Python Language Tutorial => Coroutine and Delegation Syntax

Example

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.x3.5

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())

Python 3.x3.33.5

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())

Python 3.x3.5

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)

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Python Language Asyncio Module Coroutine and Delegation Syntax

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