Christopher Barker writes:
> > The current design is that sentinels with the same name from the same
> > module will always be identical. So for example `Sentinel("name") is
> > Sentinel("name")` will be true.
>
> Hmm -- so it's a semi-singleton -- i.e. behaves like a singlton, but only
> in a particular module namespace.
I don't see why this is a problem. Sentinels are "out of band"
signals, they're invalid data *for the problem at hand*. That doesn't
mean they're *always* invalid data. For example, Unicode surrogate
characters are invalid in conforming streams of characters like Python
str, but Python uses them internally anyway as "sentinels" meaning
"byte value #XX encountered in input, but is invalid there". (You
might not agree that these are sentinels, but I'm hard put to see why
my interpretation is incorrect.) And, of course, Unicode itself uses
them in concrete representations of characters, specifically UTF-16.
So sentinel usage is fundamentally private to an application as far as
I can see, and the sentinel value is fundamentally arbitrary, as long
as it's not valid data for the application.
> I would like to see a handful of common sentinels defined. Is there a plan
> to put a few that are useful for the stdlib in the `sentinel`
> module?
I don't think this works. You end up with the problem that None has
as a sentinel: mostly it's great, but occasionally application's
interpretation gets in the way of the sentinel semantics because in
the end Python considers it a valid datum. For modules to be
composable, they need to agree on the *exact* semantics of the
sentinel. If a convention based on one of the true singletons (None,
Ellipsis, True, False) doesn't work, you probably want a private
sentinel anyway.
IMO YMMV
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