[Python-Dev] PEP-435 reference implementation (original) (raw)

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 flagcolors = set(Color) | set(ExtraColor) Now I can test "c in flagcolors" 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]

try: 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.

-- Steven

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