On Mon, Mar 15, 2021 at 10:53 AM Ethan Furman <ethan@stoneleaf.us> wrote:
On 3/12/21 5:28 PM, Guido van Rossum wrote:
> On Fri, Mar 12, 2021 at 1:52 PM Ethan Furman wrote:

>> A question that comes up quite a bit on Stackoverflow is how to test
>> to see if a value will result in an Enum member, preferably without
>> having to go through the whole try/except machinery.
>> A couple versions ago one could use a containment check:
>>     if 1 in Color:
>> but than was removed as Enums are considered containers of members,
>> not containers of the member values.
> Maybe you were a bit too quick in deleting it. Was there a serious
> bug that led to the removal? Could it be restored?

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'.)
>> At this point I see three options:
>> 1) add a `get(value, default=None)` to EnumMeta (similar to `dict.get()`
> But the way to convert a raw value to an enum value is Color(1), not
> Color[1], so Color.get(1) seems inconsistent.

Very good point.

> Maybe you can just change the constructor so you can spell this as
> Color(1, default=None) (and then check whether that's None)?

An interesting idea.

>> 2) add a recipe to the docs
> But what would the recipe say? Apparently you're looking for a one-liner,
> since you reject the try/except solution.

The recipe would be for a method that could be added to an Enum, such as:

     def get_by_value(cls, value, default=None):
         for member in cls:
             if member.value == value:
                 return member
         return default

But that's a non-solution -- people can figure out how to write such a helper just fine (although probably using try/except) but they don't want to -- they have *one* line where they want to do this check and so they're going for a local solution -- probably the try/except.
>> 3) do nothing
> Always a good option. :-)

Yes, but not always a satisfying one.  :)

>  Where's that StackOverflow item? How many upvotes does it have?

93 - How do I test if int value exists in Python Enum without using try/catch?

25 - How to test if an Enum member with a certain name exists?

  3 - Validate value is in Python Enum values

  2 - How to check if string exists in Enum of strings?

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?

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

--Guido van Rossum (python.org/~guido)