[Python-Dev] PEP 557: Data Classes

Ethan Furman ethan at stoneleaf.us
Mon Sep 11 18:16:57 EDT 2017

On 09/11/2017 03:00 PM, Barry Warsaw wrote:
> On Sep 10, 2017, at 20:08, Nathaniel Smith wrote:
>> I've sometimes wished that attrs let me control whether it generated equality methods (eq/ne/hash) separately from ordering methods (lt/gt/...). Maybe the cmp= argument should take an enum with options none/equality-only/full?
> I have had use cases where I needed equality comparisons but not ordered comparisons, so I’m in favor of the option to split them.  (atm, I can’t bring up a specific case, but it’s not uncommon.)
> Given that you only want to support the three states that Nathaniel describes, I think an enum makes the most sense, and it certainly would read well.  I.e. there’s no sense in supporting the ordered comparisons and not equality, so that’s not a state that needs to be represented.
> I’d make one other suggestion here: please let’s not call the keyword `cmp`.  That’s reminiscent of Python 2’s `cmp` built-in, which of course doesn’t exist in Python 3.  Using `cmp` is just an unnecessarily obfuscating abbreviation.  I’d suggest just `compare` with an enum like so:
> enum Compare(enum.Enum):
>      none = 1
>      unordered = 2
>      ordered = 3

I like the enum idea (suprise! ;) but I would suggest "equal" or "equivalent" instead of "unordered"; better to say what 
they are rather than what they are not.


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