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 <talei...@gmail.com> wrote:
On Thu, May 13, 2021 at 7:44 PM Ethan Furman <et...@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-le...@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/URFRF634732GRICGLRPGJEJON2BYQZM4/
Code of Conduct: http://python.org/psf/codeofconduct/
_______________________________________________
Python-Dev mailing list -- python-dev@python.org
To unsubscribe send an email to python-dev-le...@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/SJ45ED57TNPLFCWXAREUGRKSPTPPJYJI/
Code of Conduct: http://python.org/psf/codeofconduct/
