[pypy-dev] x is y <=> id(x)==id(y)

Jacob Hallén jacob at openend.se
Mon May 6 00:10:18 CEST 2013

Personally, I think that being implementation detail compatible with CPython 
is the way tio go if we want to achieve maximum popularity in the short run.

Making a sane implementation (Armins third option) is the one that I think 
will serve the Python community in the best way in the long run. Using "is" as 
a comparison when you mean "==" is a bad meme that has been very hard to get 
rid of.

The person who's opinion on this matter that I would value the most is 
Guido's. I suggest asking him.


Sunday 05 May 2013 you wrote:
> Hi all,
> On Sun, May 5, 2013 at 9:16 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> >> It's true what you're saying, but we consistently see bug reports
> >> about people comparing ints or strings with is and complaining that
> >> they work fine on cpython, but not on pypy.
> > 
> > Then their code is buggy, not PyPy. But you know that :-)
> This is precisely what this thread is about: such "buggy" code that
> uses "is" to compare two immutable objects.  At this point, the
> question is not "would it cause any trouble in existing programs to
> say that "x is not y" when CPython in the same program says that "x is
> y", because we know that the answer to that is "yes".
> We already found out a perfectly reasonable fix for "small" objects: two
> equal ints are always "is"-identical and have the same id() in PyPy.
> This is a nice way to solve the above problem.  If anything it creates
> the opposite problem: some code that works on PyPy might not work on
> CPython.  If PyPy becomes used enough, CPython will then have to care
> about that too, and we'll end up with a well-defined definition of
> "is" on immutable objects :-)
> But we're not (yet) using the same idea on *all* types of immutable
> objects.  So what we're concerned about now is whether it could be
> implemented efficiently: the answer could be "yes if we forget about
> strictly enforcing "x is y <=> id(x) == id(y)".  So, the question:
> although it's documented to be wrong, would it actually cause any
> trouble to relax this requirement?
> > a = b = X   # regardless of what X is
> > mylist = [a, None]
> > assert mylist[0] is a
> > assert mylist[0] is b
> > 
> > both assertions must pass, no matter what X is, whether mutable or
> > immutable.
> I *think* that in this case the assertions cannot fail in PyPy either.
> If X is a string, then we get as "mylist[0]" an object that is a
> different W_StringObject but containing internally the same
> RPython-level string, and as such (because we tweaked "is") they
> compare "is"-identical.  But that seems like a problem waiting to
> happen: if in the future we're using a list strategy for a list
> of single characters, then W_StringObjects containing single
> characters will be rebuilt out of an RPython list of characters, and
> not be "is"-identical under our current definition.
> In addition, the problem right now is about code like ``if x[5] is
> '.': ...`` which happens to work as expected on CPython, but not on
> PyPy.  In PyPy's case the two strings x[5] and '.' are using different
> RPython-level strings.
> A bientôt,
> Armin.
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