Concurrency and Thread Safety in Python's asyncio

Mar 17, 2024 ยท 2 min read

Python's asyncio module provides infrastructure for writing concurrent code using the async/await syntax. Asynchronous programming enables executing multiple tasks concurrently within a single thread, avoiding resource-heavy multi-threaded environments. However, special care must be taken to ensure asyncio code is thread-safe.

The Event Loop

The key to understanding asyncio's thread safety is the event loop. This is a single-threaded executor managing all coroutine scheduling. By default, there is one global event loop per process. Attempting to run multiple event loops or run coroutine code outside the main loop can introduce race conditions and break concurrency guarantees.

import asyncio

loop = asyncio.get_event_loop()

Sharing Data Between Coroutines

Since all coroutines execute within the same thread, sharing data between them is thread-safe. However, correctness still depends on proper synchronization, usually via asyncio.Lock:

lock = asyncio.Lock()

async def access_shared_state():
   async with lock:
      # do something with shared state

This prevents concurrent access to shared data. But running asyncio code in threads can break these guarantees.


Executing asyncio coroutines in threads requires creating a new event loop instance using asyncio.new_event_loop(). Cross-thread communication must use appropriate synchronization like asyncio.Queue.

Beware running blocking code in asyncio coroutines - it blocks the entire event loop since there is only one thread. For CPU-bound work, use asyncio.to_thread():

loop = asyncio.get_event_loop()
result = await loop.run_in_executor(None, blocking_func)

Key Takeaways

  • Asyncio enables concurrency within a single thread using an event loop
  • Sharing data between coroutines is thread-safe
  • Multithreading requires new event loops and explicit synchronization
  • Blocking code must execute in threads to avoid blocking the event loop
  • Following these best practices ensures efficient, thread-safe asyncio code.

    Browse by tags:

    Browse by language:

    The easiest way to do Web Scraping

    Get HTML from any page with a simple API call. We handle proxy rotation, browser identities, automatic retries, CAPTCHAs, JavaScript rendering, etc automatically for you

    Try ProxiesAPI for free

    curl ""

    <!doctype html>
        <title>Example Domain</title>
        <meta charset="utf-8" />
        <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1" />


    Don't leave just yet!

    Enter your email below to claim your free API key: