middleware.security.SecurityMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
]

The CommonMiddleware will be processed before the AuthenticationMiddleware on a request, but after on a response.

4. Summary

• Middleware in Django is a series of hooks into Django’s request/response process.
• The order middleware is defined in MIDDLEWARE setting determines the order of execution.
• Middleware classes specified first are run last upon response.

5. Practice Exercises

1. Exercise 1: Create a middleware that prints "Before view" and "After view" before and after the view respectively.
2. Exercise 2: Add the middleware created in Exercise 1 to your Django project and change its order in the MIDDLEWARE setting. Observe any changes in the execution order.
3. Exercise 3: Create a middleware that modifies the request object by adding an attribute to it. Add this middleware to your Django project and access the added attribute in one of your views.

Solution and explanation for Exercise 1

class PrintMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        print("Before view")
        response = self.get_response(request)
        print("After view")
        return response

This middleware prints "Before view" before the view is called and "After view" after the view is called.

The solution and explanation for the other exercises are left as an exercise for the reader. You can try them out and check your understanding of the concept.

6. Next Steps

• Learn about the different types of built-in Django middleware and their uses.
• Explore how to write middleware that modifies the request or response objects.
• Beyond Django, learn about how middleware works in other web frameworks.

7. Additional Resources

• Django official documentation on middleware: https://docs.djangoproject.com/en/3.2/topics/http/middleware/
• Django for Professionals by William S. Vincent
• Django for Beginners by Andrew Pinkham