RunnablePassthrough and RunnableLambda in LangChain - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does RunnablePassthrough output?

Given a RunnablePassthrough instance in LangChain, what will be the output when you run it with input "Hello"?

LangChain

from langchain_core.runnables import RunnablePassthrough
runnable = RunnablePassthrough()
result = runnable.invoke("Hello")
print(result)

A "Hello"

B "RunnablePassthrough"

C None

D Raises TypeError

💡 Hint

RunnablePassthrough returns exactly what it receives.

RunnablePassthrough is designed to pass the input through unchanged. So invoking it with "Hello" returns "Hello".

📝 Syntax

intermediate

Which RunnableLambda syntax correctly doubles a number?

Select the correct RunnableLambda code that doubles the input number.

LangChain

from langchain_core.runnables import RunnableLambda

A RunnableLambda(lambda x: x ** 2)

B RunnableLambda(lambda x: x + 2)

C RunnableLambda(lambda x: x * 2)

D RunnableLambda(lambda x: str(x) * 2)

💡 Hint

Doubling means multiplying by 2.

RunnableLambda takes a function. To double a number, the function should multiply input by 2.

❓ state_output

advanced

What is the output of this RunnableLambda chain?

Consider this code snippet using RunnableLambda and RunnablePassthrough. What is printed?

LangChain

from langchain_core.runnables import RunnableLambda, RunnablePassthrough
passthrough = RunnablePassthrough()
lambda_double = RunnableLambda(lambda x: x * 2)
chain = passthrough | lambda_double
result = chain.invoke(5)
print(result)

A 5

B 10

C TypeError

D None

💡 Hint

RunnablePassthrough passes input unchanged, then RunnableLambda doubles it.

The chain first passes 5 unchanged, then doubles it to 10.

🔧 Debug

advanced

Why does this RunnableLambda code raise an error?

This RunnableLambda is defined as RunnableLambda(lambda x: x[0]). What error occurs when invoking with input 10?

LangChain

from langchain_core.runnables import RunnableLambda
runnable = RunnableLambda(lambda x: x[0])
runnable.invoke(10)

A TypeError: 'int' object is not subscriptable

B IndexError: list index out of range

C AttributeError: 'int' has no attribute 'invoke'

D No error, output is 1

💡 Hint

Check if the input type supports indexing.

The input 10 is an integer and cannot be indexed, so accessing x[0] raises a TypeError.

🧠 Conceptual

expert

Which statement about RunnablePassthrough and RunnableLambda is true?

Choose the correct statement about RunnablePassthrough and RunnableLambda in LangChain.

A RunnablePassthrough runs a custom function, RunnableLambda returns input unchanged

B RunnablePassthrough modifies input, RunnableLambda always returns input unchanged

C Both RunnablePassthrough and RunnableLambda require async functions

D RunnablePassthrough returns input unchanged, RunnableLambda runs a custom function on input

💡 Hint

Think about what each class is designed to do with input.

RunnablePassthrough simply returns the input as is. RunnableLambda applies a user-defined function to the input.