Is there a way to instantiate a subclass from a class constructor?

[John Roth]

"Paul Rubin" <phr-n2001 at nightsong.com> wrote in message
news:7xofo2l2kk.fsf at ruckus.brouhaha.com...
> "Terry Reedy" <tjreedy at home.com> writes:
> > "> The obvious workaround is to define a function that calls the
> > appropriate
> > > constructor, e.g.
> > >   def newpoly(sides):
> > >     if sides==3: return Triangle(...)
> > >     ...
> >
> > A factory function like this is a standard solution that has been
> > discussed in several posts on this list.  Don't knock it.
>
> I'm not knocking it, I just don't see how to extend the class from
> additional base classes without having to add more special code to
> the factory function, which is against the OO spirit.  Any thoughts?

The thing is, that's exactly what a factory function is for - it does
whatever
special case logic is required to get the right class instantiated, so that
the code doesn't have to be replicated elsewhere in the system.

John Roth