[Python-3000] Add "generalized boolean" as ABC to PEP 3119

[Guido van Rossum]

On 10/16/07, David A. Wheeler <dwheeler at dwheeler.com> wrote:
> Hi, I'm a Python user who likes much in the upcoming Python 3000.  I wish you well! I have a few comments, though, that I hope are constructive.  Guido asked me to repost them to this mailing list for discussion.  I'll send my different comments as separate messages, so that they can be easily discussed separately.  So...
>
> In PEP 3119 (Abstract Base Classes): I suggest adding an ABC for a "generalized bool" (perhaps name it Gbool?).
>
> Any class defining __bool__ (formerly __nonzero__), or one implementing Sized (which implement __len__), would be a generalized boolean.  (Well, unless __len__ is no longer auto-called if there's no __bool__; if there's no auto-call, then I think just __bool__ would be checked, similar to how Sized works).   All numbers and collections are generalized bools, obviously; many user-created classes will NOT be generalized bools. Many functions accept generalized bools, not strictly bools, and it'd be very nice to be able to explicitly _denote_ that in a standard way.

This sounds misguided to me. While it is true that some types can
never be false, they can still be useful as a truth value: e.g. a
parameter could be either a Widget object (assuming Widgets are never
false) or None. This is used pretty commonly.

So there is absolutely nothing to test for in the type of an object --
*every* object is usable as a "generalized boolean". It therefore
becomes purely a matter of argument annotation, an area which is
explicitly left open for experimentation by PEP 3107.

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