class Foo(object):      # 默认metaclass = type, 当前类, 由type类创建

    a = 'aaa'

    def func(self, x):

        return x + 1

Foo = type("Foo", (object, ), {'a': "aaa", 'func': lambda self, x: x + 1})

    python3:

        class Foo(object, metaclass=type):

            pass

    python2:

        class Foo(object):

            __metaclass__=type

            pass

class MyType(type):

    def __init__(self, *args, **kwargs):

        print("创建类之前")

        super(MyType, self).__init__(*args, **kwargs)

        print("创建类之后")

class Base(object, metaclass=MyType):

    pass

Base = MyType('Base', (object, ), {})

class Foo(Base):         # 基类由MyType创建,Bar也由MyType创建

    a = 'aaa'

    def func(self, x):

        return x + 1

class MyType(type):

    def __init__(self, *args, **kwargs):

        print("MyType: __init__")

        super(MyType, self).__init__(*args, **kwargs)

    def __call__(self, *args, **kwargs):

        print('MyType: __call__')

        super(MyType, self).__call__(*args, **kwargs)

def with_metaclass(arg):

    return MyType('Base', (arg,), {})

class Foo(with_metaclass(object)):

    a = 'aaa'

    def __init__(self, *args, **kwargs):

        print('Foo: __init__')

        super(Foo, self).__init__(*args, **kwargs)

    def __new__(cls, *args, **kwargs):

        print('Foo: __new__')

        return super(Foo, cls).__new__(cls)

    def func(self, x):

        return x + 1

b = Foo()

# MyType: __init__      这个是创建Base类的时候执行MyType的__init__

# MyType: __init__      创建Foo类的时候,执行MyType的__init__

# MyType: __call__      实例化 Foo(), 先执行MyType的__call__, 再执行Foo的__new__和__init__

# Foo: __new__          然后才会执行Foo的__new__方法

# Foo: __init__　　　　  最后执行Foo的__init__方法