On Tue, Jul 7, 2020 at 6:56 AM Dominik Vilsmeier <dominik.vilsmeier@gmx.de> wrote:
Well, the point is that this "except comparisons" is not quite true:

     >>> i = {'a': []}.items()
     >>> s = {('a', 1)}
     >>> i == s
     TypeError: unhashable type: 'list'

If passed a set as `other` operand, dict_items seems to decide to
convert itself to a set, for no obvious reasons since, as you mentioned,
it does know how to compare itself to another view containing
non-hashable values:

     >>> i == {'a': {}}.items()

So if you're dealing with items views and want to compare them to a set
representing dict items, then you need an extra `try/except` in order to
handle non-hashable values in the items view. Not only does this require
an extra precautionary step, it also seems strange given that in Python
you can compare all sorts of objects without exceptions being raised. I
can't think of any another built-in type that would raise an exception
on equality `==` comparison. dict_items seems to make an exception to
that rule.

I think this really is a bug (well, missing feature). It surely, *could* be implemented to work, but maybe not efficiently or easily, so may well not be worth it -- is the use case of comparing a dict_items with another dict_items really helpful?

In fact, my first thought was that the way to do the comparison is to convert to a dict, rather than a set, and then do the compare. And then I realized that dict_items don't exist without a dict anyway, so you really should be comparing the "host" dicts anyway. Which leaves exactly no use cases for this operation.

I suppose we could add that capability by referencing the host dict if two dict_items are compared -- but really, why?

Though maybe a nicer message:

"TypeError: These views cannot be compared:  compare the dicts themselves"

 > An equality comparison between one `dict.values()` view and another
will always return `False`. This also applies when comparing
`dict.values()` to itself.

Surely that must be a relic from pre-3.7 days where dicts were unordered
and hence order-based comparison wouldn't be possible

well, maybe -- or it's not a relic, and rather a result of the fact that while dicts are now order-preserving, they are still order-not-important.

Given that:
In [90]: {'a' : 1, 'b' : 2} == {'b' : 2, 'a' : 1}                            
Out[90]: True

{'a' : 1, 'b' : 2}.values() == {'b' : 2, 'a' : 1}.values()

should also be True. but that's not possible to do correctly, since it can not be known that all the same values are there, there is no way to know that they are attached to the same keys. And if it did an order-preserving compare, then values would compare false when the dict is not the same.

So: we *could* either do an order preserving (by converting to a sequence) or not (by converting to a sequence and then sorting) comparison of the values -- but would that be at all useful? Do we ever need to ask the question:

Do these two dicts have all the same values without regard to the keys?

Now that I say that -- I suppose the answer is yes -- someone may want to ask that question -- but I think it would be better for them to ask it with their own code, so it's clear what it actually means.


Christopher Barker, PhD

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