Many of the cache operations make use of the default argument to the
.get() operation to determine whether the key was found in the cache.
The default value of the default argument is None, so this results in
these operations assuming that None is not stored in the cache when it
actually is. Adding a sentinel object solves this issue.
Unfortunately the unmaintained python-memcached library does not support
a default argument to .get(), so the previous behavior is preserved for
the deprecated MemcachedCache backend.
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