asyncio: schedule a coroutine

Once you write a coroutine, you need three things to get it running: create it, obtain a loop and schedule coroutine's execution.

The first two steps are trivial:

import asyncio
import sys

async def indication(timeout):
    while True:
        print('.', end='')
        sys.stdout.flush()
        await asyncio.sleep(timeout)
async def sleep(t, indication_t):    # steps
    coro = indication(indication_t)  # 1
    loop = asyncio.get_event_loop()  # 2

    # ...
    await asyncio.sleep(t)

loop = asyncio.get_event_loop()
loop.run_until_complete(sleep(5, 0.5))

The slightly trickier part is to schedule execution of a coroutine within an already running loop. There are two different ways to achieve that: loop.create_task(coro) and asyncio.ensure_future(coro, loop=loop). Though they both work, they have pretty different semantic.

create_task does precisely what we need: it schedules execution of a coroutine and returns a future that allows you to track the coroutine execution (likely by awaiting it). That future is strictly speaking a task (asyncio.Task), the particular type of futures that wrap a coroutine.

ensure_future(x) just ensures that x is a future or wrap it in one. If x is a coroutine, is uses create_task for such wrapping, obviously scheduling x. The previous name for ensure_future is async; it was changed to respect the fact that async is a keyword now.

import asyncio
import sys

async def indication(timeout):
    while True:
        print('.', end='')
        sys.stdout.flush()
        await asyncio.sleep(timeout)
async def sleep(t, indication_t):
    coro = indication(indication_t)
    loop = asyncio.get_event_loop()

    # # Choose one:
    # loop.create_task(coro)
    # asyncio.ensure_future(coro)

    await asyncio.sleep(t)

loop = asyncio.get_event_loop()
loop.run_until_complete(sleep(5, 0.5))

Guido explicitly recommends using create_task() since its intent is more clear.