[Python-Dev] Enumeration items: type(EnumClass.item) is EnumClass ? (original) (raw)

Guido van Rossum guido at python.org
Mon Apr 29 21:04:35 CEST 2013

On Mon, Apr 29, 2013 at 11:34 AM, Larry Hastings <larry at hastings.org> wrote:

What's the problem with overriding the isinstance checks? You mention it but seem to assume it's a bad idea. That seems to me like it'd adequately solve that problem with an acceptable level of magic.

Depending on whether you are using isinstance() to check if an enum value belongs in the set of acceptable enums, or to check it it makes sense to call a certain method, you need isinstance() to behave differently.

In particular, the default isinstance() would correctly state that MoreColor.red is not a MoreColor instance, so any methods defined only on MoreColor should not be called. OTOH it gives the wrong answer if you are checking that Color.red is an acceptable MoreColor value. But if you override isinstance() to give the right answer for the latter (so isinstance(Color.red, MoreColor) is True), then you could incorrectly conclude that it is safe to call a MoreColor method on Color.red.

Please do read the StackOverflow links I gave:

http://stackoverflow.com/questions/4604978/subclassing-an-enum http://stackoverflow.com/questions/3427947/enumeration-inheritence-in-java/3428050#3428050

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

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