INSTALLED_APPS = [
 #自带并且注册的一个组件即app
  'django.contrib.admin',
  'django.contrib.auth',
  'django.contrib.contenttypes',

urlpatterns = [
  # 自动存在的admin路由
  url(r'^admin/', admin.site.urls),
  url(r'^stark/', site.urls),

]

#在admin.py文件中有一个__init__文件 ,其中有代码如下
#意思是:程序的启动自动寻找名为admin的py文件,然后执行
def autodiscover():
  autodiscover_modules('admin', register_to=site)

#具体的方法如下:
def autodiscover_modules(*args, **kwargs):
  """
  Auto-discover INSTALLED_APPS modules and fail silently when
  not present. This forces an import on them to register any admin bits they
  may want.

  You may provide a register_to keyword parameter as a way to access a
  registry. This register_to object must have a _registry instance variable
  to access it.
  """

# 在django启动时候,系统自动加载
from django.contrib import admin
#导入该APP下的models
from DRF import models
#此处是一个单例模式 admion.site 
admin.site.register(models.Publisher)

# AdminSite 类
class AdminSite(object):
    ...  
    def __init__(self, name='admin'):
      self._registry = {} # model_class class -> admin_class instance
      self.name = name
      self._actions = {'delete_selected': actions.delete_selected}
      self._global_actions = self._actions.copy()
      all_sites.add(self)
    ....
site = AdminSite()

# AdminSite中的register 方法
  def register(self, model_or_iterable, admin_class=None, **options):
    """
    Registers the given model(s) with the given admin class.
    The model(s) should be Model classes, not instances.
    If an admin class isn't given, it will use ModelAdmin (the default
    admin options). If keyword arguments are given -- e.g., list_display --
    they'll be applied as options to the admin class.
    If a model is already registered, this will raise AlreadyRegistered.
    If a model is abstract, this will raise ImproperlyConfigured.
    """

class AdminSite(object):

  def __init__(self):
    self._registry = {}
obj1 = AdminSite()

import a
a.obj1._registry['k2'] = 666

import a
a.obj1._registry['k1'] = 123
print(a.obj1._registry)

class V1(object):
  def foo(self)
    pass
V1 = V1()

def Singleton(cls):
  _instance = {}
  def _singleton(*args, **kargs):
    if cls not in _instance:
      _instance[cls] = cls(*args, **kargs)
    return _instance[cls]
  return _singleton
@Singleton
class A(object):
  a = 1
  def __init__(self, x=0):
    self.x = x
a1 = A(2)
a2 = A(3)