[Python-Dev] Re: PEP-317

[Phillip J. Eby]

At 03:03 PM 6/9/03 -0400, Guido van Rossum wrote:
> > I guess I'm missing why this wouldn't work for new-style exceptions,
> > provided they are required to subclass Exception (which I thought
> > everything was going to be required to do eventually).
>
>I'm dithering on that, actually.  It's a Java vs. C++ thing: in C++
>you can raise any type, in Java it must be an Exception instance.
>
>Python usually doesn't care about isinstance relationships but cares
>about whether the needed protocol is implemented, and this suggests
>that requiring exceptions to subclass Exception is unPythonic.

Hm.  I suppose if we had PEP 246 in the core, we could always 
'adapt(ob,Exception)' instead, and also define an adaptation for tuples to 
become exceptions.  ;)


>But at other times, the desire to catch bogus arguments to raise
>suggests that the subclass requirement might be just fine, and I don't
>have a use case that breaks with this requirement (since you can
>always use multiple inheritance to make a desirable class raisable).

And I expect that even Jim Fulton won't argue that a subclass of an 
Exception subclass should be able to change its stripes and *not* be usable 
as an Exception any more.  In other words, I don't think it's any more of a 
wart than requiring someone to subclass datetime.tzinfo.