You have a good point (and as static typing proponent I should have thought of that).
Maybe there is not actually a use case for passing an arbitrary default? Then maybe overloading __contains__ (‘in’) might be better? The ergonomics of that seem better for the dominant use case (“is this a valid value for that enum?”).
On Mon, Mar 15, 2021 at 21:37 Matt Wozniski email@example.com wrote:
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.
On Mon, Mar 15, 2021 at 3:55 PM Guido van Rossum firstname.lastname@example.org wrote:
On Mon, Mar 15, 2021 at 12:48 PM Ethan Furman email@example.com 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
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.
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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/ _______________________________________________ Python-ideas mailing list -- email@example.com To unsubscribe send an email to firstname.lastname@example.org https://mail.python.org/mailman3/lists/python-ideas.python.org/
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