[Python-Dev] bool does not want to be subclassed? (original) (raw)

Guido van Rossum guido at python.org
Fri Feb 13 18:59:48 EST 2004

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/)

More information about the Python-Dev mailing list