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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