<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def oldest(self):
 """ Gets key, value pair for oldest item in cache :returns: tuple """

        if len(self._store) == 0:
            return

        kv = min(self._store.items(), key=lambda x: x[1][1])

        return kv[0], kv[1][0]