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

Amaury Forgeot d'Arc amauryfa at gmail.com
Sun May 5 13:38:11 CEST 2013


2013/5/5 Armin Rigo <arigo at tunes.org>

> Hi all,
> I'm just wondering again about some "bug" reports that are not bugs,
> about people misusing "is" to compare two immutable objects.  The
> current situation in PyPy is that "is" works like "==" for ints,
> longs, floats or complexes.  It does not for strs or unicodes or
> tuples.  Now of course someone on python-dev was (indirectly)
> complaining that you can compare in CPython ``x is ' '``, which works
> because single-character strings are cached, but not in PyPy.  I'm
> sure someone else has been bitten by writing in CPython ``x is ()``,
> which is also cached there.

Strings are not always cached; with CPython2.7:
>>> x = u'é'.encode('ascii', 'ignore')
>>> x == '', x is ''
(True, False)

> (Fwiw I think that there is a design flaw somewhere in Python, to
> allow "1 is 1" to be executed without any error but also without any
> well-defined result...)
> Can we fix it once and for all?  It's annoying because of id: if we
> want ``x is y`` for equal huge strings x and y, but still want
> ``id(x)==id(y)``, then we have to compute ``id(some_string)`` in a
> rather slow way, producing a huge number.  The same for tuples: if we
> always want ``(1, 2) is (1, 2)`` then we need to compute
> ``id(some_tuple)`` recursively, which can also lead to huge numbers.
> In fact such a definition can explode the memory: ``a = (); for i in
> range(100): a = (a, a); id(a)`` would likely need a 2**100-digits
> number.
> Solution 2 would be to add these hacks specially for cases that
> CPython caches: I think by now we're only missing empty or single-char
> strings or unicodes, and empty tuple.
> Solution 3 would be to drop half of the rule, keeping only
> ``id(x)==id(y) => x is y``.  This would be the easiest, as we could
> remove the complicated computations already done for longs or floats
> or complexes.  We'd clearly document it as a difference from CPython.
> The question is what kind of code might break if we drop the case ``x
> is y => id(x)==id(y)``.
> A bientôt,
> Armin.
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Amaury Forgeot d'Arc
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