input: def feature_extraction(input_str: str) -> dict:
      """Extracts specific features from a JSON string representation of a document.
      The features extracted are the 'id', 'title', and the length of the 'body'.
      >>> feature_extraction('{"id":111,"title":"This is a sample title","body":"This is a sample body text"}')
      {'id': 111, 'title': 'This is a sample title', 'body_len': 30}
      """
output:     import json
      data = json.loads(input_str)
      return {
          'id': data['id'],
          'title': data['title'],
          'body_len': len(data['body'])
      }