[Python-ideas] Boolean ABC similar to what's provided in the 'numbers' module

[Guido van Rossum]

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?

--Guido


On Feb 14, 2018 12:42 AM, "Sylvain MARIE" <
sylvain.marie at schneider-electric.com> 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.



Sylvain



*De :* Python-ideas [mailto:python-ideas-bounces+sylvain.marie=schneider-
electric.com at python.org] *De la part de* Chris Barker
*Envoyé :* mardi 13 février 2018 21:12
*À :* David Mertz <mertz at gnosis.cx>
*Cc :* python-ideas <python-ideas at python.org>
*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 <mertz at gnosis.cx> 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?"



or



 - 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.



-CHB







-- 


Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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Chris.Barker at noaa.gov


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