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.
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:
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
The 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
exception middleware.
Middleware can live anywhere on your Python path.
__init__(get_response)¶Middleware factories must accept a get_response argument. You can also
initialize some global state for the middleware. Keep in mind a couple of
caveats:
get_response argument,
so you can’t define __init__() as requiring any other arguments.__call__() method which is called once per request,
__init__() is called only once, when the Web server starts.It’s sometimes useful to determine at startup time whether a piece of
middleware should be used. In these cases, your middleware’s __init__()
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 DEBUG is True.
To activate a middleware component, add it to the MIDDLEWARE list in
your Django settings.
In 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 django-admin
startproject:
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',
]
A Django installation doesn’t require any middleware — MIDDLEWARE
can be empty, if you’d like — but it’s strongly suggested that you at least use
CommonMiddleware.
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
SessionMiddleware. See
Middleware ordering for some common hints about ordering of Django
middleware classes.
During the request phase, before calling the view, Django applies middleware in
the order it’s defined in MIDDLEWARE, top-down.
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
out.
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.
Besides the basic request/response middleware pattern described earlier, you can add three other special methods to class-based middleware:
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 the appropriate view; it’ll apply response middleware to that
HttpResponse and return the result.
Note
Accessing request.POST inside
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.
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_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 process_exception methods of the middleware classes above that
middleware won’t be called at all.
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().
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)
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
calling its 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 HttpResponse
object with a status_code of 404.
django.utils.deprecation.MiddlewareMixin¶Django provides django.utils.deprecation.MiddlewareMixin to ease creating
middleware classes that are compatible with both MIDDLEWARE and the
old 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.
The __call__() method:
self.process_request(request) (if defined).self.get_response(request) to get the response from later
middleware and the view.self.process_response(request, response) (if defined).If used with MIDDLEWARE_CLASSES, the __call__() method will
never be used; Django calls process_request() and process_response()
directly.
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 and
MIDDLEWARE_CLASSES:
MIDDLEWARE_CLASSES, every middleware will always have its
process_response method called, even if an earlier middleware
short-circuited by returning a response from its process_request
method. Under 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 MIDDLEWARE will see the
response.MIDDLEWARE_CLASSES, process_exception is applied to
exceptions raised from a middleware process_request method. Under
MIDDLEWARE, process_exception applies only to exceptions
raised from the view (or from the render method 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_response method raises
an exception, the process_response methods of all earlier middleware are
skipped and a 500 Internal Server Error HTTP response is always
returned (even if the exception raised was e.g. an
Http404). Under 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.Oct 31, 2018