[Python-3000] PEP 3119 - Introducing Abstract Base Classes

Jim Jewett jimjjewett at gmail.com
Fri Apr 27 19:10:24 CEST 2007

On 4/27/07, Barry Warsaw <barry at python.org> wrote:

> - - Attributes.  Interfaces allow you to make assertions about
> attributes, not just methods, while ABCs necessarily cover only methods.

Why can't they have data attributes as well?

> - - With interfaces, you can make assertions about individual objects
> which may be different than what their classes assert.  Interface
> proponents seem to care a lot about this and it seems there are valid
> uses cases for it.

Isn't this something that could be handled by overriding isinstance?

> Another example of separating inheritance and interface comes up when
> you want to derive a subclass to share implementation details, but
> you want to subtly change the semantics, which would invalidate an
> ABC claim by the base class.  Something like a GrowOnlyDictionary
> that derived from dict for implementation purposes, but didn't want
> to implement __delitem__ as required by the MutableMapping ABC.

OK, that makes the isubclass override trickier, so there should be an
example, but I think it can still be done.

> Finally, I'm concerned with the "weight" of adding ABCs to all the
> built-in types.

What if the builtin types did not initially derive from any ABC, but
were added (through an issubclass override) when the abc module was


More information about the Python-3000 mailing list