On Mon, Apr 23, 2012 at 1:01 PM, Paul Rubin <no.email@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. >
The "is" operator is perfectly defined. But it doesn't check to see whether two objects hold equivalent values, it checks whether they are the same thing. You're not interested in whether 20+30 and 30+20 are the same object, you're interested in whether they return equivalent values which should be checked with ==. > > 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. > -- And if I try running "from java.util import ArrayList" in CPython it will give me an import error. It doesn't mean that the import mechanism is broken because it returns different results in different implementations of Python. -- http://mail.python.org/mailman/listinfo/python-list
