Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #10958 > unrolled thread

Calling super() in __init__ of a metaclass

Back to article view | Back to comp.lang.python

Contents

  Calling super() in __init__ of a metaclass Eli Bendersky <eliben@gmail.com> - 2011-08-06 10:34 +0300
    Re: Calling super() in __init__ of a metaclass Peter Otten <__peter__@web.de> - 2011-08-06 10:11 +0200

#10958 — Calling super() in __init__ of a metaclass

Consider this standard metaclass definition:

class MyMetaclass(type):
    def __init__(cls, name, bases, dct):
        super(MyMetaclass, cls).__init__(name, bases, dct)
        # do meta-stuff

class Foo(object):
    __metaclass__ = MyMetaclass

The call "super(MyMetaclass, cls)" should returns the parent of
MyMetaclass here. But the 'cls' passed into this __init__ is *not*
MyMetaclass, but rather the created class - i.e. Foo. So how does
"super" get to the parent of MyMetaclass using this information? The
documentation of "super" says:

    If the second argument is a type, issubclass(type2, type) must be
true (this is useful for classmethods).

Yes, 'cls' is a type (it's "class Foo"), but no, it's not a subclass
of MyMetaclass, so this doesn't help.

Eli

[toc] | [next] | [standalone]

#10965

Eli Bendersky wrote:

> Consider this standard metaclass definition:
> 
> class MyMetaclass(type):
>     def __init__(cls, name, bases, dct):
>         super(MyMetaclass, cls).__init__(name, bases, dct)
>         # do meta-stuff
> 
> class Foo(object):
>     __metaclass__ = MyMetaclass
> 
> The call "super(MyMetaclass, cls)" should returns the parent of
> MyMetaclass here. But the 'cls' passed into this __init__ is *not*
> MyMetaclass, but rather the created class - i.e. Foo. So how does
> "super" get to the parent of MyMetaclass using this information? The
> documentation of "super" says:
> 
>     If the second argument is a type, issubclass(type2, type) must be
> true (this is useful for classmethods).
> 
> Yes, 'cls' is a type (it's "class Foo"), but no, it's not a subclass
> of MyMetaclass, so this doesn't help.

Don't let yourself get confused by the name 'cls' for what is normally 
called 'self'. Foo is an instance of MyMetaclass, so the situation is 
exactly the same as in

class A(object):
    def __init__(self, ...)
        super(A, self).__init__(...)

I don't know how exactly super() is implemented, but to go from an instance 
to its class you can use type(instance) or instance.__class__.

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.python

csiph-web