[Python-ideas] Fwd: Why do equality tests between OrderedDict keys/values views behave not as expected?

[Steven D'Aprano]

On Fri, Dec 18, 2015 at 10:28:52PM -0500, Franklin? Lee wrote:
> On Fri, Dec 18, 2015 at 9:34 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> > On Fri, Dec 18, 2015 at 08:37:09AM -0800, Guido van Rossum wrote:
> >
> >> ValuesView is not a set because there may be duplicates. But the identity
> >> thing feels odd. (Even though I designed this myself.) Maybe because values
> >> may not be comparable?
> >
> > Right, that makes sense now, and it's even documented that value views
> > are not treated as sets:
> >
> > https://docs.python.org/2/library/stdtypes.html#dictionary-view-objects
> >
> >
> > I'm not sure what you mean by "values may not be comparable"? Since
> > we're only talking about equality, aren't all values comparable?
> >
> >
> > --
> > Steve
> 
> See my example in the other email
> (https://mail.python.org/pipermail/python-ideas/2015-December/037498.html).
> That's a case where the order of comparison matters, so you can't do a
> conceptual "unordered comparison" without, in the worst case,
> comparing everything to everything else. This is due to custom __eq__
> (by OrderedDict, for irony's sake): a == b and b == c does not mean a
> == c.

I don't know what Guido means by "values might not be comparable", but 
your example is lack of transitivity.

Mathematical equality is transitive: if a == b, and b == c, then a == c. 
But that doesn't extend to non-numeric concepts of equality, e.g. 
preference ordering, or other forms of ranking. Since Python __eq__ can 
be overridden, we cannot assume that equality of arbitrary objects is 
necessarily transitive. And indeed, even with Mappings they are not:

py> from collections import OrderedDict as odict
py> a = odict([('a', 1), ('b', 2)])
py> b = dict(a)
py> c = odict([('b', 2), ('a', 1)])
py> a == b == c
True
py> a == c
False


-- 
Steve