[Python-3000] iostack and Oh Oh

Josiah Carlson jcarlson at uci.edu
Sat Dec 9 18:53:34 CET 2006

"Steven Bethard" <steven.bethard at gmail.com> wrote:
> On 12/9/06, Josiah Carlson <jcarlson at uci.edu> wrote:
> > I'd prefer to see...
> >
> >     class MyMapping:
> >         @implements(mapping)
> >         def __getitem__(self, key):
> >             ...
> >
> >         @implements(mapping)
> >         def __len__(self):
> >             ...
> >
> [snip]
> >
> > P.S. Here's a 10 minute implementation of the above semantics that is
> > *almost* backwards compatible with Python 2.3 (except for the decorator
> > thing, which Python 2.3 has to do without).
> > __builtins__.type = type
> Sorry, I don't understand this implementation.  I just tried it and
> the following fails because ``__getitem__`` is now an
> ``implements_wrapper`` that accepts the wrong arguments::
>     class C(object):
>         @implements(mapping)
>         def __getitem__(self, item):
>             return 42
>     C()[1]

It doesn't work because we need to replace the base metaclass from
object, which the above mechanism doesn't do.  It half-asses it by
monkey-patching __builtins__, but object.__new__ doesn't look in
__builtins__, it uses CPython calls.

If you want to get your above use to work all the time (I believe, I
hadn't tested it), you can also add the following just after the
__builtins__.type = type .

    class object(object):
        __metaclass__ = type

    __builtins__.object = object

> But I think the ``implements()`` decorator is a good way to approach
> this.  Here's a different approach to it::


>     class C(object):
>         __metaclass__ = interface_monitor
>         @implements(Mapping)
>         def __getitem__(self, key):
>             return 4
>         @implements(Mapping)
>         def __iter__(self):
>             return [4, 4, 4]
> Note that with this code, an interface is just a set of methods, so
> you can use ``supports()`` equally well to check for support of
> individual methods or to check support for whole sets of methods.

That's what I was getting at with the previous version, though instead
of using methods on the various interface objects (eg Mapping), I used
objects in the class namespace.  Using a function does allow for doing
argument list length and/or keyword name verification, but I don't know
if we want to go that far.

Using the monkey patching of __builtins__.object as I include in this
message, makes the __metaclass__ bit that you use unnecessary.

I also have a personal aversion for adding *yet another* attribute to
the methods that implement interfaces, but since we are adding
__signature__, etc., and no one else seems to be complaining, what's one

 - Josiah

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