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Using Prompt Templates in LangChain: A Detailed Guide for Generating Language Model Prompts

Using Prompt Templates in LangChain: A Detailed Guide for Generating Language Model Prompts

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Using Prompt Templates in LangChain: A Detailed Guide for Generating Language Model Prompts


Imagine you are working on a language model project and need to generate prompts that are specific to your task. You want to include instructions, examples, and context to guide the model's responses. This is where prompt templates come in handy. Prompt templates are predefined recipes for generating language model prompts, and they are an essential tool in LangChain, a powerful platform for building and fine-tuning language models.

In this article, we will delve into the world of prompt templates in LangChain. Specifically, we will focus on two types of prompt templates: PromptTemplate and ChatPromptTemplate. These templates provide a structured approach to generating prompts by incorporating instructions, few-shot examples, and specific context and questions appropriate for a given task. Whether you are working on a text completion, text classification, or chatbot project, prompt templates can help you streamline your development process and produce high-quality prompts.

Article Summary:

  • Prompt templates in LangChain are predefined recipes for generating language model prompts.
  • These templates include instructions, few-shot examples, and specific context and questions appropriate for a given task.
  • This article provides a detailed guide on how to create and use prompt templates in LangChain, with examples and explanations.
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Using Prompt Templates in LangChain:


PromptTemplate is used for creating templates for a string prompt. It allows you to define placeholders in the template that can be filled with specific values at runtime. Let's take a look at an example of creating a prompt template using the PromptTemplate class:

from langchain import PromptTemplate
template = PromptTemplate("Please write a {noun} sentence.")
formatted_prompt = template.format(noun="creative")

In this example, we create a prompt template with the text "Please write a noun sentence." The noun placeholder indicates that this part of the prompt will be replaced with a noun value when generating the prompt. We can then use the format method of the template to replace the placeholder with the desired value. In this case, we replace noun with "creative", resulting in the formatted prompt "Please write a creative sentence." This allows us to easily customize the prompt by providing different values for the placeholders.

Custom prompt templates can include any number of variables. For example, we can create a template that prompts the user to write a sentence using both a noun and an adjective:

template = PromptTemplate("Please write a {adjective} sentence using a {noun}.")
formatted_prompt = template.format(adjective="colorful", noun="flower")

Similarly, we can create more complex templates by combining multiple PromptTemplates using Prompt Template Composition. This allows us to build prompts that include multiple customizable parts:

template1 = PromptTemplate("Please write a {adjective} sentence.")
template2 = PromptTemplate("Use a {noun} in your sentence.")
composite_template = template1 + template2
formatted_prompt = composite_template.format(adjective="creative", noun="paintbrush")

In this example, we create two prompt templates, template1 and template2, and then combine them using the + operator to create a composite template. The resulting prompt template will incorporate both the adjective and noun variables, allowing us to generate prompts like "Please write a creative sentence. Use a paintbrush in your sentence."


ChatPromptTemplate, on the other hand, is used for creating templates for chat models, where the prompt is a list of chat messages. Each chat message in the prompt can have a different role, such as system, human, or AI. Here's an example of creating a chat prompt template using the ChatPromptTemplate class:

from langchain import ChatPromptTemplate
template = ChatPromptTemplate([
    ("sys", "You are an AI assistant that helps with daily tasks."),
    ("user", "What's the weather like today?"),
    ("sys", "The weather is sunny and warm."),
    ("user", "Should I wear sunscreen?"),
    ("sys", "Yes, it's always a good idea to wear sunscreen when it's sunny.")
formatted_prompt = template.format_messages()

In this example, we define a chat prompt template that includes messages from different roles: system and user. Each message is represented as a tuple with the role as the first element and the content as the second element. The format_messages method is used to format the template and generate the prompt as a list of messages.

ChatPromptTemplate provides a flexible way to represent chat conversations and simulate interactions with the language model. It allows you to define the specific roles and messages that are appropriate for your task, providing a structured context for the model's responses.

Alternative ways to represent chat messages include using MessagePromptTemplate or BaseMessage, depending on your specific requirements.

Both PromptTemplate and ChatPromptTemplate are part of the LangChain Expression Language (LCEL) and implement the Runnable interface. This means they support various invoke, stream, and batch calls for running the prompts. For example, you can invoke a prompt template with prompt variables and retrieve the generated prompt as a string or a list of messages.

