python中type和type .__ new__有什么区别? [英] What is the difference between type and type.__new__ in python?

问题描述

我正在写一个元类，却不小心是这样的:

I was writing a metaclass and accidentally did it like this:

class MetaCls(type):
    def __new__(cls, name, bases, dict):
        return type(name, bases, dict)

...而不是这样:

class MetaCls(type):
    def __new__(cls, name, bases, dict):
        return type.__new__(cls, name, bases, dict)

这两个元类之间到底有什么区别?更具体地说，是什么导致第一个不能正常工作(元类未调用某些类)?

What exactly is the difference between these two metaclasses? And more specifically, what caused the first one to not work properly (some classes weren't called into by the metaclass)?

推荐答案

在第一个示例中，您正在创建一个全新的类:

In the first example you're creating a whole new class:

>>> class MetaA(type):
...     def __new__(cls, name, bases, dct):
...         print 'MetaA.__new__'
...         return type(name, bases, dct)
...     def __init__(cls, name, bases, dct):
...         print 'MetaA.__init__'
... 
>>> class A(object):
...     __metaclass__ = MetaA
... 
MetaA.__new__
>>> 

而在第二种情况下，您将调用父级的__new__:

while in the second case you're calling parent's __new__:

>>> class MetaA(type):
...     def __new__(cls, name, bases, dct):
...         print 'MetaA.__new__'
...         return type.__new__(cls, name, bases, dct)
...     def __init__(cls, name, bases, dct):
...         print 'MetaA.__init__'
... 
>>> class A(object):
...     __metaclass__ = MetaA
... 
MetaA.__new__
MetaA.__init__
>>> 

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