On Sat, Feb 15, 2014 at 9:29 AM, Marko Rauhamaa <ma...@pacujo.net> wrote: > Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info>: >> On Sat, 15 Feb 2014 14:07:35 +0200, Marko Rauhamaa wrote: >>> Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info>: >>>> On Sat, 15 Feb 2014 12:13:54 +0200, Marko Rauhamaa wrote: >>>>> 5. id(x) == id(y) iff x is y >>>> >>>> # Counter-example >>>> py> x = 230000 >>>> py> idx = id(x) >>>> py> del x >>>> py> y = 420000 >>>> py> idy = id(y) >>>> py> idx == idy >>>> True >>> >>> I don't accept that as a counterexample. > >> Why? > > Nowhere do I see the violating "x is y".
You formulated your rule as a rule of inference. The logical inference from the above is that x is y, which is false even if it can't be directly tested in Python. >> All I need to do is show a case where two distinct objects have the >> same ID. > > How do you know objects are "distinct"? Myself, I would use the "is" > test. Eliding over details of how one knows that both of the literals above create objects, if the objects are separately created, then they are distinct objects. >>> That's the point. I don't think id() and "is" have any abstract >>> meaning on top of the formal axioms. >> >> Who is talking about "abstract meaning"? > > I am. I mean, "implementation-independent". > >> Object identity is simple and well-defined in Python. I don't know why >> you are so resistant to this. Read the documentation. > > It is not defined at all: > > Every object has an identity, a type and a value. An object's > identity never changes once it has been created; you may think of it > as the object's address in memory. The 'is' operator compares the > identity of two objects; the id() function returns an integer > representing its identity. > > Thus "x and y are identical" *means* "x is y" and nothing else. This notion of identity sounds useless, and if that is the way you prefer to understand it then you can safely ignore that it exists. I think that most users though inherently understand the concept of objects being distinct or identical and see the value in being able to test for this. -- https://mail.python.org/mailman/listinfo/python-list
