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

[Guido van Rossum]

> 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?

Among immutable built-in types, these plus Ellipsis are the only ones.
The language doesn't guarantee such a thing for numbers other than
bool, strings, unicode or tuples.

--Guido van Rossum (home page: http://www.python.org/~guido/)