[issue11610] Improved support for abstract base classes with descriptors

[Darren Dale]

Darren Dale <dsdale24 at gmail.com> added the comment:

On Sat, Jun 11, 2011 at 7:32 PM, Eric Snow <report at bugs.python.org> wrote:
>
> Eric Snow <ericsnowcurrently at gmail.com> added the comment:
> Per your last message, if a specific descriptor has an abstract setter then the descriptor should be considered abstract.  If the implementation of that attribute is not a descriptor should it raise a TypeError?  If it is a descriptor but it does not have a setter, should it likewise fail?

Consider a framework like Enthought's Traits or Riverbank Computing's
dip, where setting the value of a descriptor can result in other
objects being notified of the change. Both Traits and dip are based on
the concept of interfaces, but imagine someone wanted to develop
something similar based on ABCs. In that case, one could argue that
replacing the descriptor with a regular attribute, or another
read-only descriptor, would not satisfy the ABC specification. Then it
might be nice if the abc mechanisms could catch the error. But it
looks like this will be difficult in cases where the subclasses
replaces the descriptor, unless perhaps an AbstractDescriptor were
provided that explained how ABCMeta is going to identify abstract
methods:

    class AbstractDescriptor(metaclass=abc.ABCMeta):
        @property
        @abc.abstractmethod
        def __abstractmethods__(self):
            # it would be nice if descriptors new their own names here,
            # __abstractmethods__ could return: ('bar.fget', 'bar.fset')
            return frozenset(m for m in ('fget', 'fset', 'fdel')
                             if getattr(getattr(self, m, None),
                                        '__isabstractmethod__', False))
        @property
        def __isabstractmethod__(self):
            return True if self.__abstractmethods__

    AbstractDescriptor.register(property)

Of course, not all descriptors would be required to derive from
AbstractDescriptor. There is no intended stick, but the carrot is
better integration with with ABCs.

Having said all that, I think the above suggestion including
__abstractmethods__ for descriptors makes unreasonable demands of
conformity between various descriptor implementations, and that Nick's
suggestion of simply asking descriptors to provide an
__isabstractmethod__ descriptor is probably good enough. Sufficient
documentation of an ABC's interface can cover the rest.

The inspect module or something like it may still be needed in ABCMeta
to work around the general issue Daniel discovered with staticmethod.
That way ABCMeta could inspect the descriptors themselves and attempt
to retrieve their __isabstractmethod__ value.

(aside: unlike ABCMeta.__new__, ABCMeta.register makes no attempt to
verify that the class being registered actually meets the
specification. Why not have ABCMeta.register perform the same checks
as ABCMeta.__new__, and raise an error if the registered class does
not conform to the specification?)

My work is going to keep me pretty busy over the next three weeks, and
I'm still not accomplished with Python's C-API. If someone else wants
to take a crack at the next patch, please feel free.

Darren

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11610>
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