On Tue, Jan 13, 2015 at 9:27 AM, Andrew Robinson <andr...@r3dsolutions.com> wrote: > Huh? I'm not adding any values when I merely subclass bool ; and even if the > subclass could be instantiated -- that's doesn't mean a new value or > instance of the base class (bool) must exist. For I could happily work with > a new subclass that contains no new data, but only an already existing > instance of 'True' or 'False' as its value source. That means there is no > new value... but at most (and even that could be worked around) a new > instance of a subclass containing an existing instance of it's base class.
If you subclass bool and instantiate your subclass, you have made a new instance of bool, because every instance of a subclass is an instance of its superclass. The Python bool type has the following invariant, for any object x: assert not isinstance(x, bool) or x is True or x is False (You can fiddle with this in Py2 by rebinding the names True and False, but you could replace those names with (1==1) and (1==0) if you want to be completely safe. Likewise, the name "bool" could be replaced with (1==1).__class__ to avoid any stupidities there. But conceptually, that's the invariant.) Subclassing bool breaks this invariant, unless you never instantiate the subclass, in which case it's completely useless. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
