[Python-Dev] [Python-3000] Pre-pre PEP for 'super' keyword (original) (raw)

Calvin Spealman ironfroggy at gmail.com
Mon Apr 30 03:01:42 CEST 2007

On 4/29/07, Guido van Rossum <guido at python.org> wrote:

On 4/29/07, Jim Jewett <jimjjewett at gmail.com> wrote: > So it is a "keyword" in the sense that None is a keyword; not in the > stronger sense that "if" is a keyword?

Um, how do you see those two differ? Is 'if' a keyword in the same sense as 'or', or in a different sense? I realize that in Python 2.5, None is not a full-fledged keyword but cannot be used as an assignment target. But that's only transitional. In 3.0 I imagine it becoming a keyword in the grammar (whose only appearance would be as one of the alternatives for 'atom'). And we're talking 3.0 here.

I think any concerns about it not being fit as a keyword would fall under two catagories or varying validity:

  1. Too many keywords is a valid concern, because it complicates the language.
  2. It just doesn't "feel" like a keyword. Less valid, unless it really doesn't feel like a keyword.

It doesn't feel like a keyword. But it doesn't feel too much not like a keyword.

Anyway, I tried to address the concerns laid out, and I'm more than happy to alter the PEP to actually say "Lets implement this as a keyword", and I actually meant to keep more agnostic on that point in the proposal itself. I was more interested in covering the interface, at least to begin, than the actual implementation. Although, being able to have a solid, working reference implementation based on the frame lookups and such is nice, so we can see how it will actually work in real code, and even use it to backport code using the new super to just about any recent Python version.

I also checked and PyPy does implement a sys._getframe() and a IronPython currently doesn't, but seems to plan on it (there is a placeholder, at present). I am not sure if notes on this belongs in the PEP or not.

Draft Three follows for all. I think I'm turning off e-mail for the rest of this evening, so I'll catch up tomorrow.

PEP: XXX Title: New Super Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Calvin Spealman <ironfroggy at gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 28-Apr-2007 Python-Version: 2.6 Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2)

Abstract

The PEP defines the proposal to enhance the super builtin to work implicitly upon the class within which it is used and upon the instance the current function was called on. The premise of the new super usage suggested is as follows:

super.foo(1, 2)

to replace the old:

super(Foo, self).foo(1, 2)

Rationale

The current usage of super requires an explicit passing of both the class and instance it must operate from, requiring a breaking of the DRY (Don't Repeat Yourself) rule. This hinders any change in class name, and is often considered a wart by many.

Specification

Within the specification section, some special terminology will be used to distinguish similar and closely related concepts. "Super type" will refer to the actual builtin type named "super". "Next Class/Type in the MRO" will refer to the class where attribute lookups will be performed by super, for example, in the following, A is the "Next class in the MRO" for the use of super.

::

    class A(object):
        def f(self):
            return 'A'

    class B(A):
        def f(self):
            super(B, self).f() # Here, A would be out "Next class in the
                               # MRO", of course.

A "super object" is simply an instance of the super type, which is associated with a class and possibly with an instance of that class. Finally, "new super" refers to the new super type, which will replace the original.

Replacing the old usage of super, calls to the next class in the MRO (method resolution order) will be made without an explicit super object creation, by simply accessing an attribute on the super type directly, which will automatically apply the class and instance to perform the proper lookup. The following example demonstrates the use of this.

::

    class A(object):
        def f(self):
            return 'A'

    class B(A):
        def f(self):
            return 'B' + super.f()

    class C(A):
        def f(self):
            return 'C' + super.f()

    class D(B, C):
        def f(self):
            return 'D' + super.f()

    assert D().f() == 'DBCA'

The proposal adds a dynamic attribute lookup to the super type, which will automatically determine the proper class and instance parameters. Each super attribute lookup identifies these parameters and performs the super lookup on the instance, as the current super implementation does with the explicit invokation of a super object upon a class and instance.

The enhancements to the super type will define a new getattr classmethod of the super type, which must look backwards to the previous frame and locate the instance object. This can be naively determined by located the local named by the first argument to the function. Using super outside of a function where this is a valid lookup for the instance can be considered undocumented in its behavior. This special method will actually be invoked on attribute lookups to the super type itself, as opposed to super objects, as the current implementation works. This may pose open issues, which are detailed below.

"Every class will gain a new special attribute, super, which refers to an instance of the associated super object for that class" In this capacity, the new super also acts as its own descriptor, create an instance-specific super upon lookup.

Much of this was discussed in the thread of the python-dev list, "Fixing super anyone?" [1]_.

Open Issues

call methods ''''''''''''''''

Backward compatability of the super type API raises some issues. Names, the lookup of the call of the super type itself, which means a conflict with doing an actual super lookup of the call attribute. Namely, the following is ambiguous in the current proposal:

::

    super.__call__(arg)

Which means the backward compatible API, which involves instansiating the super type, will either not be possible, because it will actually do a super lookup on the call attribute, or there will be no way to perform a super lookup on the call attribute. Both seem unacceptable, so any suggestions are welcome.

