[Python-Dev] Re: The repr of a sentinel

May 13, 2021


      On 5/13/2021 1:39 PM, Tal Einat wrote:
...
On Thu, May 13, 2021 at 7:44 PM Ethan Furman <ethan@stoneleaf.us> 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
...
...
...
MISSING
<class '__main__.MISSING'>
I think a repr of just "MISSING", or maybe "dataclasses.MISSING" would 
be better.

Eric

[Python-Dev] Re: The repr of a sentinel

Eric V. Smith