What happens when await is called on synchronous methods?

Question

I have synchronous methods for uploading and downloading files (represented by sync_wait method). I want to do that asynchronously and endlessly in a way where I have parallel executions of uploaders and downloaders at the same time forever. I achieved this this way:

Running this code:

import time, asyncio
from functools import wraps, partial

# https://stackoverflow.com/a/50450553/3026886
def to_async(func):
    @wraps(func)
    async def run(*args, **kwargs):
        return await asyncio.get_event_loop().run_in_executor(None, partial(func, *args, **kwargs))
    return run

@to_async
def sync_wait(msg):
    time.sleep(msg)

async def producer(n, queue):
    while True:
        msg = .2
        await sync_wait(msg)
        print(f'{n}p')
        await queue.put(msg)

async def consumer(n, queue):
    while True:
        msg = await queue.get()
        print(f'{n}c')
        await sync_wait(msg)

async def main():
    queue = queue = asyncio.Queue(10)
    producers = [producer(n, queue) for n in range(2)]
    consumers = [consumer(n, queue) for n in range(4)]
    await asyncio.gather(*(producers + consumers), return_exceptions=True)

if __name__ == "__main__":
    asyncio.run(main())

Printed this output:

1p
0p
0c
1c
1p
2c
0p
3c
1p
2c
0p
1c
1p
0c
0p
3c
1p
2c
0p
3c
1p
0c
0p
3c
...

which makes sense since I have 2 producers and 4 consumers interacting with my queue. My boss told me I didn't need the to_async decorator. But after removing only the decorator from sync_wait definiton, I got no prints at all. How can i explain this new behavior?

If you remove *only* the ``sync_wait`` but not the ``await``, things should crash with a noticeable error. — MisterMiyagi, Feb 28 '22 at 11:52
I removed only the decorator from `sync_wait`, meaning I kept using `sync_wait` but without `@to_async` above its definition. no errors were thrown — rado, Feb 28 '22 at 11:54

MisterMiyagi · Accepted Answer · 2022-03-01T08:35:29.420

When you await something that cannot be awaited, things crash:

# python3 -m asyncio
asyncio REPL 3.9.9 (main, Jan 10 2022, 11:05:09) 
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> await time.sleep(.5)  # sleeps .5 secs before failing to await
Traceback (most recent call last):
...
TypeError: object NoneType can't be used in 'await' expression

However, if you put failing things into a Task or gather them, they just fail silently until you actually await them to retrieve their result.

>>> async def fail():
...     print("about to fail...")
...     print(1/0)
...
>>> t = asyncio.create_task(fail())
about to fail...
>>> await t  # only fails noticeably when retrieving result
Traceback (most recent call last):
...
ZeroDivisionError: division by zero

Consequently, if you put failing tasks into a gather(..., return_exceptions=True) that has an infinitely running, non-failing task, failures are never reported.

What happens when await is called on synchronous methods?

1 Answers1