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Artificial Intelligence
LLM
Python

Prompt Template Guide with LangChain

Photo de Romain DE LA SOUCHÈRE

Tech Lead, CTO AXI Technologies

Published on 15 avril 2025 · 10 min of reading

Introduction to prompt templates

In the world of conversational artificial intelligence, the use of prompt templates has become essential for optimizing interactions with language models. These templates serve as predefined models that guide the generation of responses by the model, ensuring that interactions are both accurate and relevant. By using well-designed prompts, developers can control the flow of conversation, steer responses, and enhance the overall user experience.

Why use prompt templates?

The use of prompt templates allows for structuring interactions and ensuring the coherence of responses from the language model. This is particularly useful for applications requiring high accuracy and consistency, such as virtual assistants or enterprise chatbots.

The benefits of prompt templates

  • Coherence: By using standardized models, the generated responses are more uniform, which is crucial for maintaining a consistent brand voice.
  • Efficiency: Templates reduce the time needed to formulate prompts, thus speeding up development.
  • Adaptability: They can be easily modified to fit different contexts or specific needs.
By adopting these practices, developers can maximize the capabilities of language models while minimizing errors and inconsistencies.

Different types of prompt templates

Prompt models allow for the conversion of user inputs and parameters into instructions for a language model. They thus guide the model's response by helping it understand the context and produce a coherent and relevant linguistic outcome.
These models use an input dictionary, where each key corresponds to a variable to be filled in the prompt model.
Prompt models generate a prompt value, which can be passed to a language model or a chat model, and converted to a string or a list of messages. This prompt value facilitates the transition between string formats and message formats.
There are different types of prompt models:

String PromptTemplates

String prompt templates (String PromptTemplates) are designed to format a single string and are often used for simple inputs. For example, a common method to create and use a prompt template follows this process:
python
In this example, the conversation prompt model generates a message upon execution. This user message will be personalized based on the variable topic provided by the user.

ChatPromptTemplates

Chat prompt templates (ChatPromptTemplates) are used to structure a list of messages. They consist of a series of models themselves. For example, a common method to create and use a conversation prompt model is as follows:
python
In this example, the conversation prompt model generates two messages upon execution. The first is a system message that requires no formatting. The second is a user message, which will be personalized based on the variable topic provided by the user.

MessagesPlaceholder

The prompt model MessagesPlaceholder allows for inserting a list of messages at a specific location. In the previous example with the conversation prompt model, we saw how to format two messages as strings. But how do we handle a situation where we want the user to provide a list of messages to be integrated at a specific point? This is where MessagesPlaceholder comes in.
python
This model produces a list of two messages: a system message followed by the provided user message. If we had passed 3 messages, the total would have been 4 messages (the system message plus the 3 provided messages). This method is convenient for inserting a list of messages at a specific location.
Another way to achieve the same result without explicitly using the MessagesPlaceholder class is:
python

Why use prompt templates?

The use of prompt templates allows for structuring interactions and ensuring the coherence of responses from the language model. This is particularly useful for applications requiring high accuracy and consistency, such as virtual assistants or enterprise chatbots.

The benefits of prompt templates

  • Coherence: By using standardized models, the generated responses are more uniform, which is crucial for maintaining a consistent brand voice.
  • Efficiency: Templates reduce the time needed to formulate prompts, thus speeding up development.
  • Adaptability: They can be easily modified to fit different contexts or specific needs.
By adopting these practices, developers can maximize the capabilities of language models while minimizing errors and inconsistencies.

Conclusion and recommendation

Prompt templates offer considerable flexibility and efficiency in optimizing interactions with language models. Whether you use String PromptTemplates for simple inputs or ChatPromptTemplates for more complex conversations, these tools allow you to structure and personalize exchanges effectively.
Using MessagesPlaceholder further expands the possibilities by allowing the integration of dynamic message lists, which is particularly useful in scenarios where the conversation needs to adapt in real-time to user inputs. This ensures that language models can respond in a more natural and contextual manner, thereby enhancing the user experience.

Recommendations

  • Start small: If you are new to prompt templates, start with String PromptTemplates to familiarize yourself with basic formatting.
  • Experiment: Don’t hesitate to experiment with different types of templates to discover what works best in your context.
  • Optimize: Analyze interactions to identify ways in which templates can be adjusted for better performance.
By following these recommendations, you can make the most of the advanced capabilities of language models and significantly improve the quality of interactions.

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Photo de Romain DE LA SOUCHÈRE

Romain DE LA SOUCHÈRE

Tech Lead, CTO AXI Technologies

Expert Data Engineering et Cloud, Romain affiche plus de 11 ans d'expérience, dont plusieurs années comme Lead Developer sur des solutions Smart Building haute performance. Il y a conçu et mis en production des moteurs de traitement capables d'absorber des centaines de milliers de données de capteurs par minute, ainsi que des bases clusterisées gérant plus de 10 millions de données dynamiques. Certifié Microsoft Azure DevOps Engineer Expert, il maîtrise aussi bien le développement back-end (Python, C#) que le DevOps (Docker, Kubernetes, Terraform) et les agents LLM. Formateur en Python, cloud, DevOps et IA générative appliquée, il forme avec une obsession : Amener chaque apprenant à concevoir et déployer des architectures réellement scalables en production.

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