code: def persist(self, storageLevel=StorageLevel.MEMORY_AND_DISK):
          
          self.is_cached = True
          javaStorageLevel = self._sc._getJavaStorageLevel(storageLevel)
          self._jdf.persist(javaStorageLevel)
          return self
docs: Sets the storage level to persist the contents of the :class:`DataFrame` across
          operations after the first time it is computed. This can only be used to assign
          a new storage level if the :class:`DataFrame` does not have a storage level set yet.
          If no storage level is specified defaults to (C{MEMORY_AND_DISK}).
  
          .. note:: The default storage level has changed to C{MEMORY_AND_DISK} to match Scala in 2.0.