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Should nested classes in an Enum be Enum members?


On 06/28/2018 01:11 AM, Steven D'Aprano wrote:
> On Wed, 27 Jun 2018 07:48:53 -0700, Ethan Furman wrote:
>
>> [Note:  there is a similar thread on Python-Ideas, if you would like to
>> respond there.]
>>
>> Consider the following Enum definition:
>>
>>     class  Color(Enum):
>>         RED = 1
>>         GREEN = 2
>>         BLUE = 3
>>         @property
>>         def lower(self):
>>             return self.name.lower()
>>         def spam(self):
>>             return "I like %s eggs and spam!" % self.lower
>>         class SomeClass:
>>             pass
>>
>> Which of the above Color attributes are enums, and which aren't?

> (In hindsight perhaps you should have called the class EnumType so that
> ambiguity would not exist. Then an enum would *always* refer to the
> members Color.RED etc, and never to Color itself.)

The problem then is the ambiguity between EnumMeta and EnumType.  :/

>> Answer:
>>
>>     - RED, GREEN, and BLUE are members
>>     - lower and spam() are not
>>     - SomeClass /is/ a member (but not its instances)
>
> Is that by accident or by design?

By design.  It is entirely possible to want an enum of types (int, float, str, etc.).


>> Question:
>>
>>     Should `SomeClass` be an enum member?  When would it be useful to
>>     have an embedded class in an Enum be an enum member?
>
> I honestly cannot think of any reason to nest a class inside of an Enum
> class. But if I did, I would probably want it to be just a regular class,
> and not an enum member.
>
> If I wanted to nest an Enum class inside an Enum class (but why???) I'd
> just inherit from Enum:
>
> class Colour(Enum):
>      class PrimaryColour(Enum):
>          RED = 1
>          GREEN = 2
>          BLUE = 3
>          OCTARINE = 8
>      class SecondaryColour(Enum):
>          PUCE = 101
>          MAUVE = 102
>          BEIGE = 103
>          TEAL = 104

This really seems to be the sticking point -- what should an Enum of Enums look like?  For example, should the above do

   --> list(Colour)
   [Colour.PrimaryColour <...>, Colour.SecondaryColour <...>]

or something else?

>> The only example I have seen so far of nested classes in an Enum is when
>> folks want to make an Enum of Enums, and the nested Enum should not
>> itself be an enum member.  Since the counter-example already works I
>> haven't seen any requests for it.  ;)
>>
>> So I'm asking the community:  What real-world examples can you offer for
>> either behavior?  Cases where nested classes should be enum members, and
>> cases where nested classes should not be members.
>
> Is this a trick question?

Heh.  Not at all.  It is entirely possible to have a real use-case which we cannot model the way we want in code.

--
~Ethan~