input: def feature_extraction(input_str: str) -> dict:
      """Extract features from a JSON string containing an id, title, and body.
      The function should return a dictionary with 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'])}