[Python-Dev] PEP 344: Exception Chaining and Embedded Tracebacks

[Guido van Rossum]

[Guido van Rossum]
> > My rule has more to do with who "owns" the namespace on the one hand,
> > and with "magic" behavior caused (or indicated) by the presence of the
> > attribute on the other. Class or instance is irrelevant; that most
> > magic attributes live on classes or modules is just because those are
> > places where most of the magic is concentrated.
> >
> > __init__ in a class is a system attribute because it has a magic
> > meaning (invoked automatically on instantiation). __file__ and
> > __name__ in a module (and __module__ and __name__ in a class!) are
> > system attributes because they are "imposing" on the user's use of the
> > namespace. (Note: next was a mistake; it should have been __next__
> > because of the "magic" rule.)

[Aahz]
> >From my POV, part of the reasoning should be the extent to which the
> attribute is intended to be publicly accessible -- part of the primary
> documented interface.  __init__ is magic, fine.  But __name__ isn't part
> of the primary use for a class, whereas these new exception attributes
> will be part of the public interface for exceptions, just like the
> methods for the Queue class.  (I'm using Queue in specific because it's
> intended to be subclassed.)

I dunno. Would you find __doc__ not part of the primary, documented
interface of classes? Or __bases__?

The Queue class is irrelevant because the VM doesn't know about it;
*almost* all system atribute are referenced by the VM (or by the
compiler) at some point(*). The reverse is not true, BTW -- many
built-in objects (e.g. tracebacks, code and function objects) have
non-system attributes.

If this were Python 3000, I'd definitely make the traceback and cause
non-system attributes. Given we're retrofitting, I'm less sure; surely
some user-defined exceptions have cause and/or traceback attributes
already.

(*)  I'd say metadata like __version__ is the exception; it's a system
attribute because it's metadata, and because it lives in a crowded
namespace, and because it was retrofitted as a convention.

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