[Python-Dev] Concerns about method overriding and subclassing with dataclasses
Eric V. Smith
eric at trueblade.com
Sun Jan 21 21:50:52 EST 2018
On 1/7/2018 12:25 PM, Guido van Rossum wrote:
> On Sun, Jan 7, 2018 at 9:09 AM, Eric V. Smith <eric at trueblade.com
> <mailto: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 <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
I've added my proposal on issue 32513:
It's long, so I won't repeat it here. The only really confusing part is
__hash__ and its interaction with __eq__.
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