AsyncIO

Question 1

What AsyncIO is a coroutine function

Answer 1

• A coroutine is a function that can be suspended and resumed.
• A coroutine can be defined via the async def expression.
• A subroutine (function) can be executed, starting at one point and finishing at another point. Whereas, a coroutine (async function) can be executed then suspended, and resumed many times before finally terminating.

Question 2

What is AsyncIO event loop

Answer 2

The asyncio event loop is a single-threaded scheduler that runs coroutine-based tasks by switching between them whenever their awaited I/O or timers become ready. It uses OS-level event notification (like epoll on linux) to drive cooperative concurrency.

Question 3

Coroutine function vs coroutine object

Answer 3

• You can think of a coroutine function as a factory for coroutine objects; more directly, remember that calling a coroutine function does not cause any user-written code to execute, but rather just builds and returns a coroutine object.
• A coroutine Python object has methods, such as send() and close(). It is a type.
• A coroutine object is an awaitable. This means it is a Python type that implements the await() method.

async def custom_coro():
    await asyncio.sleep(1)

coro = custom_coro()
print(type(coro))
# <class 'coroutine'>

Question 4

Basic way to start new event loop for a program (that will execute main)

Answer 4

import asyncio

async def main():
    await custom_coro()

asyncio.run(main())

Question 5

Create event loop (low level method)

Answer 5

loop = asyncio.new_event_loop()

Question 6

What is Task

Answer 6

• The asyncio event loop manages tasks. As such, all coroutines become and are managed as tasks within the event loop.
• The create_task() function wraps an awaitable object into a higher-level Task object that’s scheduled to run concurrently on the event loop in the background. In contrast, awaiting a coroutine runs it immediately, pausing the execution of the caller until the awaited coroutine finishes.

Question 7

How to create task for t coroutine, called "MyTask"

Answer 7

task = asyncio.create_task(task_coroutine(), name='MyTask')

Question 8

How to see if task was finished or cancelled?

Answer 8

task.done()
task.cancelled()

Question 9

How to get results of a task and handle errors

Answer 9

try:
	# Get the return value from the wrapped coroutine
	value = task.result()
except asyncio.CancelledError:
	# Task was canceled
except asyncio.InvalidStateError:
	# Task is not yet done
except Exception:
	# Task failed and there is no result

Question 10

How to get list of all tasks

Answer 10

tasks = asyncio.all_tasks()

Question 11

How to group awaitables to be treated as a single awaitable

Answer 11

results = await asyncio.gather(coro1(), asyncio.create_task(coro2()))

Question 12

Wait for list of tasks to complete.
- All of them
- Only first
- First error

Answer 12

done, pending = await asyncio.wait(tasks, return_when=asyncio.ALL_COMPLETED) # default
first_done, rest = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)
first_error, rest = await asyncio.wait(tasks, return_when=asyncio.FIRST_EXCEPTION)

Question 13

Loop over async iterator (like async generator)

Answer 13

async for i in powers_of_two(5):
    g.append(i)

Question 14

How to work with async resource with context manager (with)

Answer 14

        async with session.get(url) as response:
            print(f"{url}: status -> {response.status}")

Question 15

How to use async queue?
- Add elements
- Remove elements

Answer 15

queue = asyncio.Queue()

queue.put(user)
user = await queue.get()

Question 16

How to sleep for 1 second (async)

Answer 16

asyncio.sleep(1)