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

Joshua Marshall Joshua.Marshall at mathworks.com
Fri Feb 13 18:41:04 EST 2004


[Joshua Marshall]
> 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.

[Martin]
> 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.

Ah I misread Guido's comment--he's talking about the actual objects, not
what they represent.

So a different question...  Can it be relied upon that two expressions
which both evaluate to False both return the same object?  That is, is
it incorrect for a Python interpreter not to do this?  I find this in
the Python Reference Manual: "for immutable types, operations that
compute new values may actually return a reference to any existing
object with the same type and value" (note the "may").  Are bool and
NoneType the only types for which this is reliably the case?



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