async

Python Async

Event Loop - manage distribute tasks

• when a task await, it will be put into the event loop and another task will run

import asyncio
async def main(): # This is a coroutine function
    print('Hello')

The asyncio doesn’t work in Jupyter Notebook

await main() # Use this for Jupyter notebook

asyncio.run(main()) # This creates a new event loop and runs the coroutine
main() # this will return a coroutine object but not run

Coroutine object needs to be awaited or it won’t execute.

Tasks

Tasks allow you to run multiple coroutines concurrently (run another coroutine while waiting for the first one to finish)

# Create a task
task = asyncio.create_task(main())
task2 = asyncio.create_task(main())
await task # wait for the task to finish before continuing
# but as soon as task is delayed, task2 can be run

# Another way to create tasks is using gather
await asyncio.gather(main(), main(), main())

Semaphore

A semaphore is a counter that limits the number of tasks that can access a shared resource or perform a particular action at the same time.

Lock

Lock will block the execution of the code until the lock is released.

semaphore = asyncio.Semaphore(2)
async def my_coro():
    async with semaphore: # This will only allow 2 coroutines to run at a time
        print('Hello')
        await asyncio.sleep(1)
await asyncio.gather(*(my_coro() for i in range(10))) # only the first 2 will run at the same time

lock = asyncio.Lock()
async def my_coro():
    print("no lock") # no lock will be printed first and immediately
    await asyncio.sleep(3)
    async with lock:
        print('Hello')
        await asyncio.sleep(1)
await asyncio.gather(*(my_coro() for i in range(5))) # 

no lock
no lock
no lock
no lock
no lock
Hello
Hello
Hello
Hello
Hello

[None, None, None, None, None]