LangChain提示词模板：MessagePlaceholder详解

LangChain 提示词模板深度解析（四）：MessagePlaceholder 与少量样本示例详解

MessagePlaceholder 的核心作用

MessagePlaceholder 是 LangChain 提示词模板中的动态占位符，允许在运行时插入对话历史或中间生成内容。与静态文本占位符不同，它专为处理可变长度的消息序列设计，适用于多轮对话或需要上下文记忆的场景。

MessagePlaceholder 的参数配置

variable_name: 指定占位符的变量名，需与输入键名匹配。
optional: 控制是否为必填字段，默认 False。若为 True，未提供输入时自动忽略该占位符。

示例代码：

from langchain.prompts import ChatPromptTemplate, MessagesPlaceholder

prompt = ChatPromptTemplate.from_messages([
    ("system", "你是一个专业助手"),
    MessagesPlaceholder(variable_name="history"),
    ("human", "{input}")
])

少量样本示例（Few-shot Prompting）实现

通过 MessagePlaceholder 可动态插入示例对话，增强模型上下文理解能力。示例需以 List[BaseMessage] 格式提供：

from langchain_core.messages import HumanMessage, AIMessage

examples = [
    HumanMessage(content="如何煮咖啡？"),
    AIMessage(content="1. 研磨咖啡豆\n2. 使用热水冲泡\n3. 等待4分钟")
]

prompt.invoke({
    "history": examples,
    "input": "如何泡茶？"
})

动态对话历史管理

结合 ConversationBufferMemory 可实现自动历史记录。每次交互后，历史消息会更新至 MessagePlaceholder：

from langchain.memory import ConversationBufferMemory

memory = ConversationBufferMemory(memory_key="history", return_messages=True)
memory.save_context({"input": "你好"}, {"output": "你好，有什么可以帮忙？"})
prompt.invoke({"input": "谢谢", "history": memory.load_memory_variables()["history"]})

性能优化注意事项

历史消息过长时需截断，避免超过模型 token 限制。
少量样本示例建议控制在 3-5 条，确保相关性并减少噪音。
可通过 MessagePromptTemplate 对示例进行格式化预处理。

典型应用场景

多轮对话系统：维护对话状态，实现连贯交互。
复杂任务分解：通过历史消息记录中间步骤结果。
示例驱动生成：动态插入领域相关样本，提升输出质量。

调试技巧

使用 prompt.format() 检查渲染后的提示词结构，确认 MessagePlaceholder 是否正确替换为预期内容。若出现变量缺失错误，检查输入字典的键名与 variable_name 是否一致。

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