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Date: Tue, 13 Jan 2015 09:35:43 +1100
Subject: Re: Comparisons and sorting of a numeric class....
From: Chris Angelico <rosuav@gmail.com>
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On Tue, Jan 13, 2015 at 9:27 AM, Andrew Robinson
<andrew3@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