Regression testing for chains in LangChain - Step-by-Step Execution

Concept Flow - Regression testing for chains

Write initial chain code

↓

Create test cases with expected outputs

↓

Run chain with test inputs

↓

Compare actual output to expected

↓

Pass test

↓

Save tests

↓

Repeat tests after changes

Shows the flow of writing chain code, creating tests, running them, checking results, fixing code if needed, and repeating tests to catch regressions.

Execution Sample

LangChain

from langchain.chains import SimpleChain

def test_chain():
    chain = SimpleChain()
    output = chain.run("Hello")
    assert output == "Hello World"

A simple test runs a chain with input 'Hello' and checks if output matches expected 'Hello World'.

Execution Table

Step	Action	Input	Expected Output	Actual Output	Test Result
1	Run chain with input	"Hello"	"Hello World"	"Hello World"	Pass
2	Run chain with input	"Test"	"Test World"	"Test World"	Pass
3	Run chain with input	"Fail"	"Fail World"	"Fail Wrld"	Fail
4	Fix chain code	N/A	N/A	N/A	N/A
5	Re-run chain with input	"Fail"	"Fail World"	"Fail World"	Pass

💡 All tests pass after fixing chain code; regression testing confirms no output breaks.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
output	None	"Hello World"	"Test World"	"Fail Wrld"	None (fixed)	"Fail World"
test_result	None	Pass	Pass	Fail	N/A	Pass

Key Moments - 2 Insights

Why does the test fail at step 3 even though the input looks similar?

At step 3, the actual output "Fail Wrld" is missing the 'o' compared to expected "Fail World". This shows a bug in the chain code that regression testing catches.

Why do we re-run tests after fixing the chain code?

Re-running tests (step 5) confirms the fix works and no new bugs were introduced, ensuring the chain behaves as expected.

Visual Quiz - 3 Questions

Look at the execution table, what is the test result at step 3?

A Pass

B Fail

C Skipped

D Error

💡 Hint

Check the 'Test Result' column at step 3 in the execution_table.

At which step is the chain code fixed?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the action 'Fix chain code' in the execution_table.

If the output at step 5 was still "Fail Wrld", what would the test result be?

A Pass

B Fail

C Error

D Skipped

💡 Hint

Compare expected and actual output in the execution_table at step 3 and 5.

Concept Snapshot

Regression testing for chains:
- Write chain code
- Create tests with inputs and expected outputs
- Run chain and compare outputs
- Fix bugs if outputs differ
- Re-run tests to confirm fixes
- Ensures chain changes don't break expected behavior

Full Transcript

Regression testing for chains means writing tests that run your chain with known inputs and check if the outputs match what you expect. If outputs differ, you fix the chain code and run tests again. This process helps catch bugs early and ensures your chain keeps working correctly after changes. The flow starts with writing chain code, creating test cases, running tests, checking results, fixing code if needed, and repeating tests. The example shows a chain tested with inputs like 'Hello' and 'Fail'. The test fails when output misses a letter, so the code is fixed and tests pass again. This cycle helps keep your chain reliable.

Practice

1.

What is the main purpose of regression testing for chains in Langchain?

easy

A. To add new features to the chain

B. To improve the speed of chain execution

C. To verify that chains still produce expected outputs after changes

D. To train the chain with new data

Solution

1. Step 1: Understand regression testing concept

   Regression testing is about checking if existing functionality still works after updates.

2. Step 2: Apply to chains context

   For chains, this means verifying outputs remain correct after code or data changes.

3. Final Answer:

   To verify that chains still produce expected outputs after changes -> Option C

4. Quick Check:

   Regression testing = verify outputs after changes [OK]

Hint: Regression testing checks output correctness after updates [OK]

Common Mistakes:

• Confusing regression testing with performance tuning
• Thinking regression testing adds new features
• Assuming regression testing trains models

2.

