Debugging failed chains in LangChain - Practice Problems & Coding Challenges

Challenge - 5 Problems

🔧 Debug

intermediate

Identify the error in this LangChain chain execution

Given the following LangChain code snippet, what error will it raise when executed?

LangChain

from langchain.chains import SimpleSequentialChain, LLMChain
from langchain.llms import OpenAI
from langchain.prompts import PromptTemplate

llm = OpenAI(temperature=0)

prompt1 = PromptTemplate.from_template("Repeat: {input}")
chain1 = LLMChain(llm=llm, prompt=prompt1)

prompt2 = PromptTemplate.from_template("Again: {text}")
chain2 = LLMChain(llm=llm, prompt=prompt2)

final_chain = SimpleSequentialChain(chains=[chain1, chain2])

output = final_chain.run("Hello")

A TypeError: __init__() got an unexpected keyword argument 'llm'

B AttributeError: 'SimpleSequentialChain' object has no attribute 'run'

C ValueError: Missing input key for chain execution

D RuntimeError: Chain execution failed due to empty chains list

💡 Hint

Check the constructor parameters for SimpleSequentialChain in LangChain.

SimpleSequentialChain does not accept a 'llm' parameter in its constructor. Passing 'llm=llm' causes a TypeError.

❓ component_behavior

intermediate

What happens when a LangChain chain input key is missing?

Consider a LangChain chain expecting an input key 'question'. What happens if you run the chain with input {'query': 'What is AI?'} instead?

A The chain runs successfully, ignoring the missing 'question' key

B The chain returns None without error

C KeyError is raised indicating 'question' key is missing

D The chain outputs an empty string as the result

💡 Hint

Chains require specific input keys to function properly.

LangChain chains expect specific input keys. Missing required keys causes a KeyError during execution.

❓ state_output

advanced

What is the output of this LangChain chain with a failing LLM call?

Given a LangChain chain that calls an LLM which raises an exception during execution, what will be the chain's output?

LangChain

from langchain.llms.base import LLM

class FailingLLM(LLM):
    def _call(self, prompt, stop=None):
        raise RuntimeError('LLM failure')

    @property
    def _identifying_params(self):
        return {}

    @property
    def _llm_type(self):
        return 'failing'

failing_llm = FailingLLM()

from langchain.chains import LLMChain
from langchain.prompts import PromptTemplate

prompt = PromptTemplate(input_variables=['text'], template='Echo: {text}')
chain = LLMChain(llm=failing_llm, prompt=prompt)

try:
    output = chain.run('test')
except Exception as e:
    output = e.args[0]

A Output is 'LLM failure'

B Output is 'Echo: test'

C Output is None

D Output is an empty dictionary {}

💡 Hint

Consider what happens when the LLM call raises an exception inside the chain.

The LLM call raises a RuntimeError with message 'LLM failure'. The try-except captures it and sets output to the error message string.

📝 Syntax

advanced

Which option correctly initializes a LangChain SequentialChain with two chains?

You want to create a SequentialChain that runs two chains in order. Which code snippet is correct?

A SequentialChain(chains=(chain1, chain2), input_vars=['input'], output_vars=['output'])

B SequentialChain(chains=[chain1, chain2], input_variables=['input'], output_variables=['output'])

C SequentialChain(chain1, chain2, input_vars=['input'], output_vars=['output'])

D SequentialChain([chain1, chain2], inputs=['input'], outputs=['output'])

💡 Hint

Check the parameter names and types in SequentialChain constructor.

SequentialChain expects a 'chains' parameter as a list and 'input_variables' and 'output_variables' as lists of strings. Option B matches this.

🧠 Conceptual

expert

Why does a LangChain chain fail silently when using an incorrect input key mapping?

You have a chain expecting input key 'question' but you pass {'query': 'Hello'}. The chain runs but returns an empty string without error. Why?

A The LLM returns empty string for any input not matching 'question'

B LangChain automatically maps unknown keys to expected keys silently

C The chain uses default empty string for missing keys and does not raise errors

D The chain's prompt template uses the wrong variable name, causing empty output

💡 Hint

Consider how prompt templates use input variables and what happens if they don't match inputs.

If the prompt template references a variable name not provided in inputs, it substitutes empty string, causing empty output without error.