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 <taleinat@gmail.com> 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
- Tal _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/URFRF634... Code of Conduct: http://python.org/psf/codeofconduct/
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