[Python-ideas] Add new `Symbol` type

[Guido van Rossum]

Thanks for an interesting discussion. I would also urge people to limit the
use of such sentinels for cases where it is *really* important to
distinguish between, say, f(None) and f(). In most cases using def
f(arg=None) is fine, and often it is even a virtue that passing None or
omitting an argument has exactly the same meaning. (I do know there a cases
where this doesn't apply -- I just think they ought to be fairly unusual.)

On Fri, Jul 6, 2018 at 8:21 AM Flavio Curella <flavio.curella at gmail.com>
wrote:

> @Nathaniel Smith:
>
> > I think the name "symbol" here is pretty confusing. It comes originally
> from Lisp
> > The thing you're talking about is what Python devs call a "sentinel"
> object.
>
>  Thank you for clarifying. I don't know much about Lisp, and I definitely
> appreciate the historical context that you provided :)
>
> I will refer to the new proposed type as `sentinel` from now on.
>
> @Michael Foord
>
> > unittest.mock includes a sentinel object, with nice repr.
>
> Thank you! I didn't think of looking in the mock library.
>
> This is one step closer, but it still have a few considerations:
> 1. It doesn't follow the same behaviour as the JS spec. But honestly, we
> don't have to.
> 2. It's kinda weird to have to import `unittest.<something>` in code
> that's not tests. But I think it's just because I'm not used to see it.
>
> @Giampaolo Rodola'
> > Historically this has always been achieved by using [...] which does
> its job just fine.
>
> The main issue with that approach is that you won't get a nice repr
>
> > adding more complexity to the language for no practical benefit
>
> I'm not following. The Python language won't be modified. I'm proposing
> adding the new type _purely_ for practical benefit.
>
> I think this thread can be resolved as 'used unittest.mock.sentinel'. It
> doesn't have 'global sentinels', but I'm not convinced they are actually
> necessary, since `mock.sentinel` objects with the same name compare as
> equal. Thanks to Nathaniel, I now understand that JS has global symbols for
> historical reasons that we don't have, and I'm not convinced of their
> usefulness.
>
> Thank you everybody for you valuable feedback! I really appreciate your
> time helping me thinking this through :)
>
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-- 
--Guido van Rossum (python.org/~guido)
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