原文地址：【LangChain系列 9】Prompt模版——MessagePromptTemplate
本文速读：
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MessagePromptTemplate
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MessagesPlaceholder
在对话模型(chat model) 中， prompt主要是封装在Message中，LangChain提供了一些MessagePromptTemplate，方便我们直接使用Message生成prompt。
01 MessagePromptTemplate
LangChain提供了几种类别的MessagePromptTemplate，比较常见的有：
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AIMessagePromptTemplate
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SystemMessagePromptTemplate
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HumanMessagePromptTemplate
上面3种分别表示固定某种角色的Message模版，如果你需要自己来指定任意角色的话可以用ChatMessagePromptTemplate，这样就可以指定角色的名称，比如下面的代码，指定了角色名称为 Jedi。
from langchain.prompts import ChatMessagePromptTemplateprompt = "May the {subject} be with you"chat_message_prompt = ChatMessagePromptTemplate.from_template(role="Jedi", template=prompt)
chat_message_prompt.format(subject="force")ChatMessage(content='May the force be with you', additional_kwargs={}, role='Jedi')
02 MessagesPlaceholder
同时，LangChain还为Message提供了占用符，我们可以使用MessagesPlaceholder来作为Message在占位符，这样我们可以根据实际的需要，在格式化prompt的时候动态地插入Message。
from langchain.prompts import MessagesPlaceholderhuman_prompt = "Summarize our conversation so far in {word_count} words."
human_message_template = HumanMessagePromptTemplate.from_template(human_prompt)chat_prompt = ChatPromptTemplate.from_messages([MessagesPlaceholder(variable_name="conversation"), human_message_template])human_message = HumanMessage(content="What is the best way to learn programming?")
ai_message = AIMessage(content="""\
1. Choose a programming language: Decide on a programming language that you want to learn.2. Start with the basics: Familiarize yourself with the basic programming concepts such as variables, data types and control structures.3. Practice, practice, practice: The best way to learn programming is through hands-on experience\
""")chat_prompt.format_prompt(conversation=[human_message, ai_message], word_count="10").to_messages()[HumanMessage(content='What is the best way to learn programming?', additional_kwargs={}),AIMessage(content='1. Choose a programming language: Decide on a programming language that you want to learn. \n\n2. Start with the basics: Familiarize yourself with the basic programming concepts such as variables, data types and control structures.\n\n3. Practice, practice, practice: The best way to learn programming is through hands-on experience', additional_kwargs={}),HumanMessage(content='Summarize our conversation so far in 10 words.', additional_kwargs={})]比如在上述代码中，在chat_prompt中定义了一个名为conversation的Message占位符，然后当chat_prompt调用format方法的时候，动态地将human_message，ai_message插入到占位符位置，从而替换占位符。
本文小结
MessagePromptTemplate在对话模型有着非常重要的作用，可以通过它来生成prompt；同时还可以通过MessagesPlaceholder实现占位符功能。
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