How to partially format prompt templates

Like partially binding arguments to a function, it can make sense to "partial" a prompt template - e.g. pass in a subset of the required values, as to create a new prompt template which expects only the remaining subset of values.

LangChain supports this in two ways:

1. Partial formatting with string values.
2. Partial formatting with functions that return string values.

In the examples below, we go over the motivations for both use cases as well as how to do it in LangChain.

Prerequisites

This guide will assume familiarity with the following concepts:

• Prompt templates

Partial with strings

One common use case for wanting to partial a prompt template is if you get access to some of the variables in a prompt before others. For example, suppose you have a prompt template that requires two variables, foo and baz. If you get the foo value early on in your chain, but the baz value later, it can be inconvenient to pass both variables all the way through the chain. Instead, you can partial the prompt template with the foo value, and then pass the partialed prompt template along and just use that. Below is an example of doing this:

from langchain_core.prompts import PromptTemplate

prompt = PromptTemplate.from_template("{foo}{bar}")
partial_prompt = prompt.partial(foo="foo")
print(partial_prompt.format(bar="baz"))

foobaz

You can also just initialize the prompt with the partialed variables.

prompt = PromptTemplate(
    template="{foo}{bar}", input_variables=["bar"], partial_variables={"foo": "foo"}
)
print(prompt.format(bar="baz"))

foobaz

Partial with functions

The other common use is to partial with a function. The use case for this is when you have a variable you know that you always want to fetch in a common way. A prime example of this is with date or time. Imagine you have a prompt which you always want to have the current date. You can't hard code it in the prompt, and passing it along with the other input variables is inconvenient. In this case, it's handy to be able to partial the prompt with a function that always returns the current date.

from datetime import datetime


def _get_datetime():
    now = datetime.now()
    return now.strftime("%m/%d/%Y, %H:%M:%S")


prompt = PromptTemplate(
    template="Tell me a {adjective} joke about the day {date}",
    input_variables=["adjective", "date"],
)
partial_prompt = prompt.partial(date=_get_datetime)
print(partial_prompt.format(adjective="funny"))

Tell me a funny joke about the day 04/21/2024, 19:43:57

You can also just initialize the prompt with the partialed variables, which often makes more sense in this workflow.

prompt = PromptTemplate(
    template="Tell me a {adjective} joke about the day {date}",
    input_variables=["adjective"],
    partial_variables={"date": _get_datetime},
)
print(prompt.format(adjective="funny"))

Tell me a funny joke about the day 04/21/2024, 19:43:57

Next steps

You've now learned how to partially apply variables to your prompt templates.

Next, check out the other how-to guides on prompt templates in this section, like adding few-shot examples to your prompt templates.