[Python-3000] [Python-Dev] PEP 367: New Super

[Guido van Rossum]

On 5/31/07, Phillip J. Eby <pje at telecommunity.com> wrote:
> At 07:48 PM 5/31/2007 +0800, Guido van Rossum wrote:
> >I've updated the patch; the latest version now contains the grammar
> >and compiler changes needed to make super a keyword and to
> >automatically add a required parameter 'super' when super is used.
> >This requires the latest p3yk branch (r55692 or higher).
> >
> >Comments anyone? What do people think of the change of semantics for
> >the im_class field of bound (and unbound) methods?
>
> Please correct me if I'm wrong, but just looking at the patch it
> seems to me that the descriptor protocol is being changed as well --
> i.e., the 'type' argument is now the found-in-type in the case of an
> instance __get__ as well as class __get__.
>
> It would seem to me that this change would break classmethods both on
> the instance and class level, since the 'cls' argument is supposed to
> be the derived class, not the class where the method was
> defined.  There also don't seem to be any tests for the use of super
> in classmethods.

I've not gotten a new patch out based on a completely different
approach. (I'm afraid I didn't quite get your suggestion so this is
original work.)

It creates a cell named __class__ which is shared between all methods
defined in a particular class, and initialized to the class object
(before class decorators are applied). Only a small change is made to
super(): instead of making it a keyword, it can be invoked as a
function without arguments, and then it digs around in the frame to
find the __class__ cell and the first argument, and uses those as its
arguments. Example:

class B:
  def foo(self): return 'B'

class C(B):
  def foo(self): return 'C' + super().foo()

C().foo() will return 'CB'. The notation super() is equivalent to
super(C, self) or super(__class__, self). It works for class methods
too.

I realize this is a deviation from the PEP: you need to call
super().foo() instead of super.foo(). Looking at the examples I find
that quite acceptable; in hindsight making super a keyword smelled a
bit too magical. (Yes, I know I've been flip-flopping a lot on this
issue. Working code is convincing. :-)

This __class__ variable can also be used explicitly (thereby
implementing 33% of PEP 3130):

class C:
  def f(self): print(__class__)

C().f()

I wonder if this may meet the needs for your PEP 3124? In
particularly, earlier on, you wrote:

> Btw, PEP 3124 needs a way to receive the same class object at more or
> less the same moment, although in the form of a callback rather than
> a cell assignment.  Guido suggested I co-ordinate with you to design
> a mechanism for this.

Is this relevant at all?

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)