Python meta class with custom metaclass behavior

Question

I am playing with metaclasses in Python 2.7. So I created a code that looks like this:

class M(type):
    def __new__(meta, name, parents, attrs):
        print 'In meta new'
        return super(meta, meta).__new__(meta, name, parents, attrs)

    def __init__(cls, *args, **kwargs):
        print 'In meta init'

    def __call__(cls, *attr, **val):
        print 'In meta call'
        return super(cls, cls).__new__(cls)

class A(object):
    __metaclass__ = M

    def __new__(cls):
        print 'In class new'
        return super(cls, cls).__new__(cls)

    def __init__(self):
        print 'In object init'

    def __call__(self):
        print 'In object call'

But the output confuses me:

A()

In meta new
In meta init
In meta call

Somehow class methods __ new __ and __ init __ were overridden, so interpreter just skip them. Can anyone explain this stuff?

Thanks for your help.

Amber · Accepted Answer · 2012-10-26 07:26:08Z

1

You're calling super() incorrectly. The first argument to super() is supposed to be the class itself, not the instance of it.

return super(meta, meta).__new__(meta, name, parents, attrs)

should be...

return super(M, meta).__new__(meta, name, parents, attrs)

and so on for the other super() calls - the first argument should be the class they're within; the second is the actual instance.

answered Oct 26, 2012 at 7:26

Amber

531k89 gold badges643 silver badges558 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

alexvassel · Accepted Answer · 2012-11-15 12:54:52Z

0

It does not work because I do not use the origin Python mechanism - cls(), which guaranties automatic working of the __new__ and __init__ methods, it is overridden by metaclass __call__ method, which doesn't do the same.

answered Nov 15, 2012 at 12:54

alexvassel

10.8k3 gold badges33 silver badges31 bronze badges

Collectives™ on Stack Overflow

Python meta class with custom metaclass behavior

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related