I must admit that I do not understand the philosophy.
But apart from that: for classes inheriting from ClonableArray, which have
to implement equality (because there is no coercion to a common parent), do
they have to implement _richcmp_ or __richcmp__ or _eq_ or __eq__? (The
amount of magic in python / sagemath is extremely confusing to me.)
On Tuesday, 13 December 2022 at 01:09:52 UTC+1 Travis Scrimshaw wrote:
> +1 on what Nils said; I only find it slightly surprising initially.
> Compare this with comparing a list to a Partition as well. Equality is a
> bit different as a programming concept than a mathematical one. When you
> implement a custom __eq__, then you are separating that class from the
> coercion-based equality.
>
> +1 on removing the parent from the hash of ClonableArray and its family
> tree.
>
> Best,
> Travis
>
> On Tuesday, December 13, 2022 at 1:56:46 AM UTC+9 Nils Bruin wrote:
>
>> On Monday, 12 December 2022 at 00:22:30 UTC-8 axio...@yahoo.de wrote:
>>
>>> Ah, thank you so much!
>>>
>>> Just to make sure that I understand the philosophy: if two elements are
>>> supposed to compare equal, they should have parents which can be coerced to
>>> a common parent, right? For example, the following (which is current
>>> behaviour) is not really what we want:
>>>
>>> sage: A = SetPartitions(3)([[1,2],[3]])
>>> sage: B = SetPartition([[1,2],[3]])
>>> sage: A == B
>>> True
>>> sage: coercion_model.common_parent(A, B)
>>>
>> TypeError: no common canonical parent for objects with parents: 'Set
>>> partitions of {1, 2, 3}' and 'Set partitions'
>>>
>>
>>> I think that is a bit surprising, but if SetPartitions don't really
>> occur much in binary operations I think it will be a hardly visible quirk.
>> One has to be careful generalizing principles about how to implement
>> coercion/equality/hashing because we a;ready know that some of the things
>> we want lead to inconsistencies.
>>
>> When in doubt, the safe thing is definitely to NOT install coercions (it
>> can always be done later), so I'd hesitate to recommend to change the
>> behaviour above. I think you'd want at least a practical use case to
>> justify it.
>>
>> Note that the relevant conversions: A.parent()(B) and B.parent()(A) do
>> work.
>>
>> It seems to me the converse implication, if for two elements the coercion
>> framework is happy to find a common parent in which they compare equal,
>> then they should be equal in the first place, sounds a little more
>> probable, but I suspect one can get problematic situations from that too.
>>
>
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