method) will call upon the value creation function after this
time period has passed since the last generation.
- The expiration_time is only stored *locally*,
- within the :class:`.CacheRegion` instance, and not
- in the cache itself. Only the *creation time* of a
- particular value is stored along with it in the cache.
- This means an individual :class:`.CacheRegion` can
- be modified to have a different expiration time, which
- will have an immediate effect on data in the cache without
- the need to set new values.
:param arguments: Optional. The structure here is passed
directly to the constructor of the :class:`.CacheBackend`
in use, though is typically a dictionary.