Hi Ilya, On Sun, May 5, 2013 at 10:11 PM, Ilya Osadchiy <osadchiy.i...@gmail.com> wrote: > If the "id(x)==id(y)" requirement is removed, does it mean that "x is y" for > immutable types is simply "x==y"? > So if we have ``a = (); for i in range(100): a = (a, a); b = (a, a)`` then > "a is b" will be computationally expensive?
It's not exactly ``x==y``: for tuples it means recursively checking that items are ``is``-identical. It's possible to avoid the computational explosion, like CPython did for equality a long time ago (up to maybe 2.3?) before it was removed. You basically want to check if the a and b objects are "in a bisimulation" or not, which can be done without visiting the same object more than once, for any connexion graph. The reason I think it's a good idea (or at least not a bad idea) to reintroduce the complexity of bisimulation where CPython removed it, is that the purpose is slightly different and not visible to the user at all. If I remember correctly it was removed because it had hard-to-explain effects on when and how many times the user's ``__eq__()`` methods were called; but there is no user-overridable code involved here, merely an "implementation detail". It could equivalently be solved by aggressively caching all tuple creation. A bientôt, Armin. _______________________________________________ pypy-dev mailing list pypy-dev@python.org http://mail.python.org/mailman/listinfo/pypy-dev