In the next section, we will explore the different ways you can run prompt templates in LangChain and how you can leverage the power of prompt templates to generate high-quality prompts for your language models.

langchain prompts

Crafting Complex Prompts with LangChain

Building on the foundation of basic prompt and chat templates, LangChain offers advanced capabilities for constructing more sophisticated prompts. This flexibility is crucial for tasks requiring nuanced inputs or for simulating intricate dialogues. In this second part of the guide, we'll explore these advanced features and demonstrate how to leverage them for your projects.

Expanding on Prompt Templates

While the basic use of PromptTemplate facilitates straightforward text generation tasks, real-world scenarios often demand more complexity. Consider a scenario where you need to generate a prompt that encapsulates a scenario description, followed by a series of questions each requiring a different context. LangChain's flexibility allows for the creation of multi-part prompts that can accommodate such requirements.

Example: Generating a Scenario-Based Prompt

from langchain import PromptTemplate
scenario_description = PromptTemplate("You are a tourist guide in {city}. You need to provide information about popular tourist spots, local cuisine, and cultural norms.")
questions = [
    PromptTemplate("What are the top three tourist attractions in {city}?"),
    PromptTemplate("What is a must-try local dish in {city}?"),
    PromptTemplate("Are there any cultural norms or etiquettes tourists should be aware of in {city}?")
# Format each prompt with the specific city
city = "Paris"
formatted_scenario = scenario_description.format(city=city)
formatted_questions = [q.format(city=city) for q in questions]
full_prompt = f"{formatted_scenario}\n\n" + "\n\n".join(formatted_questions)

In this example, we create a detailed scenario description and a list of questions, each as a separate PromptTemplate. By formatting these templates with a specific city, we generate a comprehensive prompt that can guide the language model to provide targeted responses for a tourist guide application.

Leveraging ChatPromptTemplate for Dynamic Conversations

ChatPromptTemplate excels in scenarios where the prompt must reflect an ongoing conversation, with roles and contexts shifting dynamically. This is especially useful for developing AI chatbots or virtual assistants capable of handling varied user inquiries.

Example: Simulating a Customer Service Chat

from langchain import ChatPromptTemplate
chat_history = ChatPromptTemplate([
    ("sys", "Welcome to XYZ Customer Service. How can I assist you today?"),
    ("user", "I'm having trouble logging into my account."),
    ("sys", "I'm sorry to hear that. Could you please provide your username or email associated with the account?"),
    ("user", "Sure, it's user@example.com."),
    ("sys", "Thank you. One moment while I check your account details.")
additional_context = ChatPromptTemplate([
    ("sys", "I've reset your password. You should receive an email with instructions on how to set a new password shortly."),
    ("user", "Thank you! How long will this take?"),
    ("sys", "The email should arrive within the next 5 minutes. Please check your spam folder if you don't see it.")
# Combine chat history with additional context for a complete interaction
complete_chat = chat_history + additional_context
formatted_chat = complete_chat.format_messages()

In this example, we simulate a customer service interaction where the chat evolves over time. By combining multiple ChatPromptTemplate instances, we craft a realistic dialogue that can train the model to handle similar interactions autonomously.

Advanced Usage Tips

  • Contextual Variability: To generate prompts that adapt to different contexts or user inputs dynamically, consider incorporating external data sources or user input variables into your templates. This allows for the generation of prompts that are highly relevant and personalized.

  • Feedback Loops: For chat models, use the responses generated by the model to inform subsequent prompts. This iterative approach can enhance the model's ability to maintain context and coherence throughout a conversation.

  • Testing and Iteration: The effectiveness of a prompt template often depends on the specific task and model. Experiment with different formulations, incorporating feedback from test interactions to refine the prompts.


Prompt templates in LangChain offer a powerful mechanism for generating structured and dynamic prompts that cater to a wide range of language model tasks. By understanding and utilizing the advanced features of PromptTemplate and ChatPromptTemplate, developers can create complex, nuanced prompts that drive more meaningful interactions with language models. As AI and language models continue to evolve, tools like LangChain enable developers to push the boundaries of what's possible, crafting experiences that are both innovative and deeply human-centric.

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