INSTALLED_APPS = [
 #自帶并且注冊(cè)的一個(gè)組件即app
  'django.contrib.admin',
  'django.contrib.auth',
  'django.contrib.contenttypes',

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

]

#在admin.py文件中有一個(gè)__init__文件 ,其中有代碼如下
#意思是:程序的啟動(dòng)自動(dòng)尋找名為admin的py文件,然后執(zhí)行
def autodiscover():
  autodiscover_modules('admin', register_to=site)

#具體的方法如下:
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啟動(dòng)時(shí)候,系統(tǒng)自動(dòng)加載
from django.contrib import admin
#導(dǎo)入該APP下的models
from DRF import models
#此處是一個(gè)單例模式 admion.site 
admin.site.register(models.Publisher)

# AdminSite 類(lèi)
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)