code: def persist(self, storageLevel=StorageLevel.MEMORY_ONLY):
          
          self.is_cached = True
          javaStorageLevel = self.ctx._getJavaStorageLevel(storageLevel)
          self._jrdd.persist(javaStorageLevel)
          return self
docs: Set this RDD's storage level to persist its values across operations
          after the first time it is computed. This can only be used to assign
          a new storage level if the RDD does not have a storage level set yet.
          If no storage level is specified defaults to (C{MEMORY_ONLY}).
  
          >>> rdd = sc.parallelize(["b", "a", "c"])
          >>> rdd.persist().is_cached
          True