[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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