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,
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.
Writing your own middleware¶
A middleware factory is a callable that takes a
get_response callable and
returns a middleware. A middleware is a callable that takes a request and
returns a response, just like a view.
A middleware can be written as a function that looks like this:
def simple_middleware(get_response): # One-time configuration and initialization. def middleware(request): # Code to be executed for each request before # the view (and later middleware) are called. response = get_response(request) # Code to be executed for each request/response after # the view is called. return response return middleware
Or it can be written as a class whose instances are callable, like this:
class SimpleMiddleware(object): def __init__(self, get_response): self.get_response = get_response # One-time configuration and initialization. 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 is called. return response
get_response callable provided by Django might be the actual view (if
this is the last listed middleware) or it might be the next middleware in the
chain. The current middleware doesn’t need to know or care what exactly it is,
just that it represents whatever comes next.
The above is a slight simplification – the
get_response callable for the
last middleware in the chain won’t be the actual view but rather a wrapper
method from the handler which takes care of applying view middleware, calling the view with appropriate URL arguments, and
applying template-response and
Middleware can live anywhere on your Python path.
Middleware factories must accept a
get_response argument. You can also
initialize some global state for the middleware. Keep in mind a couple of
- Django initializes your middleware with only the
get_responseargument, so you can’t define
__init__()as requiring any other arguments.
- Unlike the
__call__()method which is called once per request,
__init__()is called only once, when the Web server starts.
Marking middleware as unused¶
It’s sometimes useful to determine at startup time whether a piece of
middleware should be used. In these cases, your middleware’s
method may raise
MiddlewareNotUsed. Django will
then remove that middleware from the middleware process and log a debug message
to the django.request logger when
To activate a middleware component, add it to the
MIDDLEWARE list in
your Django settings.
MIDDLEWARE, each middleware component is represented by a string:
the full Python path to the middleware factory’s class or function name. For
example, here’s the default value created by
MIDDLEWARE = [ 'django.middleware.security.SecurityMiddleware', 'django.contrib.sessions.middleware.SessionMiddleware', 'django.middleware.common.CommonMiddleware', 'django.middleware.csrf.CsrfViewMiddleware', 'django.contrib.auth.middleware.AuthenticationMiddleware', 'django.contrib.messages.middleware.MessageMiddleware', 'django.middleware.clickjacking.XFrameOptionsMiddleware', ]
The order in
MIDDLEWARE matters because a middleware can depend on
other middleware. For instance,
AuthenticationMiddleware stores the
authenticated user in the session; therefore, it must run after
Middleware ordering for some common hints about ordering of Django
Middleware order and layering¶
During the request phase, before calling the view, Django applies middleware in
the order it’s defined in
You can think of it like an onion: each middleware class is a “layer” that
wraps the view, which is in the core of the onion. If the request passes
through all the layers of the onion (each one calls
get_response to pass
the request in to the next layer), all the way to the view at the core, the
response will then pass through every layer (in reverse order) on the way back
If one of the layers decides to short-circuit and return a response without
ever calling its
get_response, none of the layers of the onion inside that
layer (including the view) will see the request or the response. The response
will only return through the same layers that the request passed in through.
Other middleware hooks¶
Besides the basic request/response middleware pattern described earlier, you can add three other special methods to class-based middleware:
process_view(request, view_func, view_args, view_kwargs)¶
request is an
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_kwargs include the first view argument
process_view() is called just before Django calls the view.
It should return either
None or an
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 the appropriate view; it’ll apply response middleware to that
HttpResponse and return the result.
middleware before the view runs or in
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.
CsrfViewMiddleware class can be
considered an exception, as it provides the
csrf_protect() decorators which allow
views to explicitly control at what point the CSRF validation should occur.
request is an
exception is an
Exception object raised by the view function.
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
process_exception. If an exception middleware returns a response,
process_exception methods of the middleware classes above that
middleware won’t be called at all.
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.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
Dealing with streaming responses¶
StreamingHttpResponse does not have a
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)
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)
Django automatically converts exceptions raised by the view or by middleware into an appropriate HTTP response with an error status code. Certain exceptions are converted to 4xx status codes, while an unknown exception is converted to a 500 status code.
This conversion takes place before and after each middleware (you can think of
it as the thin film in between each layer of the onion), so that every
middleware can always rely on getting some kind of HTTP response back from
get_response callable. Middleware don’t need to worry about
wrapping their call to
get_response in a
try/except and handling an
exception that might have been raised by a later middleware or the view. Even
if the very next middleware in the chain raises an
Http404 exception, for example, your middleware won’t see
that exception; instead it will get an
object with a
status_code of 404.
Upgrading pre-Django 1.10-style middleware¶
django.utils.deprecation.MiddlewareMixin to ease creating
middleware classes that are compatible with both
MIDDLEWARE and the
MIDDLEWARE_CLASSES. All middleware classes included with Django
are compatible with both settings.
The mixin provides an
__init__() method that accepts an optional
get_response argument and stores it in
self.get_response(request)to get the response from later middleware and the view.
self.process_response(request, response)(if defined).
- Returns the response.
If used with
__call__() method will
never be used; Django calls
In most cases, inheriting from this mixin will be sufficient to make an old-style middleware compatible with the new system with sufficient backwards-compatibility. The new short-circuiting semantics will be harmless or even beneficial to the existing middleware. In a few cases, a middleware class may need some changes to adjust to the new semantics.
These are the behavioral differences between using
MIDDLEWARE_CLASSES, every middleware will always have its
process_responsemethod called, even if an earlier middleware short-circuited by returning a response from its
MIDDLEWARE, middleware behaves more like an onion: the layers that a response goes through on the way out are the same layers that saw the request on the way in. If a middleware short-circuits, only that middleware and the ones before it in
MIDDLEWAREwill see the response.
process_exceptionis applied to exceptions raised from a middleware
process_exceptionapplies only to exceptions raised from the view (or from the
rendermethod of a
TemplateResponse). Exceptions raised from a middleware are converted to the appropriate HTTP response and then passed to the next middleware.
MIDDLEWARE_CLASSES, if a
process_responsemethod raises an exception, the
process_responsemethods of all earlier middleware are skipped and a
500 Internal Server ErrorHTTP response is always returned (even if the exception raised was e.g. an
MIDDLEWARE, an exception raised from a middleware will immediately be converted to the appropriate HTTP response, and then the next middleware in line will see that response. Middleware are never skipped due to a middleware raising an exception.