maric at aristote.info
Mon Sep 15 14:09:13 CEST 2008
Le Sunday 14 September 2008 22:25:18 Steven D'Aprano, vous avez écrit :
> On Sun, 14 Sep 2008 12:15:04 -0700, Gary Herron wrote:
> > (If you wish to consider the base class "abstract", just agree with
> > yourself to not instantiate it.)
> That's certainly the most lightweight option.
> > Please forget about Abstract Base Classes. They have nothing to do with
> > what you want, and are a *HUGE* overkill for your application. They
> > are not (yet) even part of standard Python, and are used primarily for a
> > class implementor to guarantee to a user of a class that it provides a
> > specific interface.
> You can implement a lightweight abstract base class in Python fairly
> easily, by adding two lines to the __init__ method of your base class.
> class Base(object):
> def __init__(self):
> if self.__class__ is Base:
> raise NotImplementedError("Abstract base class")
> def method(self, *args):
> return len(args) # or something useful
But this doesn't match what abstract classes are (in C++ for example), because
__init__ can, and probably will, have logic reusable in concrete classes. The
only way to do this is to call in the __int__ at least one of the methods
raising NotImplementedError, probably creating a dummy one for this purpose,
and still it doesn't satisfy with the constraint that *all* abstract methods
must be implemented in concrete classes.
> "The fact that many languages disallow instantiation of abstract types
> (and force subtypes to implement all needed functionality) further
> ensures program correctness."
> but I don't see how that follows (except possibly in very special cases).
> Given that the set of instances of class B is empty, how does that help
> you know that B.method is correct?
I agree with you on this, and the simple scheme of defining some methods
raising exceptions is obviously sufficiient where duck typing is.
ABC, as I understood it, is for resolving another problem : unrelated (by
inheritance) classes, which share the same signature, but need to be
distinguished in their behavior.
More information about the Python-list