Variables and dynamic content in LangChain

Introduction

Variables let you store and reuse information easily. Dynamic content means your program can change what it shows based on those variables.

You want to remember user input and use it later in a conversation.

You need to customize responses based on different situations.

You want to build a chatbot that adapts answers depending on previous messages.

You want to insert changing data like dates or names into your text.

You want to create templates that fill in details automatically.

Syntax

LangChain

from langchain import PromptTemplate

# Define a template with variables inside curly braces
template = "Hello, {name}! Today is {day}."

# Create a PromptTemplate object
prompt = PromptTemplate(template=template, input_variables=["name", "day"])

# Fill variables with actual values
filled_prompt = prompt.format(name="Alice", day="Monday")

Variables are placed inside curly braces {} in the template string.

You must list all variable names in input_variables when creating the PromptTemplate.

Examples

This example shows a simple template with one variable color. It prints a sentence with the color filled in.

LangChain

template = "My favorite color is {color}."
prompt = PromptTemplate(template=template, input_variables=["color"])
print(prompt.format(color="blue"))

Here, two variables greeting and name are used to create a friendly message.

LangChain

template = "{greeting}, {name}!"
prompt = PromptTemplate(template=template, input_variables=["greeting", "name"])
print(prompt.format(greeting="Hi", name="Bob"))

This example shows how to insert a single dynamic value like the day of the week.

LangChain

template = "Today is {day}."
prompt = PromptTemplate(template=template, input_variables=["day"])
print(prompt.format(day="Friday"))

Sample Program

This program creates a message template for appointment reminders. It fills in the title, last name, and date dynamically.

LangChain

from langchain import PromptTemplate

# Create a template with variables
template = "Dear {title} {last_name}, your appointment is on {date}."
prompt = PromptTemplate(template=template, input_variables=["title", "last_name", "date"])

# Fill variables with actual data
message = prompt.format(title="Dr.", last_name="Smith", date="June 10th")

print(message)

Important Notes

Always list all variables you want to use in the input_variables list.

If you forget to provide a variable value when calling format, it will raise an error.

Using variables makes your prompts flexible and reusable for many situations.

Summary

Variables let you insert changing information into text templates.

Use PromptTemplate with input_variables to define which parts change.

Fill variables with format to get the final text with dynamic content.

Practice

1. What is the main purpose of using variables in a Langchain PromptTemplate?

easy

A. To create multiple templates at once

B. To insert changing information into the text dynamically

C. To store the final output text permanently

D. To make the template text static and unchangeable

Solution

1. Step 1: Understand the role of variables in templates

   Variables are placeholders that allow parts of the text to change based on input.

2. Step 2: Connect variables to dynamic content

   Using variables lets you insert different values each time you use the template, making it dynamic.

3. Final Answer:

   To insert changing information into the text dynamically -> Option B

4. Quick Check:

   Variables = dynamic content insertion [OK]

Hint: Variables let text change based on input values [OK]

Common Mistakes:

• Thinking variables make text static
• Confusing variables with final output storage
• Believing variables create multiple templates automatically

2. Which of the following is the correct way to define a PromptTemplate with variables in Langchain?

easy

A. PromptTemplate(template="Hello {name}", input_variables=name)

B. PromptTemplate(template="Hello name", input_variables=[name])

C. PromptTemplate(template="Hello {name}", input_variables=["name"])

D. PromptTemplate(template="Hello {name}", variables=["name"])

Solution

1. Step 1: Check the syntax for defining variables

   The input_variables parameter must be a list of strings naming the variables used in the template.

2. Step 2: Match the template placeholders with variable names

   The template uses curly braces around variable names like {name} to mark where to insert values.

3. Final Answer:

   PromptTemplate(template="Hello {name}", input_variables=["name"]) -> Option C

4. Quick Check:

   Correct syntax = PromptTemplate(template="Hello {name}", input_variables=["name"]) [OK]

Hint: Use curly braces and list of strings for variables [OK]

Common Mistakes:

• Forgetting quotes around variable names
• Using wrong parameter name like variables instead of input_variables
• Not using curly braces in template

3. Given the code:

template = PromptTemplate(template="Hello {user}, today is {day}.", input_variables=["user", "day"])
result = template.format(user="Alice", day="Monday")
print(result)

What will be printed?

medium

A. Hello Alice, today is Monday.

B. Hello {user}, today is {day}.

C. Hello user, today is day.

D. Error: Missing variables

Solution

1. Step 1: Understand how format fills variables

   The format method replaces {user} with "Alice" and {day} with "Monday".

2. Step 2: Predict the final string after formatting

   The placeholders are replaced, so the printed string is "Hello Alice, today is Monday."

3. Final Answer:

   Hello Alice, today is Monday. -> Option A

4. Quick Check:

   format replaces variables correctly = Hello Alice, today is Monday. [OK]

Hint: format() replaces placeholders with given values [OK]

Common Mistakes:

• Printing template string without formatting
• Confusing variable names with values
• Expecting an error when variables are provided

4. What is wrong with this code snippet?

template = PromptTemplate(template="Hi {name}", input_variables=["name"])
result = template.format(nam="Bob")
print(result)

medium

A. input_variables should be a string, not a list

B. The template string is missing curly braces

C. format method cannot be used with PromptTemplate

D. The variable name in format is misspelled, causing a KeyError

Solution

1. Step 1: Compare variable names in template and format call

   The template expects variable "name" but format uses "nam" which is incorrect.

2. Step 2: Understand the error caused by mismatch

   This mismatch causes a KeyError because "name" is not provided in format arguments.

3. Final Answer:

   The variable name in format is misspelled, causing a KeyError -> Option D

4. Quick Check:

   Variable name mismatch = KeyError [OK]

Hint: Check variable names match exactly in template and format [OK]

Common Mistakes:

• Assuming format ignores missing variables
• Thinking input_variables must be a string
• Believing format method is invalid here

5. You want to create a PromptTemplate that dynamically inserts a user's name and their favorite color, but if the color is not provided, it should default to "blue". Which approach correctly handles this dynamic content with a default value?

hard

A. Use input_variables=["name", "color"] and call format with color="blue" if missing

B. Define template with {name} and {color}, but omit color from input_variables to default it

C. Use input_variables=["name"] only and write template as "Hello {name}, your color is blue"

D. Set input_variables=["name", "color"] and use a conditional expression inside template like {color or 'blue'}

Solution

1. Step 1: Understand how to provide default values

   Langchain's PromptTemplate requires all variables listed in input_variables to be provided when formatting.

2. Step 2: Provide default value in code when calling format

   By including color in input_variables and passing color="blue" if missing, you ensure the template fills correctly.

3. Final Answer:

   Use input_variables=["name", "color"] and call format with color="blue" if missing -> Option A

4. Quick Check:

   Default values handled in format call = Use input_variables=["name", "color"] and call format with color="blue" if missing [OK]

Hint: Provide all variables; set defaults when calling format [OK]

Common Mistakes:

• Omitting variables from input_variables to default
• Trying to use Python expressions inside template strings
• Hardcoding default text instead of using variables