Actually keeping the old super around in 2.x and creating a completely new super type seperately may be the best option. A future import or even a simple import in 2.x of the new super type from some builtin module may offer a way to choose which each module uses, even mixing uses by binding to different names. Such a builtin module might be called 'newsuper'. This module is also the reference implementation, which I will present below.

super type's new getattr ''''''''''''''''''''''''

To give the behavior needed, the super type either needs a way to do dynamic lookup of attributes on the super type object itself or define a metaclass for the builtin type. This author is unsure which, if either, is possible with C- defined types.

When should we create super attributes? '''''''''''''''''''''''''''''''''''''''''''

They either need to be created on class creation or on super attribute lookup. For the second, they could be cached, of course, which seems like it may be the best idea, if implicit creation of a super object for every class is considered too much overhead.

How does it work in inner functions? ''''''''''''''''''''''''''''''''''''

If a method defines a function and super is used inside of it, how does this work? The frame looking and instance detection breaks here. However, if there can be some unambiguous way to use both the new super form and still be able to explicitly name the type and instance, I think its an acceptable tradeoff to simply be explicit in these cases, rather than add weird super-specific lookup rules in these cases.

An example of such a problematic bit of code is:

::

    class B(A):
        def f(self):
            def g():
                return super.f()
            return g()

Should super actually become a keyword? '''''''''''''''''''''''''''''''''''''''

This would solve many of the problems and allow more direct implementation of super into the language proper. However, some are against the actual keyword- ization of super. The simplest solution is often the correct solution and the simplest solution may well not be adding additional keywords to the language when they are not needed. Still, it may solve many of the other open issues.

Can we also allow super()? ''''''''''''''''''''''''''

There is strong sentiment for and against this, but implementation and style concerns are obvious. Particularly, that its "magical" and that super() would differ from super.call(), being very unpythonic.

Reference Implementation

This implementation was a cooperative contribution in the original thread [1]_.

::

    #!/usr/bin/env python
    #
    # newsuper.py

    import sys

    class SuperMetaclass(type):
        def __getattr__(cls, attr):
            calling_frame = sys._getframe().f_back
            instance_name = calling_frame.f_code.co_varnames[0]
            instance = calling_frame.f_locals[instance_name]
            return getattr(instance.__super__, attr)

    class Super(object):
        __metaclass__ = SuperMetaclass
        def __init__(self, type, obj=None):
            if isinstance(obj, Super):
                obj = obj.__obj__
            self.__type__ = type
            self.__obj__ = obj
        def __get__(self, obj, cls=None):
            if obj is None:
                raise Exception('only supports instances')
            else:
                return Super(self.__type__, obj)
        def __getattr__(self, attr):
            mro = iter(self.__obj__.__class__.__mro__)
            for cls in mro:
                if cls is self.__type__:
                    break
            for cls in mro:
                if attr in cls.__dict__:
                    x = cls.__dict__[attr]
                    if hasattr(x, '__get__'):
                        x = x.__get__(self, cls)
                    return x
            raise AttributeError, attr

    class autosuper(type):
        def __init__(cls, name, bases, clsdict):
            cls.__super__ = Super(cls)

    if __name__ == '__main__':
        class A(object):
            __metaclass__ = autosuper
            def f(self):
                return 'A'

        class B(A):
            def f(self):
                return 'B' + Super.f()

        class C(A):
            def f(self):
                return 'C' + Super.f()

        class D(B, C):
            def f(self, arg=None):
                var = None
                return 'D' + Super.f()

        assert D().f() == 'DBCA'

Alternative Proposals

No Changes

Although its always attractive to just keep things how they are, people have sought a change in the usage of super calling for some time, and for good reason, all mentioned previously.

* Decoupling from the class name (which might not even be bound to the
  right class anymore! [2]_)
* Simpler looking, cleaner super calls would be better

super(this_class, self)

This is nearly an anti-proposal, as it basically relies on the acceptance of the this_class PEP, which proposes a special name that would always be bound to the class within which it is used. If that is accepted, this_class could simply be used instead of the class' name explicitly, solving the name binding issues [2]_.

self.super.foo(*args)

The super attribute is mentioned in this PEP in several places, and could be a candidate for the completel solution, actually using it explicitly instead of any super usage directly. However, double-underscore names are usually an internal detail, and attempted to be kept out of everyday code.

super(self, *args) or super(self, *args)

This solution only solves the problem of the type indication, does not handle differently named super methods, and is explicit about the name of the instance. It is less flexable without being able to enacted on other method names, in cases where that is needed. One use case this fails is where a base- class has a factory classmethod and a subclass has two factory classmethods, both of which needing to properly make super calls to the one in the base- class.

super.foo(self, *args)

This variation actually eliminates the problems with locating the proper instance, and if any of the alternatives were pushed into the spotlight, I would want it to be this one.

super or super()

This proposal leaves no room for different names, signatures, or application to other classes, or instances. A way to allow some similar use alongside the normal proposal would be favorable, encouraging good design of multiple inheritence trees and compatible methods.

History

29-Apr-2007 - Changed title from "Super As A Keyword" to "New Super" - Updated much of the language and added a terminology section for clarification in confusing places. - Added reference implementation and history sections.

References

.. [1] Fixing super anyone? (http://mail.python.org/pipermail/python-3000/2007-April/006667.html)

Copyright

This document has been placed in the public domain.

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