code: def process_fields_param(fields):
      
      if not fields:
          return fields
      if isinstance(fields, six.string_types):
          fields = split_strip(fields)
      if '_type' not in fields:
          fields.append('_type')
      return {
          '_source_include': fields,
          '_source': True,
      }
docs: Process 'fields' ES param.
  
      * Fields list is split if needed
      * '_type' field is added, if not present, so the actual value is
        displayed instead of 'None'