There was a discussion a while back ( a year or so?? ) on Python-ideas that
introduced the idea of having more "sentinel-like" singletons in Python --
right now, we only have None.
I can't remember the context, but the consensus seemed to be that it
was easy to create a custom sentinel object, and it was not worth adding
more "official" ones to the language. But this conversation reminded me
about that, and while I do agree that we don't need more that are elevated
to the status of None, maybe it would be good to have a couple (or only
MISSING) in the standard library somewhere "central" for everyone to use.
"central" rather than in, say, dataclasses.
I'm not sure where that should be, the operator module, maybe??
Ayway, if someone were to put one of the nifty implementations being
discussed here in the stdlib --I'd use it :-)
-CHB
On Thu, May 13, 2021 at 1:14 PM Tal Einat <talei...@gmail.com> wrote:
> On Thu, May 13, 2021 at 8:46 PM Eric V. Smith <e...@trueblade.com> wrote:
> >
> >
> > On 5/13/2021 1:39 PM, Tal Einat wrote:
> > > Here is my suggestion (also posted on the related bpo-44123), which is
> > > also simple, ensures a single instance is used, even considering
> > > multi-threading and pickling, and has a better repr:
> > >
> > > class Sentinel:
> > > def __new__(cls, *args, **kwargs):
> > > raise TypeError(f'{cls.__qualname__} cannot be instantiated')
> > >
> > > class MISSING(Sentinel):
> > > pass
> >
> > >>> MISSING
> > <class '__main__.MISSING'>
> >
> > I think a repr of just "MISSING", or maybe "dataclasses.MISSING" would
> > be better.
>
> The repr uses whatever module the class is defined in, so we'd get:
>
> >>> from dataclasses import MISSING
> >>> MISSING
> <class 'dataclasses.MISSING'>
>
> We could override that to something even cleaner with a meta-class. For
> example:
>
> class Sentinel(type):
> @classmethod
> def __prepare__(cls, name, bases, **kwds):
> d = super().__prepare__(name, bases, **kwds)
> def __new__(cls_, *args, **kwargs):
> raise TypeError(f'{cls_!r} is a sentinel and cannot be
> instantiated')
> d.update(__new__=__new__)
> return d
>
> def __repr__(cls):
> return f'{cls.__module__}.{cls.__qualname__}'
>
> Which results in:
>
> >>> from dataclasses import MISSING
> >>> MISSING
> dataclasses.MISSING
> >>> type(MISSING)
> <class 'sentinels.Sentinel'>
> >>> MISSING()
> Traceback (most recent call last): ...
> TypeError: dataclasses.MISSING is a sentinel and cannot be instantiated
>
> - Tal
>
>
> - Tal
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--
Christopher Barker, PhD (Chris)
Python Language Consulting
- Teaching
- Scientific Software Development
- Desktop GUI and Web Development
- wxPython, numpy, scipy, Cython
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