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

Guido van Rossum guido at python.org
Wed Apr 5 05:01:35 CEST 2006

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/)

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