I find the idea of having the constructor potentially return something
other than an instance of the class to be very... off-putting. Maybe it's
the best option, but my first impression of it isn't favorable, and I can't
think of any similar case that exists in the stdlib today off the top of my
head. It seems like we should be able to do better.
If I might propose an alternative before this gets set in stone: what if
`Enum` provided classmethods `from_value` and `from_name`, each with a
`default=<sentinel>`, so that you could do:
Color.from_value(1) # returns Color.RED
Color.from_value(-1) # raises ValueError
Color.from_value(-1, None) # returns None
Color.from_name("RED") # returns Color.RED
Color.from_name("BLURPLE") # raises ValueError
Color.from_name("BLURPLE", None) # returns None
That still allows each concept to be expressed in a single line, and
remains explicit about whether the lookup is happening by name or by value.
It allows spelling `default=None` as just `None`, as we desire. And instead
of being a `__contains__` with unusual semantics coupled with a constructor
with unusual semantics, it's a pair of class methods that each have fairly
unsurprising semantics.
~Matt
On Mon, Mar 15, 2021 at 3:55 PM Guido van Rossum <gu...@python.org> wrote:
> +1
>
> On Mon, Mar 15, 2021 at 12:48 PM Ethan Furman <et...@stoneleaf.us> wrote:
>
>> On 3/15/21 11:27 AM, Guido van Rossum wrote:
>> > On Mon, Mar 15, 2021 at 10:53 AM Ethan Furman wrote:
>>
>> >> Part of the reason is that there are really two ways to identify an
>> >> enum -- by name, and by value -- which should `__contains__` work with?
>> >
>> > The two sets don't overlap, so we could allow both. (Funny
>> > interpretations of `__contains__` are not unusual, e.g.
>> > substring checks are spelled 'abc' in 'fooabcbar'.)
>>
>> They could overlap if the Enum is a `str`-subclass -- although having the
>> name of one member match the value of a different member seems odd.
>>
>> >> I think I like your constructor change idea, with a small twist:
>> >>
>> >> Color(value=<sentinel>, name=<sentinel>, default=<sentinal>)
>> >>
>> >> This would make it possible to search for an enum by value or by name,
>> >> and also specify a default return value (raising an exception if the
>> >> default is not set and a member cannot be found).
>> >
>> >
>> > So specifically this would allow (hope my shorthand is clear):
>> > ```
>> > Color['RED'] --> Color.RED or raises
>> > Color(1) -> Color.RED or raises
>> > Color(1, default=None) -> Color.RED or None
>> > Color(name='RED', default=None) -> Color.RED or None
>> > ```
>> > This seems superficially reasonable. I'm not sure what
>> > Color(value=1, name='RED') would do -- insist that both value and
>> > name match? Would that have a use case?
>>
>> I would enforce that both match, or raise. Also not sure what the
>> use-case would be.
>>
>> > My remaining concern is that it's fairly verbose -- assuming we don't
>> > really need the name argument, it would be attractive if we could
>> > write Color(1, None) instead of Color(1, default=None).
>> >
>> > Note that instead of Color(name='RED') we can already write this:
>> > ```
>> > getattr(Color, 'RED') -> Color.RED or raises
>> > getattr(Color, 'RED', None) -> Color.RED or None
>>
>> Very good points.
>>
>> Everything considered, I think I like allowing `__contains__` to verify
>> both names and values, adding `default=<sentinel>` to the constructor for
>> the value-based "gimme an Enum or None" case, and recommending `getattr`
>> for the name-based "gimme an Enum or None" case.
>>
>> --
>> ~Ethan~
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>
>
> --
> --Guido van Rossum (python.org/~guido)
> *Pronouns: he/him **(why is my pronoun here?)*
> <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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