Hi Tal,
Would it make sense to have an unique singleton for such sentinel, a
built-in singleton like None or Ellipsis? I propose the name
"Sentinel".
Sentinel would be similar to None, but the main property would be that
"Sentinel is None" is false :-)
The stdlib contains tons of sentinels:
* _collections_abc: __marker__
* cgitb.__UNDEF__
* configparser: _UNSET
* dataclasses: _HAS_DEFAULT_FACTORY, MISSING, KW_ONLY
* datetime.timezone._Omitted
* fnmatch.translate() STAR
* functools.lru_cache.sentinel (each @lru_cache creates its own sentinel object)
* functools._NOT_FOUND
* heapq: temporary sentinel in nsmallest() and nlargest()
* inspect._sentinel
* inspect._signature_fromstr() invalid
* plistlib._undefined
* runpy._ModifiedArgv0._sentinel
* sched: _sentinel
* traceback: _sentinel
There are different but similar use cases:
* Optional parameter: distinguish between func() and func(arg=value),
a sentinel is useful to distinguish func() from func(arg=None)
* Look into a data structure for a value and store the result in a
value, distinguish if 'result' variable was set ("result is not None"
doesn't work since None is a value). Quick example: "missing =
object(); tmsg = self._catalog.get(message, missing); if tmsg is
missing: ..."
Special cases:
* dataclases._EMPTY_METADATA = types.MappingProxyType({})
* string._sentinel_dict = {}
* enum: _auto_null = object()
Victor
On Thu, May 13, 2021 at 7:40 PM Tal Einat <[email protected]> wrote:
>
> On Thu, May 13, 2021 at 7:44 PM Ethan Furman <[email protected]> wrote:
> >
> > Consider me complaining. ;-)
>
> +1
>
> > An actual Sentinel class would be helpful:
> >
> > >>> class Sentinel:
> > ... def __init__(self, repr):
> > ... self.repr = repr
> > ... def __repr__(self):
> > ... return self.repr
> > ...
> >
> > >>> MISSING = Sentinel('MISSING')
> > >>> MISSING
> > MISSING
> >
> > >>> implicit = Sentinel('<implicit>')
> > >>> implicit
> > <implicit>
>
> 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
>
> - Tal
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