[Python-Dev] redefining is

[Robert Brewer]

Andrew Koenig wrote:
>8
> The original author of the proposal on comp.lang.python was clearly
> disturbed at getting object identity when equivalence would 
> have made more
> sense, and was proposing to change things as follows:
> 
> 	x == y		Value equality
> 
> 	x is y		Object equivalence
> 
> 	id(x) == id(y)	Object identity
> 
> I can completely understand why this change might be 
> considered too large to make to an existing language--
> just like changing the wan C++ handles virtual
> destructors would be--but nevertheless I think it's 
> interesting to consider the abstract merits of an
> idea such as this one, perhaps for the mythical
> Python 3.0.
> 
> To put it another way:  If the three operations above 
> existed, I am finding it hard to think of many cases
> in which the third of these operations would
> be more useful than the second, which suggests that the 
> second should be easier to express.

Wonderfully well-stated, Andrew; thanks for taking the time. You've
moved this topic out of the realm of "crackpot ideas" for me. ;)

Given the above, it seems reasonable to me that, rather than change the
behavior of "is", one should introduce a new operator (or function,
etc.). That is, instead of the above, I would expect:

    Value equality: x == y

    Object identity: x is y
                 or: id(x) == id(y)

    Object equivalence: x equiv y		
                    or: equiv(x, y)

The reasons being:

1. Backward compatibility, par for the course.

2. "A is B" in English is quite specifically about identity, not
equivalence. Can you think of a case where this is not so? If you can,
then I'd bet it's because "mutual substitutability" is always true for
identical objects--the equivalence is a side-effect of being identical.
Note that I'm not talking about "A is a B" or other statements. I also
assume, for Python, that if id(x) == id(y), then (x equiv y) would also
be True.

Now if we can only find a short English word that means "mutually
substitutable". ;)



Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org