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

[Franklin? Lee]

On Fri, Dec 18, 2015 at 11:10 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Fri, Dec 18, 2015 at 10:43:24PM -0500, Franklin? Lee wrote:
>> On Fri, Dec 18, 2015 at 10:38 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>> > On Fri, Dec 18, 2015 at 05:59:39PM -0500, Franklin? Lee wrote:
>> >
>> >> I see two options:
>> >> - comparison is explicitly NotImplemented. Any code that used it
>> >> should've used `is`.
>> >
>> > We're still talking about equality between Mapping.values() views,
>> > correct?
> [...]
>> First, failing fast. I see this as a silent error waiting to happen.
>
> Franklin, could you please try to be a little bit more explicit about
> what "it" and "this" is when you describe something? I find it very hard
> to understand what *specific* thing you are referring to when you refer
> to it using short-hand.
>
> You say that "this" is a silent error waiting to happen. Does "this"
> refer to your suggestion that "comparison is explicitly NotImplemented",
> or something else? I can't tell.

Sorry, bad habit. I see dict().values().__eq__ as an error.


>> Second, "NotImplemented" allows the other side to try its __eq__.
>
> Okay, that makes better sense. I tought you meant that
> Mapping.ValuesView *did not implement* an __eq__ method, so that it
> raised an exception if you tried to compare them:
>
> # What I thought you meant
> a.values() == b.values()
> => raises an exception
>
>
> Now I understand that you mean they should return NotImplemented
> instead. But in practice, that doesn't actually change the behaviour
> that much: if both sides return NotImplemented, Python will fall back to
> the default behaviour, which is identity.

I didn't realize this.

Since explicitly raising NotImplemented would cause it NOT to try the
reflected method, I withdraw the suggestion of making the comparison
fail in any way that it doesn't already.