Using Asyncio Conditions for Stateful Coroutines

Mar 25, 2024 ยท 2 min read

Asyncio conditions allow coroutines to wait for certain states or events during execution. They are useful for scenarios where you need to coordinate or synchronize several coroutines based on shared state.

Why Conditions Over Events/Queues?

Unlike events or queues, conditions are stateful. A condition has an internal flag that gets set() and cleared() to denote state changes. Coroutines can then wait() for the flag to be set indicating a certain state has been reached.

This makes them perfect for scenarios like:

  • Waiting for a shared resource to become available
  • Pausing execution until a certain state is reached
  • Blocking a coroutine until certain work is complete
  • A Condition Usage Example

    Here's a simple consumer/producer example using a condition:

    import asyncio
    import random
    async def producer(cond):
        """Set random delay before setting condition"""
        wait_for = random.randint(1, 5) 
        await asyncio.sleep(wait_for)
    async def consumer(cond):
        """Wait for condition to be set by producer""" 
        await cond.wait()
        print('Resource is now available!')
    async def main():
        cond = asyncio.Condition()
        prod_task = asyncio.create_task(producer(cond))
        cons_task = asyncio.create_task(consumer(cond))
        await asyncio.gather(prod_task, cons_task)

    The key thing to note is that consumer() waits for the condition to be set in producer() indicating some shared state change.

    Other Useful Methods

  • cond.notify() - Resume one coroutine waiting on the condition
  • cond.notify_all() - Resume all coroutines waiting
  • These allow you to precisely control how many/which coroutines are resumed when the state changes.

    Common Pitfalls

  • Forgetting await cond.wait() - this suspends the coroutine
  • Managing access to shared state - mutexes are often still needed
  • Using asyncio conditions correctly takes some practice to get right! Start simple and test thoroughly.

    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: