Before Python 3.5+ was released, the asyncio module used generators to mimic asynchronous calls and thus had a different syntax than the current Python 3.5 release.

Python 3.5 introduced the async and await keywords. Note the lack of parentheses around the await func() call.

import asyncio

async def main():
    print(await func())

async def func():
    # Do time intensive stuff...
    return "Hello, world!"

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Before Python 3.5, the @asyncio.coroutine decorator was used to define a coroutine. The yield from expression was used for generator delegation. Note the parentheses around the yield from func().

import asyncio

@asyncio.coroutine
def main():
    print((yield from func()))

@asyncio.coroutine
def func():
    # Do time intensive stuff..
    return "Hello, world!"

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Here is an example that shows how two functions can be run asynchronously:

import asyncio

async def cor1():
    print("cor1 start")
    for i in range(10):
        await asyncio.sleep(1.5)
        print("cor1", i)

async def cor2():
    print("cor2 start")
    for i in range(15):
        await asyncio.sleep(1)
        print("cor2", i)

loop = asyncio.get_event_loop()
cors = asyncio.wait([cor1(), cor2()])
loop.run_until_complete(cors)

Note: Uses the Python 3.5+ async/await syntax

asyncio supports the use of Executor objects found in concurrent.futures for scheduling tasks asynchronously. Event loops have the function run_in_executor() which takes an Executor object, a Callable, and the Callable's parameters.

Scheduling a task for an Executor

import asyncio
from concurrent.futures import ThreadPoolExecutor

def func(a, b):
    # Do time intensive stuff...
    return a + b

async def main(loop):
    executor = ThreadPoolExecutor()
    result = await loop.run_in_executor(executor, func, "Hello,", " world!")
    print(result)

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main(loop))

Each event loop also has a "default" Executor slot that can be assigned to an Executor. To assign an Executor and schedule tasks from the loop you use the set_default_executor() method.

import asyncio
from concurrent.futures import ThreadPoolExecutor

def func(a, b):
    # Do time intensive stuff...
    return a + b

async def main(loop):
    # NOTE: Using `None` as the first parameter designates the `default` Executor.
    result = await loop.run_in_executor(None, func, "Hello,", " world!")
    print(result)

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.set_default_executor(ThreadPoolExecutor())
    loop.run_until_complete(main(loop))

There are two main types of Executor in concurrent.futures, the ThreadPoolExecutor and the ProcessPoolExecutor. The ThreadPoolExecutor contains a pool of threads which can either be manually set to a specific number of threads through the constructor or defaults to the number of cores on the machine times 5. The ThreadPoolExecutor uses the pool of threads to execute tasks assigned to it and is generally better at CPU-bound operations rather than I/O bound operations. Contrast that to the ProcessPoolExecutor which spawns a new process for each task assigned to it. The ProcessPoolExecutor can only take tasks and parameters that are picklable. The most common non-picklable tasks are the methods of objects. If you must schedule an object's method as a task in an Executor you must use a ThreadPoolExecutor.

uvloop is an implementation for the asyncio.AbstractEventLoop based on libuv (Used by nodejs). It is compliant with 99% of asyncio features and is much faster than the traditional asyncio.EventLoop. uvloop is currently not available on Windows, install it with pip install uvloop.

import asyncio
import uvloop

if __name__ == "__main__":
    asyncio.set_event_loop(uvloop.new_event_loop())
    # Do your stuff here ...

One can also change the event loop factory by setting the EventLoopPolicy to the one in uvloop.

import asyncio
import uvloop

if __name__ == "__main__":
    asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
    loop = asyncio.new_event_loop()

Concept

Use an Event to synchronize the scheduling of multiple coroutines.

Put simply, an event is like the gun shot at a running race: it lets the runners off the starting blocks.

Example

import asyncio

# event trigger function
def trigger(event):
    print('EVENT SET')
    event.set()  # wake up coroutines waiting

# event consumers
async def consumer_a(event):
    consumer_name = 'Consumer A'
    print('{} waiting'.format(consumer_name))
    await event.wait()
    print('{} triggered'.format(consumer_name))

async def consumer_b(event):
    consumer_name = 'Consumer B'
    print('{} waiting'.format(consumer_name))
    await event.wait()
    print('{} triggered'.format(consumer_name))

# event
event = asyncio.Event()

# wrap coroutines in one future
main_future = asyncio.wait([consumer_a(event),
                            consumer_b(event)])

# event loop
event_loop = asyncio.get_event_loop()
event_loop.call_later(0.1, functools.partial(trigger, event))  # trigger event in 0.1 sec

# complete main_future
done, pending = event_loop.run_until_complete(main_future)

Output:

Consumer B waiting
Consumer A waiting
EVENT SET
Consumer B triggered
Consumer A triggered

Here we make a simple echo websocket using asyncio. We define coroutines for connecting to a server and sending/receiving messages. The communcations of the websocket are run in a main coroutine, which is run by an event loop. This example is modified from a prior post (opens new window).

import asyncio
import aiohttp

session = aiohttp.ClientSession()                          # handles the context manager
class EchoWebsocket:
    
    async def connect(self):
        self.websocket = await session.ws_connect("wss://echo.websocket.org")
        
    async def send(self, message):
        self.websocket.send_str(message)

    async def receive(self):
        result = (await self.websocket.receive())
        return result.data

async def main():
    echo = EchoWebsocket()
    await echo.connect()
    await echo.send("Hello World!")
    print(await echo.receive())                            # "Hello World!"


if __name__ == '__main__':
    # The main loop
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

probably the most common misconception about asnycio is that it lets you run any task in parallel - sidestepping the GIL (global interpreter lock) and therefore execute blocking jobs in parallel (on separate threads). it does not!

asyncio (and libraries that are built to collaborate with asyncio) build on coroutines: functions that (collaboratively) yield the control flow back to the calling function. note asyncio.sleep in the examples above. this is an example of a non-blocking coroutine that waits 'in the background' and gives the control flow back to the calling function (when called with await). time.sleep is an example of a blocking function. the execution flow of the program will just stop there and only return after time.sleep has finished.

a real-live example is the requests (opens new window) library which consists (for the time being) on blocking functions only. there is no concurrency if you call any of its functions within asyncio. aiohttp (opens new window) on the other hand was built with asyncio in mind. its coroutines will run concurrently.