Async HTTP client calls in FastAPI - Mini Project: Build & Apply

Async HTTP Client Calls with FastAPI

📖 Scenario: You are building a FastAPI application that fetches data from an external API asynchronously. This helps your app stay fast and responsive, even when waiting for other servers.

🎯 Goal: Create a FastAPI app that uses an async HTTP client to get JSON data from a public API and returns it to the user.

📋 What You'll Learn

Use FastAPI to create a web server

Use the httpx.AsyncClient to make asynchronous HTTP GET requests

Create an endpoint /joke that fetches a random joke from the external API

Return the JSON response from the external API directly to the client

💡 Why This Matters

🌍 Real World

Many web apps need to get data from other servers without freezing or slowing down. Async HTTP clients let your app stay fast and handle many users at once.

💼 Career

Knowing how to use async HTTP calls in FastAPI is important for backend developers building scalable APIs and microservices.

1

Set up FastAPI app and import httpx

Create a FastAPI app by importing FastAPI and httpx. Then create an instance called app.

FastAPI

from fastapi import FastAPI
import httpx

# Create FastAPI app instance
# Your code here

Hint

Use app = FastAPI() to create the app instance.

2

Define the external API URL

Create a variable called joke_api_url and set it to the string "https://official-joke-api.appspot.com/random_joke".

FastAPI

from fastapi import FastAPI
import httpx

app = FastAPI()

# Define the external joke API URL
# Your code here

Hint

Assign the URL string exactly to joke_api_url.

3

Create async endpoint to fetch joke

Define an async function called get_joke with the route decorator @app.get("/joke"). Inside, use async with httpx.AsyncClient() as client to make a GET request to joke_api_url. Await the response and return response.json().

FastAPI

from fastapi import FastAPI
import httpx

app = FastAPI()

joke_api_url = "https://official-joke-api.appspot.com/random_joke"

# Create async endpoint to fetch joke
# Your code here

Hint

Remember to use async def and await for the HTTP call.

4

Add main guard to run app with Uvicorn

Add the code block if __name__ == "__main__": and inside it call import uvicorn and then uvicorn.run(app, host="127.0.0.1", port=8000) to run the FastAPI app.

FastAPI

from fastapi import FastAPI
import httpx

app = FastAPI()

joke_api_url = "https://official-joke-api.appspot.com/random_joke"

@app.get("/joke")
async def get_joke():
    async with httpx.AsyncClient() as client:
        response = await client.get(joke_api_url)
        return response.json()

# Add main guard to run app with uvicorn
# Your code here

Hint

This lets you run the app by executing the script directly.

Practice

1. What is the main benefit of using async HTTP client calls in a FastAPI application?

easy

A. They allow the app to handle other tasks while waiting for the HTTP response.

B. They make the HTTP response faster from the server side.

C. They automatically cache all HTTP responses.

D. They convert HTTP calls into database queries.

Solution

1. Step 1: Understand async behavior in FastAPI

   Async calls let the app continue working without waiting for the HTTP response, improving responsiveness.

2. Step 2: Identify the benefit of async HTTP client calls

   This means the app can handle other requests or tasks while waiting, making it efficient.

3. Final Answer:

   They allow the app to handle other tasks while waiting for the HTTP response. -> Option A

4. Quick Check:

   Async calls improve responsiveness = B [OK]

Hint: Async means non-blocking wait for HTTP response [OK]

Common Mistakes:

• Thinking async makes server response faster
• Confusing async with caching
• Believing async converts HTTP to database calls

2. Which of the following is the correct way to create an async HTTP client call using httpx in FastAPI?

easy

A. async with httpx.AsyncClient() as client: response = await client.get('https://example.com')

B. response = httpx.get('https://example.com')

C. client = httpx.Client(); response = client.get('https://example.com')

D. client = httpx.AsyncClient(); response = client.get('https://example.com')

Solution

1. Step 1: Identify async client usage

   Using async with ensures proper resource management for async clients.

2. Step 2: Use await to wait for the async HTTP call

   The call to client.get() must be awaited to get the response asynchronously.

