why () is () and [] is [] work in other way?

[Ian Kelly]

On Mon, Apr 23, 2012 at 11:01 AM, Paul Rubin <no.email at nospam.invalid> wrote:
> Kiuhnm <kiuhnm03.4t.yahoo.it> writes:
>> I can't think of a single case where 'is' is ill-defined.
>
> If I can't predict the output of
>
>    print (20+30 is 30+20)  # check whether addition is commutative
>    print (20*30 is 30*20)  # check whether multiplication is commutative
>
> by just reading the language definition and the code, I'd have to say
> "is" is ill-defined.

It seems to me that one could equally argue that "+" is ill-defined,
because the output of "x + y" could just as easily be 1729 or 42.  In
either case the problem is not in the operator, but in determining
exactly what the left and right sides evaluate to.