LangServe for API deployment in LangChain - Mini Project: Build & Apply

Deploy a Simple API with LangServe

📖 Scenario: You want to create a simple API that answers questions using a language model. LangServe helps you deploy this API easily.

🎯 Goal: Build a basic LangServe API that accepts a question and returns an answer from a language model.

📋 What You'll Learn

Create a LangChain language model instance

Set up LangServe API configuration

Write the main API logic to handle questions

Complete the API deployment code

💡 Why This Matters

🌍 Real World

LangServe lets developers quickly deploy language model APIs for chatbots, assistants, or data queries.

💼 Career

Knowing LangServe helps you build and deploy AI-powered APIs, a valuable skill in AI and backend development roles.

1

Create the language model instance

Create a variable called llm that is an instance of OpenAI with temperature=0.

LangChain

# Your code here

Hint

Use OpenAI(temperature=0) to create the language model instance.

2

Set up LangServe API configuration

Create a variable called app that is an instance of LangServe with the llm you created.

LangChain

from langchain.llms import OpenAI

llm = OpenAI(temperature=0)
# Your code here

Hint

Use LangServe(llm=llm) to create the API app.

3

Write the main API logic

Define a function called answer_question that takes a parameter question and returns app.llm(question).

LangChain

from langchain.llms import OpenAI
from langchain.server import LangServe

llm = OpenAI(temperature=0)
app = LangServe(llm=llm)
# Your code here

Hint

Define a function that calls app.llm with the question.

4

Complete the API deployment code

Add the line app.run() at the end to start the LangServe API server.

LangChain

from langchain.llms import OpenAI
from langchain.server import LangServe

llm = OpenAI(temperature=0)
app = LangServe(llm=llm)

def answer_question(question: str) -> str:
    return app.llm(question)
# Your code here

Hint

Call app.run() to start the API server.

Practice

1. What is the main purpose of LangServe in LangChain?

easy

A. To quickly turn language models into web APIs

B. To train new language models from scratch

C. To visualize language model outputs in charts

D. To store large datasets for language models

Solution

1. Step 1: Understand LangServe's role

   LangServe is designed to make language models accessible as web APIs easily.

2. Step 2: Compare options with LangServe's function

   Only To quickly turn language models into web APIs matches this purpose; others describe unrelated tasks.

3. Final Answer:

   To quickly turn language models into web APIs -> Option A

4. Quick Check:

   LangServe = API deployment [OK]

Hint: LangServe = language model + web API [OK]

Common Mistakes:

• Confusing LangServe with model training tools
• Thinking LangServe is for data storage
• Assuming LangServe creates visualizations

2. Which of the following is the correct minimal structure for a LangServe class?

easy

A. def MyAPI(input):
return input.upper()

B. class MyAPI:
def __call__(self, input):
return input.upper()

C. class MyAPI:
def call(self, input):
return input.upper()

D. class MyAPI:
def __init__(self, input):
return input.upper()

Solution

1. Step 1: Identify required method for LangServe

   LangServe requires a class with a __call__ method to handle requests.

2. Step 2: Check each option's method name and structure

   Only class MyAPI: def __call__(self, input): return input.upper() uses __call__ correctly; others use wrong method names or invalid return in __init__.

3. Final Answer:

   class with __call__ method -> Option B

4. Quick Check:

   __call__ method = correct structure [OK]

Hint: LangServe needs __call__, not call or __init__ [OK]

Common Mistakes:

• Using call instead of __call__
• Returning values from __init__ method
• Defining a function instead of a class

3. Given this LangServe class:

class EchoAPI:
    def __call__(self, input):
        return f"Echo: {input}"

What will be the output when calling EchoAPI()('hello')?

medium

A. "hello"

B. TypeError: 'EchoAPI' object is not callable

C. "Echo: hello"

D. "EchoAPI: hello"

Solution

1. Step 1: Understand __call__ method behavior

   The __call__ method formats the input by prefixing 'Echo: ' to it.

2. Step 2: Evaluate the call EchoAPI()('hello')

   Creating EchoAPI instance and calling it with 'hello' returns 'Echo: hello'.

3. Final Answer:

   "Echo: hello" -> Option C

4. Quick Check:

   __call__ returns formatted string [OK]

Hint: Calling instance runs __call__ method [OK]

Common Mistakes:

• Expecting raw input without prefix
• Thinking instance is not callable
• Confusing class name with output

4. What is wrong with this LangServe class?

class BadAPI:
    def call(self, input):
        return input[::-1]

medium

A. The return statement should convert input to uppercase

B. The input slicing syntax is incorrect

C. The class must inherit from a base LangServe class

D. The method should be named __call__, not call

Solution

1. Step 1: Check method name required by LangServe

   LangServe expects a __call__ method to make the class callable.

2. Step 2: Analyze method name in BadAPI

   BadAPI uses call instead of __call__, so it won't work as expected.

3. Final Answer:

   The method should be named __call__, not call -> Option D

4. Quick Check:

   __call__ method required [OK]

Hint: Method must be __call__, not call [OK]

Common Mistakes:

• Using call instead of __call__
• Assuming inheritance is mandatory
• Thinking input slicing is invalid

5. You want to deploy a LangServe API that reverses input text but only if the input is a non-empty string. Which class correctly implements this?

hard

A. class ReverseAPI:
def __call__(self, input):
if input is None or input == "":
return "Empty input"
return input[::-1]

B. class ReverseAPI:
def __call__(self, input):
return input[::-1] if input != None else "Empty input"

C. class ReverseAPI:
def __call__(self, input):
if input == "":
return "Empty input"
else:
return input[::-1]

D. class ReverseAPI:
def __call__(self, input):
if input != "":
return input[::-1]
return "Empty input"

Solution

1. Step 1: Identify conditions for input validation

   We must check if input is None or empty string to handle empty input properly.

2. Step 2: Evaluate each option's condition

   class ReverseAPI: def __call__(self, input): if input is None or input == "": return "Empty input" return input[::-1] checks both None and empty string correctly before reversing input.

3. Final Answer:

   Checks both None and empty string before reversing -> Option A

4. Quick Check:

   Check None and empty string before processing [OK]

Hint: Check None and empty string explicitly [OK]

Common Mistakes:

• Only checking for empty string, missing None
• Using != None instead of is None
• Not handling empty input cases