Overriding (factory?) methods and inheritance ...

[Jp Calderone]

On Mon, Dec 30, 2002 at 04:43:35PM +0000, Rocco Rossi wrote:
> I posted a message yesterday about problems I was having when inheriting
> from base classes that contained methods (would it be correct to call them
> "factory" methods) which produced instances of the same class (type), like a
> RationalNumber class or a ComplexNumber class or Vector class, and so on.
> 
> The inherited specialized class would of course have those same methods
> return values of the base class type, so that in the end one would not be
> able to exploit the new features of the inherited class for those values ...
> no code re-use!!!
> 
> One way out, of course, (which was also suggested by Laotseu --- BTW
> thanks ---), would be to override precisely those methods, and like I said,
> we would be losing the benefits of object-oriented programming that way, but
> it certainly still is possible to let the base class do the majority of the
> work ... Has this problem ever come up before? It seems to me that it is
> non-trivial, but then again maybe I'm not confronting it in the appropriate
> manner. Need some help.
> 
> Thanks.
> 

  I missed yesterday's post, but perhaps...

class Base:
    def commonMethod(self, args):
        self.variousThings(args)
        return self.__class__(appropriateVariables)

  or if your class is new-style...

class Base(object):
    def commonMethod(self, args):
        return type(self)(appropriateVariables)

  or even...

class Base(object):
    def __new__(klass, args):
        if someCondition(args):
            return FirstSubClass(args)
        elif anotherCondition(args):
            return SecondSubClass(args)
        else:
            return LastSubClass(args)

    def commonMethod(self, args):
        return Base(args)

  The last is probably the least desirable since it is somewhat inflexible. 

  Jp

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