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.
:meth:`.CacheRegion.cache_on_arguments` - applies :meth:`.get_or_create`