详解django.contirb.auth-认证

首先看middleware的定义：

auth模块有两个middleware：AuthenticationMiddleware和SessionAuthenticationMiddleware。

AuthenticationMiddleware负责向request添加user属性

class AuthenticationMiddleware(object):  def process_request(self, request):    assert hasattr(request, 'session'), (      "The Django authentication middleware requires session middleware "      "to be installed. Edit your MIDDLEWARE_CLASSES setting to insert "      "'django.contrib.sessions.middleware.SessionMiddleware' before "      "'django.contrib.auth.middleware.AuthenticationMiddleware'."    )    request.user = SimpleLazyObject(lambda: get_user(request))

可以看见AuthenticationMiddleware首先检查是否由session属性，因为它需要session存储用户信息。

user属性的添加，被延迟到了get_user()函数里。SimpleLazyObject是一种延迟的技术。

在来看SessionAuthenticationMiddleware的定义：

它负责session验证

class SessionAuthenticationMiddleware(object):  """  Middleware for invalidating a user's sessions that don't correspond to the  user's current session authentication hash (generated based on the user's  password for AbstractUser).  """  def process_request(self, request):    user = request.user    if user and hasattr(user, 'get_session_auth_hash'):      session_hash = request.session.get(auth.HASH_SESSION_KEY)      session_hash_verified = session_hash and constant_time_compare(        session_hash,        user.get_session_auth_hash()      )      if not session_hash_verified:        auth.logout(request)

通过比较user的get_session_auth_hash方法，和session里面的auth.HASH_SESSION_KEY属性，判断用户的session是否正确。

至于request里面的user对象，由有什么属性，需要看看get_user()函数的定义。

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

显然get_user方法在request增加了_cached_user属性，用来作为缓存。

因为用户认证需要查询数据库，得到用户的信息，所以减少开销是有必要的。

注意，这种缓存只针对同一个request而言的，即在一个view中多次访问request.user属性。

每次http请求都是新的request。

再接着看auth.get_user()方法的定义，深入了解request.user这个对象：

def get_user(request):  """  Returns the user model instance associated with the given request session.  If no user is retrieved an instance of `AnonymousUser` is returned.  """  from .models import AnonymousUser  user = None  try:    user_id = request.session[SESSION_KEY]    backend_path = request.session[BACKEND_SESSION_KEY]  except KeyError:    pass  else:    if backend_path in settings.AUTHENTICATION_BACKENDS:      backend = load_backend(backend_path)      user = backend.get_user(user_id)  return user or AnonymousUser()