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 

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 

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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