Variables and dynamic content in LangChain - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does LangChain handle variable substitution in prompt templates?

Consider a LangChain prompt template that uses variables to dynamically insert user input. What will be the output if the variable is not provided when calling the template?

LangChain

from langchain.prompts import PromptTemplate

prompt = PromptTemplate(
    input_variables=["name"],
    template="Hello, {name}! Welcome to LangChain."
)

output = prompt.format()

A Raises a KeyError because 'name' variable is missing

B Outputs 'Hello, {name}! Welcome to LangChain.' literally without substitution

C Outputs 'Hello, ! Welcome to LangChain.' with empty string for missing variable

D Outputs 'Hello, None! Welcome to LangChain.' substituting None for missing variable

💡 Hint

Think about how Python string formatting behaves when a required key is missing.

LangChain's PromptTemplate uses Python's str.format method internally. If a required variable is missing, it raises a KeyError.

❓ state_output

intermediate

What is the output of a LangChain prompt with multiple variables?

Given the following prompt template and variable inputs, what is the final rendered string?

LangChain

from langchain.prompts import PromptTemplate

prompt = PromptTemplate(
    input_variables=["user", "task"],
    template="User {user} wants to perform the task: {task}."
)

output = prompt.format(user="Alice", task="data analysis")

A "User Alice wants to perform the task: {task}."

B "User {user} wants to perform the task: {task}."

C "User None wants to perform the task: data analysis."

D "User Alice wants to perform the task: data analysis."

💡 Hint

Check how variables are replaced in the template string.

The format method replaces all variables with the provided values, producing the full string with substitutions.

📝 Syntax

advanced

Identify the syntax error in this LangChain prompt template definition

Which option contains a syntax error that will prevent the LangChain PromptTemplate from being created?

LangChain

from langchain.prompts import PromptTemplate

A PromptTemplate(input_variables=["name"], template="Hello, {name}!")

B PromptTemplate(input_variables="name", template="Hello, {name}!")

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

D )"!}eman{ ,olleH"=etalpmet ,]"eman"[=selbairav_tupni(etalpmeTtpmorP

💡 Hint

Check the type expected for input_variables parameter.

input_variables must be a list of strings, not a single string. Passing a string causes a type error or unexpected behavior.

🔧 Debug

advanced

Why does this LangChain prompt fail to substitute variables correctly?

Given the code below, why does the output still contain the variable placeholders instead of the values?

LangChain

from langchain.prompts import PromptTemplate

prompt = PromptTemplate(
    input_variables=["city"],
    template="Welcome to {city}!"
)

output = prompt.format(city=None)
print(output)

A Because None is passed, the placeholder is replaced with 'None' string

B Because None is passed, the placeholder remains unchanged

C Because the variable 'city' is missing, it raises a KeyError

D Because the template string is invalid, it raises a SyntaxError

💡 Hint

Consider how Python's str.format handles None values.

Passing None replaces the placeholder with the string 'None', so the output is 'Welcome to None!'.

🧠 Conceptual

expert

How does LangChain support dynamic content with nested variables in prompt templates?

LangChain allows prompt templates to include variables whose values are themselves templates or require runtime evaluation. Which approach correctly supports this dynamic nested content?

A Using PromptTemplate with input_variables and passing variables as plain strings only

B Using string concatenation before passing variables to PromptTemplate.format()

C Using PromptTemplate with input_variables and passing another PromptTemplate instance as a variable value

D Using PromptTemplate with input_variables and passing a dictionary as a variable value without further formatting

💡 Hint

Think about how LangChain composes prompts for complex workflows.

LangChain supports nested prompt templates by allowing PromptTemplate instances as variable values, enabling dynamic content generation.