This is a small class that improves upon using a plain object as a sentinel for function arguments when None is a valid parameter and can't be used as the default. The improvements vs. a plain object are:
- Better
__str__and__repr__for better messages in logs and stacktraces. Instead of something like'<object object at 0x7ffb4d50e830>'you get'<not_set>' - Sentinels are picklable
See: https://en.wikipedia.org/wiki/Sentinel_value Inspired by: https://pypi.org/project/sentinels/
.. autoclass:: collections_extended.sentinel