Python asyncio task ordering

Question

I have a question about how the event loop in python's asyncio module manages outstanding tasks. Consider the following code:

import asyncio

@asyncio.coroutine
def a():
   for i in range(0, 3):
      print('a.' + str(i))
      yield


@asyncio.coroutine
def b():
   for i in range(0, 3):
      print('b.' + str(i))
      yield


@asyncio.coroutine
def c():
   for i in range(0, 3):
      print('c.' + str(i))
      yield


tasks = [
   asyncio.Task(a()),
   asyncio.Task(b()),
   asyncio.Task(c()),
]

loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait([t1, t2, t3]))

Running this will print:

a.0
b.0
c.0
a.1
b.1
c.1
a.2
b.2
c.2

Notice that it always prints out 'a' then 'b' then 'c'. I'm guessing that no matter how many iterations each coroutine goes through it will always print in that order. So you'd never see something like

b.100
c.100
a.100

Coming from a node.js background, this tells me that the event loop here is maintaining a queue internally that it uses to decide which task to run next. It initially puts a() at the front of the queue, then b(), then c() since that's the order of the tasks in the list passed to asyncio.wait(). Then whenever it hits a yield statement it puts that task at the end of the queue. I guess in a more realistic example, say if you were doing an async http request, it would put a() back on the end of the queue after the http response came back.

Can I get an amen on this?

Answer 1

Currently your example doesn't include any blocking I/O code. Try this to simulate some tasks:

import asyncio


@asyncio.coroutine
def coro(tag, delay):
    for i in range(1, 8):
        print(tag, i)
        yield from asyncio.sleep(delay)


loop = asyncio.get_event_loop()

print("---- await 0 seconds :-) --- ")
tasks = [
    asyncio.Task(coro("A", 0)),
    asyncio.Task(coro("B", 0)),
    asyncio.Task(coro("C", 0)),
]

loop.run_until_complete(asyncio.wait(tasks))

print("---- simulate some blocking I/O --- ")
tasks = [
    asyncio.Task(coro("A", 0.1)),
    asyncio.Task(coro("B", 0.3)),
    asyncio.Task(coro("C", 0.5)),
]

loop.run_until_complete(asyncio.wait(tasks))

loop.close()

As you can see, coroutines are scheduled as needed, and not in order.