RunnablePassthrough and RunnableLambda in LangChain - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is RunnablePassthrough in Langchain?

RunnablePassthrough is a simple component that returns the input it receives without any changes. Think of it like a mirror that just reflects what you give it.

beginner

How does RunnableLambda differ from RunnablePassthrough?

RunnableLambda lets you run any custom function you write on the input. Unlike RunnablePassthrough, it can change or process the input before giving back a result.

intermediate

Show a simple example of RunnableLambda usage.

You can create a RunnableLambda that adds 1 to a number:
runnable = RunnableLambda(lambda x: x + 1)
runnable.invoke(5) returns 6.

intermediate

Why use RunnablePassthrough in a chain?

RunnablePassthrough is useful when you want to pass data through a chain step without changing it. It helps keep the flow simple and clear.

beginner

What method do you call to run the logic inside RunnableLambda or RunnablePassthrough?

You call the 'invoke' method with your input to run the logic and get the output.

What does RunnablePassthrough do with the input it receives?

A Deletes the input

B Transforms the input using a function

C Returns the input unchanged

D Saves the input to a file

RunnablePassthrough simply returns the input as it is without any changes.

Which Runnable lets you run a custom function on the input?

A RunnableLambda

B RunnableOutput

C RunnableChain

D RunnablePassthrough

RunnableLambda runs any custom function you provide on the input.

How do you execute the logic inside a RunnableLambda?

A call invoke()

B call execute()

C call run()

D call start()

The 'invoke' method runs the logic inside RunnableLambda or RunnablePassthrough.

If you want to pass data through a chain without changing it, which Runnable should you use?

A RunnableModify

B RunnableLambda

C RunnableTransform

D RunnablePassthrough

RunnablePassthrough passes data unchanged through the chain.

What is a real-life analogy for RunnablePassthrough?

A A blender mixing ingredients

B A mirror reflecting your image

C A chef cooking food

D A printer printing a document

RunnablePassthrough acts like a mirror, returning exactly what it receives.

Explain the difference between RunnablePassthrough and RunnableLambda in Langchain.

Think about whether the input changes or not.

Describe a scenario where you would use RunnablePassthrough in a chain.

Imagine passing a message along without editing it.

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