On Tue, Jan 13, 2015 at 12:59 PM, Andrew Robinson <andr...@r3dsolutions.com> wrote: > There is no need to copy data from an initialized superclass instance into a > subclass instance that has no new data, but only rebind -- or add a > binding/proxy object -- to bind the superclass instance to the subclass > methods. > > eg: what is now standard practice to create a new copy of the superclass: > > class myFalse( bool ): __new__( self, data ): return super( myFalse, self > ).__new__(self,data) > > Could be replaced by a general purpose proxy meant to handle singleton > subclassing: > > class myFalse( bool ): __new__( self ): return > bind_superinstance_to_subclass( False, myFalse )
I don't understand. What do you expect this to be doing? Are you trying to replace the original False with your new subclass? >> he Python bool type has the following >> invariant, for any object x: >> >> assert not isinstance(x, bool) or x is True or x is False > I mean, even right now -- with the language as-is -- let's define something > that blatantly creates a new instance of something neither an actual > instance of True nor False, and make that x -- and see if your assertion > catches it: > > Python 2.7.5 (default, May 29 2013, 02:28:51) > [GCC 4.8.0] on linux2 > Type "help", "copyright", "credits" or "license" for more information. >>>> x=(False,) >>>> assert not isinstance(x, bool) or x is True or x is False >>>> > > LOL ... no exception was raised... and we know if the assertion Failed, an > exception ought to be raised: The assertion did not fail. There are three parts, and as long as one of them is true, the assertion will pass: 1) x isn't an instance of bool 2) x is the object known as True 3) x is the object known as False You just gave an example of the first part of the invariant. That's an instance of tuple, which is not a subclass of bool, ergo isinstance(x, bool) returns False, negating that makes True, and the assertion passes. [1] > So -- your assertion, at least as shown, is pretty useless in helping > determine why subclassing is not allowed, or instances of subclasses that > are not distinct from their superclasses existing instance. It exactly defines the nature of Python's bool type: there are precisely two instances of it. ChrisA [1] Which puts me in mind of https://www.youtube.com/watch?v=D0yYwBzKAyY -- https://mail.python.org/mailman/listinfo/python-list
