[Python-3000] Updated and simplified PEP 3141: A Type Hierarchy for Numbers (original) (raw)

Guido van Rossum guido at python.org
Thu May 17 19:08:11 CEST 2007

On 5/17/07, Collin Winter <collinw at gmail.com> wrote:

ABCs can define concrete methods. These concrete methods provide functionality that the child classes do not themselves provide.

You seem to be misreading my intention here. ABCs serve two purposes: they are interface specifications, and they provide "default" or "mix-in" implementations of some of the methods they specify. The pseudo-inheritance enabled by the register() call uses only the specification part, and requires that the registered class implement all the specified methods itself. In order to benefit from the "mix-in" side of the ABC, you must subclass it directly.

Let's imagine that Python didn't have the readlines() method, and that I wanted to define one. I could create an ABC that provides a default concrete implementation of readlines() in terms of readline().

class ReadlinesABC(metaclass=ABCMeta): def readlines(self): # some concrete implementation @abstractmethod def readline(self): pass If I register a Python-language class as implementing this ABC, "isinstance(x, ReadlinesABC) == True" means that I can now call the readlines() method. However, if I register a C-language extension class as implementing this ABC, "isinstance(x, ReadlinesABC) == True" may or may not indicate that I can call readlines(), making the test of questionable value. You can say that I shouldn't have registered a C extension class with this ABC in the first place, but that's not the point.

No, it is exactly the point. If you want to have functionality that is not provided by some class, you should use an adaptor.

The point is that for consumer code "isinstance(x, ReadlinesABC) == True" is an unreliable test that may or may not accurately reflect the object's true capabilities.

Maybe attempting to use partially-concrete ABCs in tandem with C classes should raise an exception. That would make this whole issue go away.

The register() call could easily verify that the registered class implements the specified set of methods -- this would include methods that are concrete for the benefit of direct subclassing.

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

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