We’ll come to know about middlewares in django but first, let’s understand what is a middleware.

What is a middleware?

Middleware is a software that acts as a bridge between an operating system and applications running on it. It provides common services and capabilities to applications outside what’s available from the operating system.

What is a django middleware?

• A middleware is a component that sits between the Django view and the Django template.
• It is a framework that allows you to hook into Django’s request/response processing at various points.
• It’s a lightweight, low-level, plug-in system for globally altering Django’s input or output.
• It can be used to modify the incoming request before it reaches the view, or the outgoing response before it reaches the browser.
• It can also be used to execute some code that doesn’t fit neatly into any of your existing views or models.

How middleware works in django?

• When a request comes to the server, it is first processed by the middleware.
• The middleware then passes the request to the view.
• The view processes the request and returns a response.
• The middleware then processes the response and returns it to the browser.

Ordering and layering of middlewares

• Django middlewares are executed in the order they are defined in the MIDDLEWARE setting.
• The order of middlewares is important because each middleware can modify the request and response.
• The order of middlewares is also important because some middlewares depend on the output of other middlewares. For example, the SessionMiddleware depends on the AuthenticationMiddleware to populate the request.user attribute.
• Middlewares are run in reverse order when the response is returned to the browser.
• The SecurityMiddleware is the first middleware in the list of middlewares. It is the first middleware to process the request and the last middleware to process the response.

How to write a custom middleware in pre-Django 1.10-style?

• In pre-Django 1.10-style, we have to write a middleware class that implements the process_request() and process_response() methods.
• You need to inherit the MiddlewareMixin class to make sure that your middleware is compatible with both pre-Django 1.10-style and post-Django 1.10-style.
• The process_request() method is called before the view is called.
• The process_response() method is called after the view is called.
• The process_exception() method is called if an exception occurs in the view. This method is optional.
• The process_template_response() method is called after the view is called and the response is rendered. This method is optional.
• The process_view() method is called just before Django calls the view.. This method is optional.

Example:

from django.utils.deprecation import MiddlewareMixin


class MyMiddleware(MiddlewareMixin):
    def process_request(self, request):
        print("process_request() is called")

    def process_response(self, request, response):
        print("process_response() is called")
        return response

    def process_exception(self, request, exception):
        print("process_exception() is called")

    def process_template_response(self, request, response):
        print("process_template_response() is called")
        return response

    def process_view(self, request, view_func, view_args, view_kwargs):
        print("process_view() is called")

How to write a custom middleware as a function?

• In Django 1.10+, we can write a middleware as a function.
• The function takes a get_response argument and returns a callable.
• The function is called once when the server starts.
• The callable is called for every request.
• The callable must return a HttpResponse object.
• The get_response argument is a callable that takes a request and returns a response.
• The get_response argument is used to call the next middleware in the chain.
• The get_response argument is used to call the view if the middleware is the last one in the chain.
• The get_response argument is used to return the response if the middleware is the first one in the chain.

Example:

def my_middleware(get_response):
    def middleware(request):
        print("middleware() is called")
        # Code to be executed for each request before
        # the view (and later middleware) are called.
        response = get_response(request)
        print("middleware() is called")
        # Code to be executed for each request/response after
        # the view is called.
        return response

    return middleware

How to write a custom middleware as a class?

• In Django 1.10+, we can write a middleware as a class.
• The __init__() method is called once when the server starts, and it takes a get_response argument.
• The __init__() method must store the get_response argument as an instance attribute and not call it.
• The class must implement the __call__() method.
• The __call__() method is called for every request and must return a HttpResponse object.
• The __call__() method must call the get_response argument and return the response.
• The __call__() method can modify the request before calling the get_response argument and modify the response before returning it.
• The __call__() method can raise an exception to stop the request/response chain.

Example:

class MyMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
        print("init() is called")

    def __call__(self, request):
        print("call() is called")
        # Code to be executed for each request before
        # the view (and later middleware) are called.
        response = self.get_response(request)
        print("call() is called")
        # Code to be executed for each request/response after
        # the view is called.
        return response

A real-world example of a custom middleware

• In this example, we will write a middleware that will log the request and response.
• The middleware will log the request and response only if the DEBUG setting is True.

from django.conf import settings


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

    def __call__(self, request):
        if settings.DEBUG:
            print("Request: ", request)
        response = self.get_response(request)
        if settings.DEBUG:
            print("Response: ", response)
        return response

So that’s how you can write a custom middleware in Django. I hope you found this tutorial useful. Thanks for reading!