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PEP 3135 -- New Super

PEP: 3135
Title: New Super
Author: Calvin Spealman <ironfroggy at gmail.com>, Tim Delaney <timothy.c.delaney at gmail.com>, Lie Ryan <lie.1296 at gmail.com>
Status: Final
Type: Standards Track
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 flexible 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.
Source: https://github.com/python/peps/blob/master/pep-3135.txt