[Python-3000] Implementations: A syntax for extending method dispatching beyond type/class inheritance

Hasan Diwan hasan.diwan at gmail.com
Thu Dec 7 16:06:29 CET 2006

The parametrized version of the pass keyword could be used to indicate a
formal interface.
On 07/12/06, Dave Anderson <python3000 at davious.org> wrote:
> Has-Method Contracts
> --------------------
> The first thing I loved about python (coming from java), was this
> realization that I was no longer need be caught up in the inheritance
> game.  If I wanted to use an unspoken convention that I could get a
> special string from every object just by defining name() in each of
> those objects, that's all I had to do.  It didn't matter which class
> tree it came from.
> Looking at the syntax sketch, this can be formalized; but, its a low
> level of formality that fits the context.
> # (I used Generic.name in my original post, but that was a mistake.
> #  Sorry for my confusion, Phillip)
> def get_name(arg: AnyClass.name):       # AnyClass will magically have
>         return arg.name()               # a reference to any method name
>                                         # somehow
> further refinement of this would be:
> class HasName:
>         def name(self):
>                 ...
> def get_name(arg: HasName):
>         return arg.name()
> but this refinement (in the syntax sketch), would force implementers to
> declare the following implementation:
> class X:
>         implements HasName
>         def name(self):

I may be missing something here, but we already have a mechanism for
defining that a object defines certain methods. Could we not keep it
orthogonal and have, e. g.:

class super():
   def __init__(self): pass

   def typeOfSuper(self): pass(subclass)

The way pass is defined now would remain the default. A new builtin pass()
would be added to support your notion of interfaces.

Has Class-specific Method Contracts
> -----------------------------------
> Similar to Has-Method Contracts are class-specific method contracts.
> This is when you use a well-defined class as a way to indicate the
> context of a method and separate it from homonyms.
> def add_key_value_pair(arg: MutableContainer.add, key, value):
>         arg[key] = value

def __add__(self, onemore):
   MutableContainer.add (self, onemore.key, onemore.value):

If you need more than one function to implement '+', you can use two

> Implement (As a) vs Inherit (Is a)
> ----------------------------------
> This is nuanced.
> Basically, if I have some object Car and I want to be able to use it
> like a dict, I don't necessarily consider it inheriting from dict (or
> even UserDict).  That is, I don't think a Car is a dict.
> To me, even if Car happens to represent a dict by being able to be used
> as a dict, the expression of this fact feels better expressed as merely
> implementing dict.
> compare:
> class Car(dict):        # car... is a dict? hmm, a bad ring to it
>         ...
> class Car:
>         implements dict # oh, Cars can be used like a dict, got it
>         ...
> In addition to that subjective differentiation, there is a technical
> argument.  Without something like an implements declaration, if I ever
> want to express a relationship to another class, but with different
> internals, I am forced to first define an abstract class and then
> inherit from that.  With an implements declaration, I can say implements
> dict and be free of inheriting any of its internal implementation.
> (I concede here that I could, in fact, inherit UserDict, override all
> the methods, and just not call super in my init.)
> Even with the nicely done ABCs, is there an easy way for a class to
> declare that it can be used like a dict with its own implementation of
> dict methods?  With this syntax sketch, UserDict would remain the same
> except:
> class UserDict:
>         implements dict         # dispatch can rely on this declaration
>         ...                     # rather than method-checking
> The Consequences of Going Outside of the Hierarchy
> --------------------------------------------------
> For all the above reasons, I want to see python support something more
> flexible than type-based dispatching.  We can look at the method-scoped
> units of Implementations as a limited version of Guido's Abilities, and
> we can look at the class-scoped implementations as types, or classes, or
>   also as abstract interfaces from concrete classes.
> Unfortunately, I am now outside of the hierarchy tree; an implementation
> tree would be a superset of a class's hierarchy tree.  I suppose that
> using dispatch this way is more complex compared to a strict class-based
> set of relationships (because I am asking for checks not just on the
> class-tree, but also on the implementations off that class-tree); but,
> I'd rather fold current python usage into a future of type /
> protocol-aware methods than force current pythonists into choosing
> between class or no class, to type or not to type.
> (Small Caveat:) Using Implementations as a Development Tool
> ------------------------------------------------------------
> Btw, if we formally differentiate that I'm declaring an implementation
> (versus inherit and override), I could programatically double-check my
> implementation with tools...
> class A:
>         implements Container
>         def len(self)
>                 ...
> >>> interface_compliance(A)

This would just scan for a  parametrised "pass" keyword and check that all
subclasses inheriting from it implemented that method.

(If I missed the gist of these questions.  Please forgive me and clarify
> them for me.)

Ditto for me.
Hasan Diwan <hasan.diwan at gmail.com>
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