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 <me...@gnosis.cx>
Cc : python-ideas <email@example.com>
Objet : Re: [Python-ideas] Boolean ABC similar to what's provided in the
On Mon, Feb 12, 2018 at 10:07 PM, David Mertz
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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