关于python的元类的问题

Question

最近在学习元类。

#python3
# coding=utf-8
class ModelMetaclass(type):

    def __new__(cls, name, bases, attrs):
        print("ainia")
        return type.__new__(cls, name, bases, attrs)
class BBB:
    def __new__(cls, name, bases, attrs):
        print("BBBBB")
        return object.__new__(cls, name, bases, attrs)
class Model(metaclass=BBB):
    pass
class Model2(metaclass=ModelMetaclass):
    pass

#python2
# coding=utf-8
class ModelMetaclass(type):
    def __new__(cls, name, bases, attrs):
        print("ainia")
        return type.__new__(cls, name, bases, attrs)
class BBB(object):
    def __new__(cls, name, bases, attrs):
        print("BBBBB")
        return object.__new__(cls, name, bases, attrs)
class Model(object):
    __metaclass__ = BBB
    pass
class Model2(object):
    __metaclass__ = ModelMetaclass
    pass

以上代码另存为test.py，中运行时提示：

Traceback (most recent call last):
  File "test2.py", line 18, in <module>
    class Model(metaclass=BBB):
  File "test2.py", line 15, in __new__
    return object.__new__(cls, name, bases, attrs)
TypeError: object() takes no parameters

但是如果我把return object.__new__(cls, name, bases, attrs)修改为return object.__new__(cls)却可以正常输出。
还请各位指教？

Answer 1

metaclass必须要从type继承。上面的Model在Python2.7下有语法错误，在Python3中可以产生Model Class但无法进行实例话。在Python3中可以顺利产生Model Class的原因我猜想可能是 “object是type的基类”的原因。

Answer 2

改成这样，new方法里object改成type.

class BBB(type):

    def __new__(cls, name, bases, attrs):
        print("BBBBB")
        return type.__new__(cls, name, bases, attrs)


class Model():
    __metaclass__ = BBB

Answer 3

这是我修改过的代码：
python2下通过

# coding=utf-8
class ModelMetaclass(type):
    def __new__(cls, name, bases, attrs):
        print("ainia")
        return type.__new__(cls, name, bases, attrs)
class BBB(type):
    def __new__(cls, name, bases, attrs):
        print("BBBBB")
        return type.__new__(cls, name, bases, attrs)
class Model:
    __metaclass__ = BBB
class Model2:
    __metaclass__ = ModelMetaclass

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