[Python-Dev] Pre-PEP: Exception Reorganization for Python 3.0

[Brett Cannon]

On 7/29/05, Robert Brewer <fumanchu at amor.org> wrote:
> Brett Cannon wrote:
> > New Hierarchy
> > =============
> >
> > Raisable (new; rename BaseException?)
> > +-- CriticalException (new)
> >     +-- KeyboardInterrupt
> >     +-- MemoryError
> >     +-- SystemExit
> >     +-- SystemError (subclass SystemExit?)
> 
> I'd recommend not subclassing SystemExit--there are too many programs
> out there which expect the argument (e.g. sys.exit(3)) to mean something
> specific, but that expectation doesn't apply at all to SystemError.
> 

Don't forget this is Python 3.0; if it makes more sense we can break code.

> > +-- Exception (replaces StandardError)
> >...
> >     +-- ControlFlowException (new)
> 
> I'd definitely appreciate ControlFlowException--there are a number of
> exceptions in CherryPy which should subclass from that. Um, CherryPy
> 3.0, that is. ;)
> 

=)  Well, assuming Guido thinks this is enough of a possible use case.

> >     +-- LookupError (better name?)
> 
> SubscriptError, perhaps? But LookupError could become the "I didn't find
> obj X in the container you specified" superclass, in which case
> LookupError is perfect.
> 

That's what it's meant for and renaming it GetitemError doesn't
exactly clarify it for newbies.  =)  I doubt something better is going
to be found.

> >     +-- TypeError
> >         +-- AttributeError (subclassing new)
> 
> "Since most attribute access errors can be attributed to an object not
> being the type that one expects, it makes sense for AttributeError to
> be considered a type error."
> 
> Very hmmm. I would have thought it would be a LookupError instead, only
> because I favor the duck-typing parts of Python. Making AttributeError
> subclass from TypeError leans toward stronger typing models a bit too
> much, IMO.
> 

This idea has been brought up before on several separate occasions and
this subclassing was what was suggested.

It seems a decision needs to be made as to whether the lack of an
attribute is a failure of using the wrong type of object, just a
failure to find something in an object, or a failure to find a name in
a namespace.  I lean towards the first one, you like the second, and
Guido seems to like the third.  $20 says Guido's opinion in the end
will matter more than ours.  =)

> >     +-- WeakReferenceError (rename for ReferenceError)
> 
> This also has a LookupError feel to it.
> 

Nope, as Fred explains in his own email.

> > It has been argued that Raisable is a bad name to use since it is not
> > an actual word [python-dev1]_.
> 
> Perhaps, but compare http://www.onelook.com/?w=raisable to
> http://www.onelook.com/?w=raiseable. The only sources "raiseable" has
> going for it are Dictionary.com (which aggregates lots of questionable
> sources), Rhymezone, and LookWAYup. I think "raisable" is the clear
> winner.
> 

And Guido seems fine with it, so unless someone can convince Guido
otherwise the PEP will go with Raisable.

-Brett