RunnablePassthrough and RunnableLambda in LangChain - Step-by-Step Execution

Concept Flow - RunnablePassthrough and RunnableLambda

Input Data

↓

Pass input as-is

↓

Output same as input

↓

Next step or result

RunnablePassthrough sends input through unchanged. RunnableLambda applies a function to input and outputs the result.

Execution Sample

LangChain

passthrough = RunnablePassthrough()
lambda_run = RunnableLambda(lambda x: x * 2)

output1 = passthrough.invoke(5)
output2 = lambda_run.invoke(5)

Shows how RunnablePassthrough returns input unchanged and RunnableLambda doubles the input.

Execution Table

Step	Runnable Type	Input	Action	Output
1	RunnablePassthrough	5	Pass input as-is	5
2	RunnableLambda	5	Apply lambda x*2	10
3	RunnablePassthrough	Hello	Pass input as-is	Hello
4	RunnableLambda	Hello	Apply lambda x*2 (string repeat)	HelloHello
5	End	-	-	-

💡 All inputs processed; RunnablePassthrough outputs input unchanged; RunnableLambda outputs function result.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
passthrough	RunnablePassthrough instance	Same instance	Same instance	Same instance	Same instance	Same instance
lambda_run	RunnableLambda instance with lambda x*2	Same instance	Same instance	Same instance	Same instance	Same instance
output1	None	5	5	5	5	5
output2	None	None	10	10	10	10

Key Moments - 3 Insights

Why does RunnablePassthrough output exactly what I input?

RunnablePassthrough is designed to return the input unchanged, as shown in execution_table rows 1 and 3.

How does RunnableLambda change the input?

RunnableLambda applies the function you give it to the input, like doubling a number or repeating a string, as seen in rows 2 and 4.

What happens if the lambda function is not compatible with the input?

An error would occur during the invoke call because the function cannot process the input type; this is not shown here but is important to handle.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of RunnableLambda when input is 5?

A 5

B 10

C 25

D None

💡 Hint

Check row 2 in the execution_table where RunnableLambda processes input 5.

At which step does RunnablePassthrough output the string 'Hello'?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at execution_table row 3 for RunnablePassthrough with input 'Hello'.

If the lambda function was changed to lambda x: x + 1, what would be the output for input 5 at step 2?

A 5

B 10

C 6

D Error

💡 Hint

Consider how lambda x: x + 1 changes input 5; see execution_table row 2 for original output.

Concept Snapshot

RunnablePassthrough: returns input unchanged.
RunnableLambda: applies a function to input and returns result.
Use invoke(input) to run.
RunnableLambda needs a function compatible with input type.
Useful for chaining operations in LangChain.

Full Transcript

RunnablePassthrough and RunnableLambda are simple tools in LangChain to process data. RunnablePassthrough just sends the input through without changing it. RunnableLambda takes a function you give it and applies that function to the input, then returns the result. For example, if you give RunnableLambda a function that doubles a number, and input 5, it returns 10. This helps you build chains of operations easily. The execution table shows step-by-step how inputs are handled and outputs produced. Remember, RunnableLambda needs a function that works with the input type or it will cause errors.