Blocking Call in Async Context

Why This Happens

The asyncio event loop runs in a single thread. Calling blocking functions like time.sleep(), requests.get(), or CPU-intensive code blocks the entire loop.

The Problem

import asyncio, time, requests

async def fetch():
    response = requests.get('https://api.example.com')  # Blocks!
    time.sleep(1)  # Blocks!
    return response.json()

The Fix

import asyncio, httpx

async def fetch():
    async with httpx.AsyncClient() as client:
        response = await client.get('https://api.example.com')
    await asyncio.sleep(1)
    return response.json()

# Or wrap sync code:
async def fetch():
    response = await asyncio.to_thread(requests.get, 'https://api.example.com')
    return response.json()

Step-by-Step Fix

1. 1

   Use async libraries

   Replace requests with httpx/aiohttp. Replace time.sleep with asyncio.sleep.

2. 2

   Use asyncio.to_thread()

   Wrap blocking calls: await asyncio.to_thread(func, args).

3. 3

   Use run_in_executor()

   For Python <3.9: await loop.run_in_executor(None, func).