My point is just that today, I use the ‘numbers’ package classes (Integral, Real, …) for PEP484 type-hinting, and I find it quite useful in term of input type validation (in combination with PEP484-compliant type checkers, whether static or dynamic). Adding a Boolean ABC with a similar behavior would certainly add consistency to that ‘numbers’ package – only for users who already find it useful, of course.


Note that my use case is not about converting an object to a Boolean, I’m just speaking about type validation of a ‘true’ boolean object, for example to be received as a function argument for a flag option. This is for example for users who want to define strongly-typed APIs for interaction with the ‘outside world’, and keep using duck-typing for internals.




De : Python-ideas [] De la part de Chris Barker
Envoyé : mardi 13 février 2018 21:12
À : David Mertz <>
Cc : python-ideas <>
Objet : Re: [Python-ideas] Boolean ABC similar to what's provided in the 'numbers' module




On Mon, Feb 12, 2018 at 10:07 PM, David Mertz <> wrote:

I'm not sure I'm convinced by Sylvain that Boolean needs to be an ABC in the standard library; Guido expresses skepticism.  Of course it is possible to define it in some other library that actually needs to use `isinstance(x, Boolean)` as Sylvain demonstraits in his post.  I'm not sure I'm unconvinced either, I can see a certain value to saying a given value is "fully round-trippable to bool" (as is np.bool_).


But is an ABC the way to do it? Personally, I'm skeptical that ABCs are a solution to, well, anything (as apposed to duck typing and EAFTP). Take Nick's example:



The other comparison that comes to mind would be the distinction
between "__int__" ("can be coerced to an integer, but may lose
information in the process") and "__index__" ("can be losslessly
converted to and from a builtin integer").



I suppose we could have had an Index ABC -- but that seems painful to me.


so maybe we could use a __true_bool__ special method? 


(and an operator.true_bool() function ???)


(this all makes me wish that python bools were more pure -- but way to late for that!)


I guess it comes down to whether you want to:


 - Ask the question: "is this object a boolean?"




 - Make this object a boolean


__index__ (and operator.index())  is essentially the later -- you want to make an index out of whatever object you have, if you can do so.







Christopher Barker, Ph.D.

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

This email has been scanned by the Symantec Email service.