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.

Practice

1. What does RunnablePassthrough do with the input it receives?

easy

A. Ignores the input and returns a fixed value

B. Transforms the input using a custom function

C. Returns the input exactly as it is without any changes

D. Throws an error if input is not a string

Solution

1. Step 1: Understand RunnablePassthrough behavior

   RunnablePassthrough is designed to return whatever input it receives without modifying it.

2. Step 2: Compare options with behavior

   Only Returns the input exactly as it is without any changes matches this behavior exactly; others describe transformations or errors which RunnablePassthrough does not do.

3. Final Answer:

   Returns the input exactly as it is without any changes -> Option C

4. Quick Check:

   RunnablePassthrough returns input unchanged = A [OK]

Hint: RunnablePassthrough just passes input through unchanged [OK]

Common Mistakes:

• Thinking RunnablePassthrough modifies input
• Confusing RunnablePassthrough with RunnableLambda
• Assuming it throws errors on certain inputs

2. Which of the following is the correct way to create a RunnableLambda that doubles a number input?

easy

A. RunnableLambda(lambda x: x + 2)

B. RunnableLambda(lambda x: x * 2)

C. RunnableLambda(lambda x: x / 2)

D. RunnableLambda(lambda x: x - 2)

Solution

1. Step 1: Identify the doubling function

   Doubling means multiplying the input by 2, so the function should be lambda x: x * 2.

2. Step 2: Match with options

   RunnableLambda(lambda x: x * 2) matches the doubling function exactly; others perform addition, division, or subtraction.

3. Final Answer:

   RunnableLambda(lambda x: x * 2) -> Option B

4. Quick Check:

   Doubling function uses multiplication by 2 = D [OK]

Hint: Doubling means multiply input by 2 in lambda [OK]

Common Mistakes:

• Using addition instead of multiplication
• Confusing division or subtraction for doubling
• Incorrect lambda syntax

3. What will be the output of this code?

passthrough = RunnablePassthrough()
lambda_runner = RunnableLambda(lambda x: x.upper())
result = lambda_runner.invoke(passthrough.invoke('hello'))
print(result)

medium

A. 'HELLO'

B. 'hello'

C. Error: RunnablePassthrough cannot be invoked

D. 'Hello'

Solution

1. Step 1: Trace RunnablePassthrough output

   Calling passthrough.invoke('hello') returns 'hello' unchanged.

2. Step 2: Apply RunnableLambda function

   The lambda converts input to uppercase, so 'hello'.upper() returns 'HELLO'.

3. Final Answer:

   'HELLO' -> Option A

4. Quick Check:

   Passthrough returns input, lambda uppercases it = C [OK]

Hint: Passthrough returns input, lambda transforms it [OK]

Common Mistakes:

• Expecting passthrough to modify input
• Confusing case conversion result
• Assuming runtime error on invoke

4. Identify the error in this code snippet:

lambda_runner = RunnableLambda(lambda x: x + 1)
result = lambda_runner.invoke('5')
print(result)

medium

A. TypeError because string '5' cannot be added to integer 1

B. SyntaxError in lambda function

C. RunnableLambda cannot be invoked with strings

D. No error; output will be '51'

Solution

1. Step 1: Analyze lambda operation on input

   The lambda tries to add 1 to input '5', which is a string, causing a type mismatch.

2. Step 2: Identify error type

   Adding integer 1 to string '5' raises a TypeError in Python.

3. Final Answer:

   TypeError because string '5' cannot be added to integer 1 -> Option A

4. Quick Check:

   Adding int to string causes TypeError = B [OK]

Hint: Adding int to string causes TypeError in Python [OK]

Common Mistakes:

• Assuming implicit string to int conversion
• Thinking output is string concatenation
• Confusing error types

5. You want to create a LangChain workflow that takes a list of numbers, passes it unchanged, then doubles each number. Which combination of RunnablePassthrough and RunnableLambda is correct?

hard

A. Use RunnablePassthrough twice, no lambda needed

B. Use RunnableLambda with lambda x: x, then RunnablePassthrough to double numbers

C. Use RunnableLambda with lambda x: x*2, then RunnablePassthrough to pass list

D. Use RunnablePassthrough to pass the list, then RunnableLambda with lambda x: [i*2 for i in x]

Solution

1. Step 1: Pass list unchanged with RunnablePassthrough

   RunnablePassthrough returns the list as is, so it fits the first step.

2. Step 2: Double each number with RunnableLambda

   RunnableLambda with lambda x: [i*2 for i in x] correctly doubles each element in the list.

3. Final Answer:

   Use RunnablePassthrough to pass the list, then RunnableLambda with lambda x: [i*2 for i in x] -> Option D

4. Quick Check:

   Passthrough passes list, lambda doubles elements = A [OK]

Hint: Passthrough passes input; lambda transforms list elements [OK]

Common Mistakes:

• Trying to double list with passthrough
• Using lambda that multiplies list object, not elements
• Reversing order of passthrough and lambda