python装饰器TypeError: 'NoneType' object is not callable

#定义这个装饰器
def decorator(func):
   print("start")
   func()#运行原函数
   print(func) #<function add at 0x000002167897E830>
   print(func())# None
   print("end")
   return func  #不能返回为空，所以不能用func()

@decorator
def  add():
   print("add")
add()

#再定义一个装饰器
def  delete(func):
     print("delete")
     return func
#定义一个装饰器
def  cal(function=None):



      def decorator(func):
         print("start")
         # func()#运行原函数
         print(func) #<function add at 0x000002167897E830>

         print("end")
         return func

      if function==None:
         return decorator
      else:
         return decorator(function)
#装饰的装饰
@cal(delete)
def  add():
   print("add")
add()

start
<function delete at 0x00000234CAA03F40>
end
delete
add