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

[Crutcher Dunnavant]

On 4/4/06, Guido van Rossum <guido at python.org> wrote:
> 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.

In other, statically typed languges. Someone hands me X, I want to
know if X is a subclass of Y. Yes or No. If X is not a class, then it
is not a subclass of Y, hence issubclass(X, Y) should return False.

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

No, but it makes many applications (such as using it as a test in list
comprehensions) difficult enough to not be worth it.

> 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/)
>


--
Crutcher Dunnavant <crutcher at gmail.com>
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