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

[Franklin? Lee]

On Fri, Dec 18, 2015 at 10:57 PM, Steven D'Aprano <steve at pearwood.info> wrote:

>> 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

Well, that's my point AND my example.

If this were lists, then a lack of transitivity of elements would mean
a lack of transitivity for lists of those elements. You get what you
put in.

But since dict views are unordered collections, a lack of transitivity
for elements would mean INCONSISTENCY: the comparison of two views as
multisets would depend on the exact order of comparison. Unless you
are willing to compare everything to everything else in the worst
case.