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: >>> 0. x is x >>> 1. if x is y then y ix x >>> 2. if x is y and y is z then x is z >>> 3. after x = y, x is y >>> 4. if x is y and x == x, then x == y >>> 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? Do you think I lied and just faked the output I put into my post? It is a clear case where two distinct objects, in this case 230000 and 420000, have the same ID. You cut out the explanation I gave explaining the example, which is crucial. Your description of identity leaves out a critical factor, namely that the objects being discussed must exist simultaneously. If you don't specify that factor, your description includes a great big hole, just as I show above. > You will have to produce: > > (id(x) == id(y)) == (x is y) > > False I don't have to produce anything of the sort. All I need to do is show a case where two distinct objects have the same ID. That is quite easy in CPython, since IDs can be re-used after objects are garbage-collected. >> (This is *implementation dependent* so your mileage my vary.) >> >>> Does that cover it? >> >> No. Your definition describes some properties of identity-equivalence, >> but doesn't explain what identity actually means. > > 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"? They have concrete meaning in Python, and extremely simple meaning at that. * id() is a function which returns an abstract implementation- dependent identity number which is unique for each object during the object's lifetime. * The "is" operator compares the two operands for identity, returning True if, and only if, they are the same object, otherwise returning False. Object identity is simple and well-defined in Python. I don't know why you are so resistant to this. Read the documentation. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
