The two "isinstance" checks that bother me are the ones for int and float. However, given that the JSON serialiser *already* includes those explicit checks, I now think it makes sense to just do the minimal fix of coercing subclasses to the base type for both of those cases, especially since such a change should be able to be ported to simplejson as well.

Anything more elaborate (including making JSON explicitly enum aware, or else offering a new encoder that supports singledispatch) should be handled in a separate issue.