当你调用 object.__new__ 时会发生什么？

Question

1. 老式类

   class Person():
       _names_cache = {}
       def __init__(self,name):
           self.name = name
       def __new__(cls,name):
           return cls._names_cache.setdefault(name,object.__new__(cls,name))
   
   ahmed1 = Person("Ahmed")
   ahmed2 = Person("Ahmed")
   print ahmed1 is ahmed2
   print ahmed1
   print ahmed2
   
   
   >>> False
   <__main__.Person instance at 0xb74acf8c>
   <__main__.Person instance at 0xb74ac6cc>
   >>>
   

2. 新型类

   class Person(object):
       _names_cache = {}
       def __init__(self,name):
           self.name = name
       def __new__(cls,name):
           return cls._names_cache.setdefault(name,object.__new__(cls,name))
   
   ahmed1 = Person("Ahmed")
   ahmed2 = Person("Ahmed")
   print ahmed2 is ahmed1
   print ahmed1
   print ahmed2
   
   >>> True
   <__main__.Person object at 0xb74ac66c>
   <__main__.Person object at 0xb74ac66c>
   >>>
   

我想了解调用object.__new__时两个版本的代码会发生什么？

Answer 1

旧式类不支持 __new__ 方法：

>>> class Foo:
...     def __new__(self, *args, **kw):
...         print 'Nope'
... 
>>> Foo()
<__main__.Foo instance at 0x101f5c908>

__new__ 只是一个新的类特性。