There are a lot of examples about SQL in comments. In the SQL case you want something like:

  def process_template(template: Template) -> tuple[str, tuple]:
    sql_parts = []
    args = []
    for item in template:
      if isinstance(item, str):
        sql_parts.append(item)
      else:
        sql_parts.append("?")
        args.append(process_value(item.value))
    return "".join(sql_parts), tuple(args)

(of course it would be more nuanced, but I hope you get the point)