Can you show some sample code that you have written that shows where this would be useful?

Note that using the numbers package actually makes static type checking through e.g. mypy difficult. So I presume you are talking about dynamic checking?


On Feb 14, 2018 12:42 AM, "Sylvain MARIE" <> wrote:

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.

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