:param expiration_time: optional expiration time which will overide
the expiration time already configured on this :class:`.CacheRegion`
if not None. To set no expiration, use the value -1.
+ the expiration_time argument here is **not guaranteed** to be
+ effective if multiple concurrent threads are accessing the same
+ key via :meth:`get_or_create` using different values
+ for ``expiration_time`` - the first thread within a cluster
+ of concurrent usages establishes the expiration time within a
+ :class:`.Dogpile` instance for the duration of those usages.
+ It is advised that all access to a particular key within a particular
+ :class:`.CacheRegion` use the **same** value for ``expiration_time``.
+ Sticking with the default expiration time configured for
+ the :class:`.CacheRegion` as a whole is expected to be the
+ usual mode of operation.