Using oslo_cache lib the only way to compare some cached_value with NO_VALUE is
to add bad import like 'from oslo_cache.backends.dictionary import NO_VALUE'.
NO_VALUE is now added to core.py file and the improved import
'oslo_cache.core import NO_VALUE' will look much better in other projects.
NO_VALUE is now public, it is added to __all__ and is used everywhere from core
instead of dogpile.cache.api.NO_VALUE.
Co-Authored-By: Sergey Nikitin <snikitin@mirantis.com>
Change-Id: I8fea4a9eb088089289ba1dd35377fb10465c313a
5881ad7cb1