[Python-Dev] Should issubclass() be more like isinstance()?

[Guido van Rossum]

On 4/4/06, Crutcher Dunnavant <crutcher at gmail.com> wrote:
> On 4/4/06, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> > Crutcher Dunnavant wrote:
> >
> > > A) issubclass() throws a TypeError if the object being checked is not
> > > a class, which seems very strange.
> >
> > If I ever pass a non-class to issubclass() it's almost
> > certainly a bug in my code, and I'd want to know about
> > it.
>
> Perhaps, but is it worth distorting a predicate?

Certainly. In other languages this would be a compile-time error.

There's no rule that predicate cannot raise an exception.

If you're not sure whether something is a class or not, you should
first sniff it out for its class-ness before checking whether it's a
subclass of something. I recommend hasattr(x, "__bases__") which is
more likely to recognize classes than isinstance(x, type) -- the
latter only works for standard new-style classes.

> > On the rare occasions when I don't want this, I'm
> > happy to write
> >
> >    isinstance(c, type) and issubclass(c, d)
>
> This doesn't work, did you mean?
>   isinstance(c, types.ClassType) and issubclass(c, d)
>
>
> > > B) issubclass() won't work on a list of classs,
> >  > the way isinstance() does.
> >
> > That sounds more reasonable. I can't think of any
> > reason why it shouldn't work.

Agreed.

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