# Django 1.8.18.dev20170123152445 documentation

# Middleware

Middleware is a framework of hooks into Django’s request/response processing. It’s a light, low-level “plugin” system for globally altering Django’s input or output.

Each middleware component is responsible for doing some specific function. For example, Django includes a middleware component, `AuthenticationMiddleware`, that associates users with requests using sessions.

This document explains how middleware works, how you activate middleware, and how to write your own middleware. Django ships with some built-in middleware you can use right out of the box. They’re documented in the built-in middleware reference.

## Activating middleware

To activate a middleware component, add it to the `MIDDLEWARE_CLASSES` tuple in your Django settings.

In `MIDDLEWARE_CLASSES`, each middleware component is represented by a string: the full Python path to the middleware’s class name. For example, here’s the default value created by `django-admin startproject`:

    MIDDLEWARE_CLASSES = (
        'django.middleware.security.SecurityMiddleware',
        'django.contrib.sessions.middleware.SessionMiddleware',
        'django.middleware.common.CommonMiddleware',
        'django.middleware.csrf.CsrfViewMiddleware',
        'django.contrib.auth.middleware.AuthenticationMiddleware',
        'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
        'django.contrib.messages.middleware.MessageMiddleware',
        'django.middleware.clickjacking.XFrameOptionsMiddleware',
    )

A Django installation doesn’t require any middleware — `MIDDLEWARE_CLASSES` can be empty, if you’d like — but it’s strongly suggested that you at least use `CommonMiddleware`.

The order in `MIDDLEWARE_CLASSES` matters because a middleware can depend on other middleware. For instance, `AuthenticationMiddleware` stores the authenticated user in the session; therefore, it must run after `SessionMiddleware`. See Middleware ordering for some common hints about ordering of Django middleware classes.

## Hooks and application order

During the request phase, before calling the view, Django applies middleware in the order it’s defined in `MIDDLEWARE_CLASSES`, top-down. Two hooks are available:

- `process_request()`
- `process_view()`

During the response phase, after calling the view, middleware are applied in reverse order, from the bottom up. Three hooks are available:

- `process_exception()` (only if the view raised an exception)
- `process_template_response()` (only for template responses)
- `process_response()`

If you prefer, you can also think of it like an onion: each middleware class is a “layer” that wraps the view.

The behavior of each hook is described below.

## Writing your own middleware

Writing your own middleware is easy. Each middleware component is a single Python class that defines one or more of the following methods:

### `process_request`

`process_request`(request)

`request` is an `HttpRequest` object.

`process_request()` is called on each request, before Django decides which view to execute.

It should return either `None` or an `HttpResponse` object. If it returns `None`, Django will continue processing this request, executing any other `process_request()` middleware, then, `process_view()` middleware, and finally, the appropriate view. If it returns an `HttpResponse` object, Django won’t bother calling any other request, view or exception middleware, or the appropriate view; it’ll apply response middleware to that `HttpResponse`, and return the result.

### `process_view`

`process_view`(request, view_func, view_args, view_kwargs)

`request` is an `HttpRequest` object. `view_func` is the Python function that Django is about to use. (It’s the actual function object, not the name of the function as a string.) `view_args` is a list of positional arguments that will be passed to the view, and `view_kwargs` is a dictionary of keyword arguments that will be passed to the view. Neither `view_args` nor `view_kwargs` include the first view argument (`request`).

`process_view()` is called just before Django calls the view.

It should return either `None` or an `HttpResponse` object. If it returns `None`, Django will continue processing this request, executing any other `process_view()` middleware and, then, the appropriate view. If it returns an `HttpResponse` object, Django won’t bother calling any other view or exception middleware, or the appropriate view; it’ll apply response middleware to that `HttpResponse`, and return the result.

Note

Accessing `request.POST` or `request.REQUEST` inside middleware from `process_request` or `process_view` will prevent any view running after the middleware from being able to modify the upload handlers for the request, and should normally be avoided.

The `CsrfViewMiddleware` class can be considered an exception, as it provides the `csrf_exempt()` and `csrf_protect()` decorators which allow views to explicitly control at what point the CSRF validation should occur.

