Using Asyncio Conditions for Stateful Coroutines

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:

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)
    cond.set()

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)

asyncio.run(main())

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

These allow you to precisely control how many/which coroutines are resumed when the state changes.

Common Pitfalls

Using asyncio conditions correctly takes some practice to get right! Start simple and test thoroughly.