django中的中间件机制和执行顺序

from django.conf import settings
from django.contrib import auth
from django.contrib.auth import load_backend
from django.contrib.auth.backends import RemoteUserBackend
from django.core.exceptions import ImproperlyConfigured
from django.utils.deprecation import MiddlewareMixin
from django.utils.functional import SimpleLazyObject


def get_user(request):
    if not hasattr(request, ‘_cached_user‘):
        request._cached_user = auth.get_user(request)
    return request._cached_user


class AuthenticationMiddleware(MiddlewareMixin):
    def process_request(self, request):
        assert hasattr(request, ‘session‘), (
            "The Django authentication middleware requires session middleware "
            "to be installed. Edit your MIDDLEWARE%s setting to insert "
            "‘django.contrib.sessions.middleware.SessionMiddleware‘ before "
            "‘django.contrib.auth.middleware.AuthenticationMiddleware‘."
        ) % ("_CLASSES" if settings.MIDDLEWARE is None else "")
        request.user = SimpleLazyObject(lambda: get_user(request))


class RemoteUserMiddleware(MiddlewareMixin):
    """
    Middleware for utilizing Web-server-provided authentication.
    If request.user is not authenticated, then this middleware attempts to
    authenticate the username passed in the ``REMOTE_USER`` request header.
    If authentication is successful, the user is automatically logged in to
    persist the user in the session.
    The header used is configurable and defaults to ``REMOTE_USER``.  Subclass
    this class and change the ``header`` attribute if you need to use a
    different header.
    """

    # Name of request header to grab username from.  This will be the key as
    # used in the request.META dictionary, i.e. the normalization of headers to
    # all uppercase and the addition of "HTTP_" prefix apply.
    header = "REMOTE_USER"
    force_logout_if_no_header = True

    def process_request(self, request):
        # AuthenticationMiddleware is required so that request.user exists.
        if not hasattr(request, ‘user‘):
            raise ImproperlyConfigured(
                "The Django remote user auth middleware requires the"
                " authentication middleware to be installed.  Edit your"
                " MIDDLEWARE setting to insert"
                " ‘django.contrib.auth.middleware.AuthenticationMiddleware‘"
                " before the RemoteUserMiddleware class.")
        try:
            username = request.META[self.header]
        except KeyError:
            # If specified header doesn‘t exist then remove any existing
            # authenticated remote-user, or return (leaving request.user set to
            # AnonymousUser by the AuthenticationMiddleware).
            if self.force_logout_if_no_header and request.user.is_authenticated:
                self._remove_invalid_user(request)
            return
        # If the user is already authenticated and that user is the user we are
        # getting passed in the headers, then the correct user is already
        # persisted in the session and we don‘t need to continue.
        if request.user.is_authenticated:
            if request.user.get_username() == self.clean_username(username, request):
                return
            else:
                # An authenticated user is associated with the request, but
                # it does not match the authorized user in the header.
                self._remove_invalid_user(request)

        # We are seeing this user for the first time in this session, attempt
        # to authenticate the user.
        user = auth.authenticate(request, remote_user=username)
        if user:
            # User is valid.  Set request.user and persist user in the session
            # by logging the user in.
            request.user = user
            auth.login(request, user)

    def clean_username(self, username, request):
        """
        Allow the backend to clean the username, if the backend defines a
        clean_username method.
        """
        backend_str = request.session[auth.BACKEND_SESSION_KEY]
        backend = auth.load_backend(backend_str)
        try:
            username = backend.clean_username(username)
        except AttributeError:  # Backend has no clean_username method.
            pass
        return username

    def _remove_invalid_user(self, request):
        """
        Remove the current authenticated user in the request which is invalid
        but only if the user is authenticated via the RemoteUserBackend.
        """
        try:
            stored_backend = load_backend(request.session.get(auth.BACKEND_SESSION_KEY, ‘‘))
        except ImportError:
            # backend failed to load
            auth.logout(request)
        else:
            if isinstance(stored_backend, RemoteUserBackend):
                auth.logout(request)


class PersistentRemoteUserMiddleware(RemoteUserMiddleware):
    """
    Middleware for Web-server provided authentication on logon pages.
    Like RemoteUserMiddleware but keeps the user authenticated even if
    the header (``REMOTE_USER``) is not found in the request. Useful
    for setups when the external authentication via ``REMOTE_USER``
    is only expected to happen on some "logon" URL and the rest of
    the application wants to use Django‘s authentication mechanism.
    """
    force_logout_if_no_header = False

import re

from django.utils.cache import patch_vary_headers
from django.utils.deprecation import MiddlewareMixin
from django.utils.text import compress_sequence, compress_string

re_accepts_gzip = re.compile(r‘\bgzip\b‘)


class GZipMiddleware(MiddlewareMixin):
    """
    Compress content if the browser allows gzip compression.
    Set the Vary header accordingly, so that caches will base their storage
    on the Accept-Encoding header.
    """
    def process_response(self, request, response):
        # It‘s not worth attempting to compress really short responses.
        if not response.streaming and len(response.content) < 200:
            return response

        # Avoid gzipping if we‘ve already got a content-encoding.
        if response.has_header(‘Content-Encoding‘):
            return response

        patch_vary_headers(response, (‘Accept-Encoding‘,))

        ae = request.META.get(‘HTTP_ACCEPT_ENCODING‘, ‘‘)
        if not re_accepts_gzip.search(ae):
            return response

        if response.streaming:
            # Delete the `Content-Length` header for streaming content, because
            # we won‘t know the compressed size until we stream it.
            response.streaming_content = compress_sequence(response.streaming_content)
            del response[‘Content-Length‘]
        else:
            # Return the compressed content only if it‘s actually shorter.
            compressed_content = compress_string(response.content)
            if len(compressed_content) >= len(response.content):
                return response
            response.content = compressed_content
            response[‘Content-Length‘] = str(len(response.content))

        # If there is a strong ETag, make it weak to fulfill the requirements
        # of RFC 7232 section-2.1 while also allowing conditional request
        # matches on ETags.
        etag = response.get(‘ETag‘)
        if etag and etag.startswith(‘"‘):
            response[‘ETag‘] = ‘W/‘ + etag
        response[‘Content-Encoding‘] = ‘gzip‘

        return response