[Python-3000] Revised PEP 3119 (Abstract Base Classes) (original) (raw)

Guido van Rossum guido at python.org
Wed May 16 03:34:52 CEST 2007

On 5/15/07, Collin Winter <collinw at gmail.com> wrote:

On 5/11/07, Guido van Rossum <guido at python.org> wrote: > - Overloading isinstance and issubclass is now a key mechanism rather > than an afterthought; it is also the only change to C code required > > - Built-in (and user-defined) types can be registered as "virtual > subclasses" (not related to virtual base classes in C++) of the > standard ABCs, e.g. Sequence.register(tuple) makes issubclass(tuple, > Sequence) true (but Sequence won't show up in bases or mro).

(The bit about "issubclass(tuple, Sequence)" currently isn't true with the sandbox prototype, but let's assume that it is/will be.)

Perhaps you tried it without the patch (reference [12] from PEP 3119) applied? It works for me:

guido at pythonic:abc$ python3.0 Python 3.0x (p3yk, May 10 2007, 17:05:42) [GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2 Type "help", "copyright", "credits" or "license" for more information.

import abc isinstance((), abc.Sequence) True

Given:

class MyABC(metaclass=ABCMeta): def foo(self): # A concrete method return 5 class MyClass(MyABC): # Mark as implementing the ABC's interface pass >>> a = MyClass() >>> isinstance(a, MyABC) True # Good, I can call foo() >>> a.foo() 5 >>> MyABC.register(list) >>> isinstance([], MyABC) True # Good, I can call foo() >>> [].foo() Traceback (most recent call last): AttributeError: 'list' object has no attribute 'foo' Have I missed something? It would seem that when dealing with ABCs that provide concrete methods, "isinstance(x, SomeABC) == True" is useless.

The intention is that you shouldn't register such cases. This falls under the consenting-adults rule.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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