def authenticate(request=None, **credentials):
    #__get_backends獲取當前系統(tǒng)中定義的認證后端，并依次迭代
    for backend, backend_path in _get_backends(return_tuples=True):
        try:
            inspect.getcallargs(backend.authenticate, request, **credentials)
        except TypeError:
            #此后端不接受這些憑據(jù)作為參數(shù)。返回繼續(xù)執(zhí)行循環(huán)
            continue
        try:
             #通過當前的認證后端嘗試獲取 User，若獲取不到就會拋出異常！
            user = backend.authenticate(request, **credentials)
        except PermissionDenied:
            #拋出異常Permission
            break
         #如果沒有返回，繼續(xù)執(zhí)行下一個認證
        if user is None:
            continue
        #添加一個屬性標志，代表后端認證成功
        user.backend = backend_path
        return user
    # 所提供的憑據(jù)對所有后端、觸發(fā)信號無效
    user_login_failed.send(sender=__name__, credentials=_clean_credentials(credentials), request=request)

def _get_backends(return_tuples=False):
    backends = []
    #AUTHENTICATION_BACKENDS 定義了當前系統(tǒng)可以用的身份認證列表
    for backend_path in settings.AUTHENTICATION_BACKENDS:
        #加載后端
        backend = load_backend(backend_path)
        backends.append((backend, backend_path) if return_tuples else backend)
    #如果未定義后端列表拋出異常
    if not backends:
        raise ImproperlyConfigured(
            'No authentication backends have been defined. Does '
            'AUTHENTICATION_BACKENDS contain anything?'
        )
    return backends

AUTHENTICATION_BACKENDS=['django.contib.auth.backends.ModelBackend']  #當前系統(tǒng)默認認證后端ModelBackend

def get_user_model():
    """
    返回一個處于激活狀態(tài)的 User
    """
    try:
        return django_apps.get_model(settings.AUTH_USER_MODEL, require_ready=False)
    except ValueError:
        raise ImproperlyConfigured("AUTH_USER_MODEL must be of the form 'app_label.model_name'")
    except LookupError:
        raise ImproperlyConfigured(
            "AUTH_USER_MODEL refers to model '%s' that has not been installed" % settings.AUTH_USER_MODEL
        )

# 使用默認User model時
>>> from django.contrib.auth import get_user_model
>>> get_user_model()
<class 'django.contrib.auth.models.User'>
# 使用自定義User model時
>>> from django.contrib.auth import get_user_model
>>> get_user_model()
<class 'xxx.models.NewUser'>
# get_user_model()實際獲取的是settings.AUTH_USER_MODEL指定的User model

#格式： "<django_app名>.<model名>"
AUTH_USER_MODEL = "appname.NewUser" 
#在models.py編寫示例
from django.conf import settings
from django.db import models
class Article(models.Model):
    author = models.ForeignKey(settings.AUTH_USER_MODEL)
    title = models.CharField(max_length=100)