Uploading lists with bulkupload in Google App Engine

In my 开发者_JS百科data model, one of my properties is a list. How do I tell the bulk loader to convert the string into a list delimited by ' ' (space)?

- property: districts
  external_name: DISTRICTS
  # Type: String Stats: 6000 properties of this type in this kind.
  import_transform: "lambda x: x.split(' ')"
  export_transform: "' '.join"