Greg Ward wrote:
Hi all --
I seem to have discovered an inconsistency in the semantics of object comparison between plain old Python instances and ExtensionClass instances. (I've cc'd python-dev because it looks as though one *could* blame Python for the inconsistency, but I don't really understand the guts of either Python or ExtensionClass enough to know.)
(demonstration snipped)
The first one and the last two are obvious, but the second one only works thanks to a trick in PyObject_Compare():
if (PyInstance_Check(v) || PyInstance_Check(w)) { ... if (!PyInstance_Check(v)) return -PyObject_Compare(w, v); ... }
However, if I make Simple an ExtensionClass:
from ExtensionClass import Base
class Simple (Base):
Then the "swap v and w and use w's comparison method" no longer works. Here's the output of the script with Simple as an ExtensionClass:
v1 == v2? no v2 == v1? Simple.__cmp__: self=<Simple at 1b51c0: 36>, other=36 yes v1 == v2.data? yes v2.data == v1? yes
It looks as though ExtensionClass would have to duplicate the trick in PyObject_Compare() that I quoted, since Python has no idea that ExtensionClass instances really should act like instances. This smells to me like a bug in ExtensionClass. Comments?
Technically, this *is* a bug in ExtensionClass, however, Python *does* deserve some of the blame. The intepreter actually uses the following rule: Instances of two different "python types" are not compared using type-supplied comparison methods unless: - At least one of the types is InstanceType or - Both of the types are numeric. This rule is rather inconvenient for ExtensionClass. It's also pretty inconvenient for non-ExtensionClass non-numeric extension types that want to be comparable with other things. You can work around this by making your ExtensionClass a numeric type. This should be possible by providing a numeric method. Jim -- Jim Fulton mailto:jim@digicool.com Python Powered! Technical Director (888) 344-4332 http://www.python.org Digital Creations http://www.digicool.com http://www.zope.org Under US Code Title 47, Sec.227(b)(1)(C), Sec.227(a)(2)(B) This email address may not be added to any commercial mail list with out my permission. Violation of my privacy with advertising or SPAM will result in a suit for a MINIMUM of $500 damages/incident, $1500 for repeats.