input: def get_current_datetime_string() -> str:
      """Generate a string that represents the current date and time in the format 'YYYY-MM-DD HH:MM:SS'.
      
      >>> get_current_datetime_string()
      '2023-10-05 14:30:45'
      """
output:     from datetime import datetime
      return datetime.now().strftime('%Y-%m-%d %H:%M:%S')