[FIXED] How to block progression in asyncio

Issue

So I understand that using async ... await python can prevent blocking and accomplish xyz to follow. But what about the opposite, where I want python to block xyz until an process has completed?

For example, suppose I have three functions, A,B & C where A should not block B and vice versa. But C should be blocked by both A and B?

async def A():
   # some code


async def B():
   # some code

def C():
   # some code

async def handler():
   # A and B not to block each other
   await A() 
   await B()
   # Must complete before C
   C()

As I understand it, asyncio will implicitly infer that C should be blocked by A and B if C is defined in terms of them (or their outputs.)

But in the event that C is not defined in terms of A & B, how can it be ensured that C will only commence when A and B are complete?

Solution

If I understand you correctly, you want to run A, B in parallel and if they both finish, call C(). You can create tasks from A(), B(), use asyncio.gather() to wait for them to finish and then call C():

import asyncio


async def A():
    await asyncio.sleep(1)
    print("A finished")


async def B():
    await asyncio.sleep(2)
    print("B finished")


def C():
    print("In C!")


async def handler():
    # A and B not to block each other
    tasks = {asyncio.create_task(A()), asyncio.create_task(B())}
    await asyncio.gather(*tasks)
    # Must complete before C
    C()


asyncio.run(handler())

Prints (and finishes in 2 seconds):

A finished
B finished 
In C!

Answered By - Andrej Kesely