Tal Einat <taleinat+pyt...@gmail.com> added the comment:
Here is what I suggest working with sentinels would look like:
>>> from dataclasses import MISSING
>>> MISSING
dataclasses.MISSING
>>> M2 = pickle.loads(pickle.dumps(MISSING))
>>> M2
dataclasses.MISSING
>>> M2 is MISSING
True
Here's an implementation which ensures a single instance is used, even
considering multi-threading and pickling, which sets a nice repr according to
the module and class name:
try:
from threading import Lock
except ImportError:
class Lock:
def __enter__(self):
pass
def __exit__(self, exc_type, exc_value, traceback):
pass
class Sentinel:
_instance = None
_lock = Lock()
def __new__(cls):
if cls._instance is None:
with cls._lock:
if cls._instance is None:
cls._instance = super().__new__(cls)
return cls._instance
def __repr__(self):
*path_parts, classname = self.__class__.__qualname__.split('.')
return '.'.join([self.__class__.__module__, *path_parts,
classname.removeprefix('_')])
class _MISSING(Sentinel):
pass
MISSING = _MISSING()
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44123>
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