On 2012-04-23, Paul Rubin 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.
"is" was never designed for comparing literals or expressions. the expression 20+30 is 30+20 maybe syntactically correct but is nevertheless pretty senseless and you can, of course, not check commutativity with it. For checking commutativity you have to use: 20+30 == 30+20 > >> 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. >>> a = something >>> b = a >>> a is b True Should be guaranteed and implementation-independent. Bernd -- "Die Antisemiten vergeben es den Juden nicht, dass die Juden Geist haben - und Geld." [Friedrich Nietzsche] -- http://mail.python.org/mailman/listinfo/python-list
