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

[Paul Rubin]

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.

> You're blaming 'is' for revealing what's really going on. 'is' is
> /not/ implementation-dependent. It's /what's going on/ that's
> implementation-dependent.
> "a is b" is true iff 'a' and 'b' are the same object. Why should 'is'
> lie to the user?

Whether a and b are the same object is implementation-dependent.