[Python-Dev] bool does not want to be subclassed?

["Martin v. Löwis"]

Joshua Marshall wrote:
> I don't think I'm convinced; the same argument could be used for
> integers (if it doesn't make sense to create a sort of boolean which
> isn't in the set { true, false }, then it doesn't make sense to
> create a sort of integer which isn't in the set { ..., -2, -1, 0, 1,
> 2, ... }).  And maybe it doesn't, but this isn't the only reason for
> subclassing.  Another reason for subclassing is to create items which
> can act like existing objects, but which have some additional
> behavior.

And indeed, for int, it is possible to have subclasses which have new
instances whose values are in the set {..., -2, -1, 0, 1, 2, ...}.
Indeed, it is possible to have multiple instances of int *itself* whose
value is, say, 1000:

 >>> 500+500 is 500+500
False

The same is not true for bool: There are only two *instances* of the
type, not just two equivalence classes of equal values:

 >>> (4>5) is (3>9)
True

So bool guarantees: a) there are only two distinct values, and b)
there are only two different objects representing these values.
It is property b) which prohibits subclassing.

Regards,
Martin