old_contents: def url_path_join(url, path_part):
      """Join a path part to the end of a URL adding/removing slashes as necessary."""
      result = url + '/' if url[-1] != '/' else url
      index = 1 if path_part[0] == '/' else 0
      return result + path_part[index:]
  
new_contents: def url_path_join(url, path_part):
      """Join a path part to the end of a URL adding/removing slashes as necessary."""
      result = url + '/' if not url.endswith('/') else url
      index = 1 if path_part.startswith('/') else 0
      return result + path_part[index:]
  
subject: Use startswith/endswith instead of indices for readability.