[Python-3000] adding @abstractmethods after class creation

Calvin Spealman ironfroggy at gmail.com
Wed Apr 25 04:42:26 CEST 2007

On 4/24/07, Steven Bethard <steven.bethard at gmail.com> wrote:
> On 4/24/07, guido.van.rossum <python-checkins at python.org> wrote:
> > +We define a new built-in decorator, ``@abstractmethod``
> [snip]
> > +**Implementation:** The ``@abstractmethod`` decorator sets the
> > +function attribute ``__isabstractmethod__`` to the value ``True``.
> > +The ``type.__new__`` method computes the type attribute
> > +``__abstractmethods__`` as the set of all method names that have an
> > +``__isabstractmethod__`` attribute whose value is true.  It does this
> > +by combining the ``__abstractmethods__` attributes of the base
> > +classes, adding the names of all methods in the new class dict that
> > +have a true ``__isabstractmethod__`` attribute, and removing the names
> > +of all methods in the new class dict that don't have a true
> > +``__isabstractmethod__`` attribute.  If the resulting
> > +``__abstractmethods__`` set is non-empty, the class is considered
> > +abstract, and attempts to instantiate it will raise ``TypeError``.
> (Hope this wasn't covered in that really long thread. I couldn't see
> an answer in the PEP.)
> So what happens in a situation like::
>     class C:
>         pass
>     C.foo = abstractmethod(foo)
> It seems from the description above like ``C.__abstractmethods__``
> would be empty, and therefore C could be instantiated even though it
> has an abstract method.
> I don't mind at all if we just say "don't do that", but then
> @abstractmethod should probably be documented explicitly as only being
> usable within a class body.  (Note that this is different from
> @classmethod and @staticmethod which can be used after the fact.)
> I guess the other option would be to have __setattr__ on classes
> append to the __abstractmethods__ list when passed a value with
> __isabstractmethod__ == True. But this would probably make setting
> attributes on classes (though not on other objects) slower.
> STeVe
> --
> I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
> tiny blip on the distant coast of sanity.
>         --- Bucky Katt, Get Fuzzy
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Or could __abstractmethods__ be created dynamically? Which would be
used less, __abstractmethods__ or setting class attributes, that it
would be OK to be a bit slower?

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