[Python-Dev] Fighting the theoretical randomness of "is" on immutables

Antoine Pitrou solipsis at pitrou.net
Mon May 6 15:26:56 CEST 2013

Le Mon, 6 May 2013 23:18:54 +1000,
Nick Coghlan <ncoghlan at gmail.com> a écrit :
> IIRC, Jython just delays calculating the object id() until it is
> called, and lives with it potentially being incredibly expensive to
> calculate. Is there some way PyPy can run with a model where "is" is
> defined in terms of values for immutable objects, with a lazily
> populated mapping from values to numeric ids if you're forced to
> define them through an explicit call to id()?

This sounds reasonable. Actually, for small ints, id() could simply be
a tagged pointer (e.g. "1 + 2 * myint.value").

> We're not going to change the language design because people don't
> understand the difference between "is" and "==" and then wrongly blame
> PyPy for breaking their code.

Well, if I'm doing:

  mylist = [x]

and ``mylist[0] is x`` returns False, then I pretty much consider the
Python implementation to be broken, not my code :-)



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