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python-peps / pep-3135.txt

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PEP: 3135
Title: New Super
Version: $Revision$
Last-Modified: $Date$
Author: Calvin Spealman <ironfroggy@gmail.com>,
        Tim Delaney <timothy.c.delaney@gmail.com>,
        Lie Ryan <lie.1296@gmail.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Apr-2007
Python-Version: 3.0
Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007, 12-Mar-2009

Numbering Note
==============

This PEP started its life as PEP 367.  Since it is now targeted
for Python 3000, it has been moved into the 3xxx space.

Abstract
========

This PEP proposes syntactic sugar for use of the ``super`` type to automatically
construct instances of the super type binding to the class that a method was
defined in, and the instance (or class object for classmethods) that the method
is currently acting upon.

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 class" will refer to
the actual builtin class named "super". A "super instance" is simply an
instance of the super class, which is associated with another class and
possibly with an instance of that class.

The new ``super`` semantics are only available in Python 3.0.

Replacing the old usage of super, calls to the next class in the MRO (method
resolution order) can be made without explicitly passing the class object
(although doing so will still be supported). Every function
will have a cell named ``__class__`` that contains the class object that the
function is defined in.

The new syntax::

    super()

is equivalent to::

    super(__class__, <firstarg>)

where ``__class__`` is the class that the method was defined in, and
``<firstarg>`` is the first parameter of the method (normally ``self``
for instance methods, and ``cls`` for class methods). For functions
defined outside a class body, ``__class__`` is not defined, and will
result in runtime ``SystemError``.

While ``super`` is not a reserved word, the parser recognizes the use
of ``super`` in a method definition and only passes in the
``__class__`` cell when this is found.  Thus, calling a global alias
of ``super`` without arguments will not necessarily work.

Closed Issues
=============

Determining the class object to use
-----------------------------------

The class object is taken from a cell named ``__class__``.


Should ``super`` actually become a keyword?
-------------------------------------------

No. It is not necessary for super to become a keyword.

super used with __call__ attributes
-----------------------------------

It was considered that it might be a problem that instantiating super instances
the classic way, because calling it would lookup the __call__ attribute and
thus try to perform an automatic super lookup to the next class in the MRO.
However, this was found to be false, because calling an object only looks up
the __call__ method directly on the object's type. The following example shows
this in action.

::

    class A(object):
        def __call__(self):
            return '__call__'
        def __getattribute__(self, attr):
            if attr == '__call__':
                return lambda: '__getattribute__'
    a = A()
    assert a() == '__call__'
    assert a.__call__() == '__getattribute__'

In any case, this issue goes away entirely because classic calls to
``super(<class>, <instance>)`` are still supported with the same meaning.

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!)
- Simpler looking, cleaner super calls would be better

Dynamic attribute on super type
-------------------------------

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 instance upon a class and instance.

This proposal relies on sys._getframe(), which is not appropriate for anything
except a prototype implementation.

self.__super__.foo(\*args)
--------------------------

The __super__ attribute is mentioned in this PEP in several places, and could
be a candidate for the complete 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(\*p, \*\*kw)
------------------

There has been the proposal that directly calling ``super(*p, **kw)`` would
be equivalent to calling the method on the ``super`` object with the same name
as the method currently being executed i.e. the following two methods would be
equivalent:

::

    def f(self, *p, **kw):
        super.f(*p, **kw)

::

    def f(self, *p, **kw):
        super(*p, **kw)

There is strong sentiment for and against this, but implementation and style
concerns are obvious. Guido has suggested that this should be excluded from
this PEP on the principle of KISS (Keep It Simple Stupid).



History
=======
12-Mar-2009 - Updated to reflect the current state of implementation.

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.

06-May-2007 - Updated by Tim Delaney to reflect discussions on the python-3000
              and python-dev mailing lists.

References
==========

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

.. [2] PEP 3130: Access to Module/Class/Function Currently Being Defined (this)
   (http://mail.python.org/pipermail/python-ideas/2007-April/000542.html)


Copyright
=========

This document has been placed in the public domain.



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