On Thu, May 13, 2021 at 8:46 PM Eric V. Smith <[email protected]> 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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