code: def _prettify_list(items):
      
      assert isinstance(items, list)
  
      keys_list = 'Available Keys:'
      for item in items:
          keys_list += '\n  - {0}'.format(item)
      return keys_list
docs: Return a human readable format of a list.
  
      Example:
  
      Available Keys:
        - my_first_key
        - my_second_key