[Python-Dev] Second post: PEP 557, Data Classes

[Eric V. Smith]

On 11/26/2017 3:35 AM, Eric V. Smith wrote:
> On 11/26/2017 12:23 AM, Carl Meyer wrote:
>> A couple minor
>> questions:
>>
>>> If compare is True, then eq is ignored, and __eq__ and __ne__ will be
>> automatically generated.
>>
>> IMO it's generally preferable to make nonsensical parameter combinations
>> an immediate error, rather than silently ignore one of them. Is there a
>> strong reason for letting nonsense pass silently here?
>>
>> (I reviewed the previous thread; there was a lot of discussion about
>> enums/flags vs two boolean params, but I didn't see explicit discussion
>> of this issue; the only passing references I noticed said the invalid
>> combo should be "disallowed", e.g. Guido in [1], which to me implies "an
>> error.")
> 
> I think you're right here. I'll change it to a ValueError.

While creating an issue for this 
(https://github.com/ericvsmith/dataclasses/issues/88), it occurs to me 
that the class-level parameter really should be "order" or "orderable", 
not "compare". It made more sense when it was called "cmp", but 
"compare" now seems wrong.

Because "eq" says "can I compare two instances", and what's currently 
called "compare" is "can I order two instances". Nick had a similar 
suggestion before the PEP was written 
(https://mail.python.org/pipermail/python-ideas/2017-May/045740.html).

The field-level parameter should stay "compare", because it's used for 
both __gt__ and friends, as well as __eq__ and __ne__. It's saying "is 
this field used in all of the comparison methods".

Eric.