Which of the following is the correct way to run a regression test on a Langchain chain named my_chain with input {"text": "Hello"} and expected output {"result": "Hi"}?

easy

A. assert my_chain.invoke({"text": "Hello"}) == {"result": "Hi"}

B. my_chain.test({"text": "Hello"}, {"result": "Hi"})

C. my_chain.run({"text": "Hello"}) == {"result": "Hi"}

D. my_chain.regression_test({"text": "Hello"}, {"result": "Hi"})

Solution

1. Step 1: Identify correct method to run chain and compare output

   Langchain chains use invoke or run to get output; to test, use assert to compare.

2. Step 2: Check options for syntax correctness

   assert my_chain.invoke({"text": "Hello"}) == {"result": "Hi"} uses assert with invoke and compares to expected output correctly.

3. Final Answer:

   assert my_chain.invoke({"text": "Hello"}) == {"result": "Hi"} -> Option A

4. Quick Check:

   Use assert with invoke for regression test [OK]

Hint: Use assert with invoke to compare outputs in regression tests [OK]

Common Mistakes:

• Using non-existent methods like regression_test
• Comparing outputs without assert
• Confusing run and test methods

3.

Given the following code snippet, what will be the output of the regression test?

class EchoChain:
    def invoke(self, inputs):
        return {"echo": inputs["message"]}

my_chain = EchoChain()
input_data = {"message": "Test"}
expected_output = {"echo": "Test"}
result = my_chain.invoke(input_data) == expected_output
print(result)

medium

A. True

B. False

C. SyntaxError

D. RuntimeError

Solution

1. Step 1: Understand the EchoChain invoke method

   The method returns a dictionary with key "echo" and value from inputs["message"].

2. Step 2: Compare the returned output with expected output

   Input is {"message": "Test"}, so output is {"echo": "Test"}, which matches expected_output.

3. Final Answer:

   True -> Option A

4. Quick Check:

   Output matches expected = True [OK]

Hint: Check returned dict matches expected dict exactly [OK]

Common Mistakes:

• Assuming method returns input unchanged
• Confusing keys in output dictionary
• Expecting errors from correct code

4.

Identify the error in this regression test code snippet for a Langchain chain my_chain:

input_data = {"query": "Hello"}
expected = {"answer": "Hi"}
result = my_chain.invoke(input_data) == expected
print(result)

Assuming my_chain.invoke returns {"response": "Hi"}, what is the problem?

medium

A. The print statement syntax is wrong

B. The input_data dictionary is missing required keys

C. The invoke method is called incorrectly

D. The expected output keys do not match the actual output keys

Solution

1. Step 1: Compare expected and actual output keys

   Expected output has key "answer" but actual output has key "response".

2. Step 2: Understand impact on regression test

   Mismatch in keys causes the equality check to fail, so test result is False.

3. Final Answer:

   The expected output keys do not match the actual output keys -> Option D

4. Quick Check:

   Output keys mismatch causes test failure [OK]

Hint: Check keys in expected vs actual output carefully [OK]

Common Mistakes:

• Assuming input_data is wrong without checking
• Thinking invoke method call is incorrect
• Blaming print statement for logic errors

5.

You want to create a regression test suite for a Langchain chain that processes user questions and returns answers. Which approach best ensures your tests catch unintended changes in the chain's behavior?

hard

A. Test the chain with random inputs and manually check outputs each time

B. Store a set of input questions and their exact expected answers, then assert equality on each test run

C. Update expected answers after every chain change without verification

D. Only check that the chain runs without errors, ignoring output correctness

Solution

1. Step 1: Understand regression test goal

   Regression tests should detect if outputs change unexpectedly after updates.

2. Step 2: Evaluate options for reliability

   Store a set of input questions and their exact expected answers, then assert equality on each test run uses fixed input-output pairs and asserts equality, which reliably detects changes.

3. Final Answer:

   Store a set of input questions and their exact expected answers, then assert equality on each test run -> Option B

4. Quick Check:

   Fixed input-output pairs catch unintended changes [OK]

Hint: Use fixed input-output pairs for reliable regression tests [OK]

Common Mistakes:

• Ignoring output correctness in tests
• Blindly updating expected outputs
• Relying on manual checks only