[Zope-dev] A summary of "Interfaces vs ZCA concepts"

[Ethan Jucovy]

Hi,

On Thu, Dec 17, 2009 at 10:15 AM, Martijn Faassen
<faassen at startifact.com> wrote:
> * It'd be nice if __call__ came back with a LookupError instead of a
> TypeError, but how to get from A to B without breakage?

Maybe I've misunderstanding, but what's the advantage of making
IFoo(x) raise a LookupError instead of a TypeError?  I've tried to
follow the thread but I've been confused about this.  I do rely on
catching TypeErrors quite often in my code -- I had thought it was
intended as part of the API.

I like that treating it as typecasting instead of lookup blurs the
conceptual distinction between "adapting x to IFoo" and "asserting x
implements IFoo directly" -- when I actually want to know one or the
other I can use explicit adaptation or check providedBy.  In other
words I use IFoo(x) when I don't care whether IFoo(x) == x.

I suppose it's not a big deal but catching LookupErrors instead would
feel somewhat less semantic for the way I think about this.  But from
what I've caught of the larger thread it sounds like I'm in the
minority here.

-Ethan