On 14. 05. 21 10:55, Victor Stinner wrote:
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 :-)
If you need your Sentinel to be different from one particular sentinel (None), you'll usually want it to be different from all the other ones as well. A sentinel for an optional parameter shouldn't really be used at all outside of the function it's defined for. That's why it's usually defined as a private module-level variable. Perhaps it would be beneficial to provide a common base class or factory, so we get a good repr. But I don't think another common value like None and Ellipsis would do much good.
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
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