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@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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