[Python-Dev] A proposal has surfaced on comp.lang.pythontoredefine "is"

[Andrew Koenig]

> Ah, but that would be definite incompatible change in semantics.  I
> would be very suprised if this printed "IS":
> 
> a = []
> b = []
> if (a, b) is (a, b): print "IS"
> else: print "ISN'T"

Indeed, it would be a change.  And I can go along with an argument that an
incompatible change of that magnitude should be rejected for that reason
alone.  But why would the change cause a problem?  Consider:

	a = []
	b = []
	x = (a, b)
	y = (a, b)

Can you think of a program that can make productive use of the value of
"x is y"?  It seems to me that x and y are mutually substitutable.