理解Django 中Call Stack机制的小Demo

class SimpleMiddleware:
 def __init__(self, get_response):
  self.get_response = get_response
  # One-time configuration and initialization.
 
 def __call__(self, request):
  # Code to be executed for each request before
  # the view (and later middleware) are called.
 
  response = self.get_response(request)
 
  # Code to be executed for each request/response after
  # the view is called.
 
  return response

import time
class Base:
 def __init__(self,get_response):
  self.get_response=get_response
 def __call__(self,request=None):
  self.pre()
  response=self.get_response(request)
  self.after()
  return response
 def pre(self):
  pass
 def after(self):
  pass
class Response:
 def __init__(self):
  pass
def view(request):
 return Response()
def startServer(callstackSets,view):
 callstackSets.reverse()
 last=callstackSets.pop(0)(view)
 for i in range(len(callstackSets)):
  last=callstackSets.pop(0)(last)
 return last
class A(Base):
 def pre(self):
  print('In A')
 def after(self):
  print('Out A ')
class B(Base):
 def pre(self):
  print('In B')
 def after(self):
  print('Out B ')
class C(Base):
 def pre(self):
  print('In C')
 def after(self):
  print('Out C ')
if __name__=='__main__':
 callstackSets=[A,B,C]
 calls=startServer(callstackSets,view)
 calls('req')