### `process_template_response`

`process_template_response`(request, response)

`request` is an `HttpRequest` object. `response` is the `TemplateResponse` object (or equivalent) returned by a Django view or by a middleware.

`process_template_response()` is called just after the view has finished executing, if the response instance has a `render()` method, indicating that it is a `TemplateResponse` or equivalent.

It must return a response object that implements a `render` method. It could alter the given `response` by changing `response.template_name` and `response.context_data`, or it could create and return a brand-new `TemplateResponse` or equivalent.

You don’t need to explicitly render responses – responses will be automatically rendered once all template response middleware has been called.

Middleware are run in reverse order during the response phase, which includes `process_template_response()`.

### `process_response`

`process_response`(request, response)

`request` is an `HttpRequest` object. `response` is the `HttpResponse` or `StreamingHttpResponse` object returned by a Django view or by a middleware.

`process_response()` is called on all responses before they’re returned to the browser.

It must return an `HttpResponse` or `StreamingHttpResponse` object. It could alter the given `response`, or it could create and return a brand-new `HttpResponse` or `StreamingHttpResponse`.

Unlike the `process_request()` and `process_view()` methods, the `process_response()` method is always called, even if the `process_request()` and `process_view()` methods of the same middleware class were skipped (because an earlier middleware method returned an `HttpResponse`). In particular, this means that your `process_response()` method cannot rely on setup done in `process_request()`.

Finally, remember that during the response phase, middleware are applied in reverse order, from the bottom up. This means classes defined at the end of `MIDDLEWARE_CLASSES` will be run first.

#### Dealing with streaming responses

Unlike `HttpResponse`, `StreamingHttpResponse` does not have a `content` attribute. As a result, middleware can no longer assume that all responses will have a `content` attribute. If they need access to the content, they must test for streaming responses and adjust their behavior accordingly:

    if response.streaming:
        response.streaming_content = wrap_streaming_content(response.streaming_content)
    else:
        response.content = alter_content(response.content)

Note

`streaming_content` should be assumed to be too large to hold in memory. Response middleware may wrap it in a new generator, but must not consume it. Wrapping is typically implemented as follows:

    def wrap_streaming_content(content):
        for chunk in content:
            yield alter_content(chunk)

### `process_exception`

`process_exception`(request, exception)

`request` is an `HttpRequest` object. `exception` is an `Exception` object raised by the view function.

Django calls `process_exception()` when a view raises an exception. `process_exception()` should return either `None` or an `HttpResponse` object. If it returns an `HttpResponse` object, the template response and response middleware will be applied, and the resulting response returned to the browser. Otherwise, default exception handling kicks in.

Again, middleware are run in reverse order during the response phase, which includes `process_exception`. If an exception middleware returns a response, the middleware classes above that middleware will not be called at all.

### `__init__`

Most middleware classes won’t need an initializer since middleware classes are essentially placeholders for the `process_*` methods. If you do need some global state you may use `__init__` to set up. However, keep in mind a couple of caveats:

- Django initializes your middleware without any arguments, so you can’t define `__init__` as requiring any arguments.
- Unlike the `process_*` methods which get called once per request, `__init__` gets called only once, when the Web server responds to the first request.

#### Marking middleware as unused

It’s sometimes useful to determine at run-time whether a piece of middleware should be used. In these cases, your middleware’s `__init__` method may raise `django.core.exceptions.MiddlewareNotUsed`. Django will then remove that piece of middleware from the middleware process and a debug message will be logged to the `django.request` logger when `DEBUG` is set to `True`.

Previously, `MiddlewareNotUsed` exceptions weren’t logged.

### Guidelines

- Middleware classes don’t have to subclass anything.
- The middleware class can live anywhere on your Python path. All Django cares about is that the `MIDDLEWARE_CLASSES` setting includes the path to it.
- Feel free to look at Django’s available middleware for examples.
- If you write a middleware component that you think would be useful to other people, contribute to the community! Let us know, and we’ll consider adding it to Django.