[Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

On Sun, Jan 7, 2018 at 9:09 AM, Eric V. Smith <eric at trueblade.com> wrote:

> On 1/3/2018 1:17 PM, Eric V. Smith wrote:
>
>> I’ll open an issue after I have time to read this thread and comment on
>> it.
>>
>
> https://bugs.python.org/issue32513
> I need to think though how __eq__ and __ne__ work, as well as the ordering
> operators.
>
> My specific concern with __ne__ is that there's one flag to control their
> generation, but python will use "not __eq__" if you don't provide __ne__. I
> need to think through what happens if the user only provides __eq__: does
> dataclasses do nothing, does it add __ne__, and how does this interact with
> a base class that does provide __ne__.


Maybe dataclasses should only ever provide __eq__ and always assume
Python's default for __ne__ kicks in? If that's not acceptable (maybe there
are cases where a user did write an explicit __ne__ that needs to be
overridden) I would recommend the following rule:

- If there's an __eq__, don't do anything (regardless of whether there's an
__ne__)
- If there no __eq__ but there is an __ne__, generate __eq__ but don't
generate __ne__
- If neither exists, generate both

-- 
--Guido van Rossum (python.org/~guido)
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