3. Final Answer:

   async with httpx.AsyncClient() as client: response = await client.get('https://example.com') -> Option A

4. Quick Check:

   Use async with and await for async calls = C [OK]

Hint: Use async with and await for async HTTP calls [OK]

Common Mistakes:

• Using httpx.Client instead of AsyncClient for async calls
• Not using await with async client.get()
• Not using async with for client context

3. What will be the output of this FastAPI async function when called?

import httpx
from fastapi import FastAPI
app = FastAPI()

@app.get('/data')
async def get_data():
    async with httpx.AsyncClient() as client:
        response = await client.get('https://httpbin.org/get')
    return {'status_code': response.status_code}

medium

A. SyntaxError in async function

B. {'status_code': '200'}

C. RuntimeError due to missing await

D. {'status_code': 200}

Solution

1. Step 1: Analyze the async HTTP call

   The function uses await with client.get(), so it correctly waits for the response.

2. Step 2: Check the returned data

   The response status code is an integer 200 for a successful HTTP GET, returned inside a dictionary.

3. Final Answer:

   {'status_code': 200} -> Option D

4. Quick Check:

   Awaited async call returns status 200 = A [OK]

Hint: Await async calls to get response object correctly [OK]

Common Mistakes:

• Returning status code as string instead of int
• Forgetting await causing runtime errors
• Syntax errors from incorrect async usage

4. Identify the error in this FastAPI async HTTP client code:

import httpx
from fastapi import FastAPI
app = FastAPI()

@app.get('/info')
async def get_info():
    client = httpx.AsyncClient()
    response = client.get('https://httpbin.org/get')
    return {'status': response.status_code}

medium

A. Using httpx.Client instead of AsyncClient

B. Missing await before client.get() call

C. Missing async keyword in function definition

D. Incorrect return type, should return string

Solution

1. Step 1: Check async call usage

   The client.get() is an async method and must be awaited to get the response.

2. Step 2: Identify missing await

   Without await, the code returns a coroutine object, causing errors when accessing status_code.

3. Final Answer:

   Missing await before client.get() call -> Option B

4. Quick Check:

   Async calls require await = A [OK]

Hint: Always await async client.get() calls [OK]

Common Mistakes:

• Not awaiting async HTTP calls
• Using sync client in async function
• Forgetting async keyword on function

5. You want to fetch data from two different URLs concurrently in a FastAPI async endpoint. Which code correctly achieves this using httpx.AsyncClient?

hard

A. async with httpx.AsyncClient() as client:
r1 = await client.get(url1)
r2 = await client.get(url2)
return {'r1': r1.status_code, 'r2': r2.status_code}

B. async with httpx.AsyncClient() as client:
r1, r2 = await client.get(url1), await client.get(url2)
return {'r1': r1.status_code, 'r2': r2.status_code}

C. async with httpx.AsyncClient() as client:
import asyncio
r1, r2 = await asyncio.gather(client.get(url1), client.get(url2))
return {'r1': r1.status_code, 'r2': r2.status_code}

D. client = httpx.AsyncClient()
r1 = client.get(url1)
r2 = client.get(url2)
return {'r1': r1.status_code, 'r2': r2.status_code}

Solution

1. Step 1: Understand concurrency with async HTTP calls

   To run multiple async calls concurrently, use asyncio.gather() to await them together.

2. Step 2: Check code correctness

   async with httpx.AsyncClient() as client: import asyncio r1, r2 = await asyncio.gather(client.get(url1), client.get(url2)) return {'r1': r1.status_code, 'r2': r2.status_code} uses asyncio.gather() inside async with context, correctly awaiting both calls concurrently.

3. Final Answer:

   async with httpx.AsyncClient() as client: import asyncio r1, r2 = await asyncio.gather(client.get(url1), client.get(url2)) return {'r1': r1.status_code, 'r2': r2.status_code} -> Option C

4. Quick Check:

   Use asyncio.gather for concurrent async calls = D [OK]

Hint: Use asyncio.gather to await multiple async calls concurrently [OK]

Common Mistakes:

• Awaiting calls sequentially losing concurrency
• Not awaiting calls causing coroutine errors
• Not using async with for client context