[Python-Dev] PEP-435 reference implementation

Steven D'Aprano steve at pearwood.info
Thu May 2 04:33:06 CEST 2013

On 02/05/13 02:43, Guido van Rossum wrote:

> Here's how I would implement "extending" an enum if subclassing were
> not allowed:
> class Color(Enum):
>    red = 1
>    white = 2
>    blue = 3
> class ExtraColor(Enum):
>    orange = 4
>    yellow = 5
>    green = 6
> flag_colors = set(Color) | set(ExtraColor)
> Now I can test "c in flag_colors" to check whether c is a flag color.

Earlier you argued that testing for enums should be done with isinstance, not "in". Or did I misunderstood? So I would have thought that isinstance(c, (Color, ExtraColor)) would be the way to check c.

I would prefer to write "c in ExtraColor", assuming c extends Color.

Lookups by value also become more complex. Instead of c = ExtraColor[value], this leads to two choices, both of which are equally ugly in my opinion:

c = [c for c in flag_colors if c.value == value][0]

     c = ExtraColor[value]
except:  # I'm not sure what exception you get here
     c = Color[value]

There is a further problem if the two enum classes have duplicate values, by accident or design. Accident being more likely, since now you have no warning when ExtraColor defines a value that duplicates something in Color. flag_colors will now contain both duplicates, since enum values from different enums never compare equal, but that's probably not what you want.


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