Python 3.5 - Name 'await' is not defined

Question

I'm trying to experiment with the new await syntax for coroutines in Python 3.5.

I had a simple example like this:

#! /usr/bin/env python
import asyncio

@asyncio.coroutine
def adder(*args, delay):
    while True:
        yield from asyncio.sleep(delay)
        print(sum(args))


def main():
    asyncio.Task(adder(1, 2, 3, delay=5))
    asyncio.Task(adder(10, 20, delay=3))

    loop = asyncio.get_event_loop()
    loop.run_forever()

if __name__ == "__main__":
    main()

I changed the yield from line to use the await keyword:

await asyncio.sleep(delay)

And I get SyntaxError:

  File "./test.py", line 8
    await asyncio.sleep(delay)
                ^
SyntaxError: invalid syntax

So I try await (asyncio.sleep(delay)) just to see what happens:

Task exception was never retrieved
future: <Task finished coro=<adder() done, defined at c:\python35\Lib\asyncio\coroutines.py:198> exception=NameError("name 'await' is not defined",)>
Traceback (most recent call last):
  File "c:\python35\Lib\asyncio\tasks.py", line 239, in _step
    result = coro.send(value)
  File "c:\python35\Lib\asyncio\coroutines.py", line 200, in coro
    res = func(*args, **kw)
  File "./test.py", line 8, in adder
    await (asyncio.sleep(delay))
NameError: name 'await' is not defined
Task exception was never retrieved
future: <Task finished coro=<adder() done, defined at c:\python35\Lib\asyncio\coroutines.py:198> exception=NameError("name 'await' is not defined",)>
Traceback (most recent call last):
  File "c:\python35\Lib\asyncio\tasks.py", line 239, in _step
    result = coro.send(value)
  File "c:\python35\Lib\asyncio\coroutines.py", line 200, in coro
    res = func(*args, **kw)
  File "./test.py", line 8, in adder
    await (asyncio.sleep(delay))
NameError: name 'await' is not defined

Am I using the keyword wrong? Why is await not defined? I got my await syntax from this post.

Just to cover all my bases:

$ /usr/bin/env python --version

Python 3.5.0

Edit:

I suppose adding the parens to the await line is trying to call a function await() - so that's why that doesn't work and gives me a NameError. But why doesn't the keyword get recognized in either example?

Answer 1

You must declare your function as async to use await. Replacing yield from is not sufficient.

#! /usr/bin/env python
import asyncio

@asyncio.coroutine
async def adder(*args, delay):
    while True:
        await asyncio.sleep(delay)
        print(sum(args))


def main():
    asyncio.Task(adder(1, 2, 3, delay=5))
    asyncio.Task(adder(10, 20, delay=3))
    loop = asyncio.get_event_loop()
    loop.run_forever()

if __name__ == "__main__":
    main()

Answer 2

You need to declare your function as async to make this work. The await keyword will only be enabled if you include an a function declared with async def in your file – see PEP 492 for details.