Read file line by line with asyncio

Question

I wish to read several log files as they are written and process their input with asyncio. The code will have to run on windows. From what I understand from searching around bot

Answer 1

From what I understand from searching around both stackoverflow and the web, asynchronous file I/O is tricky on most operating systems (select will not work as intended, for example). While I'm sure I could do this with other methods (e.g. threads), I though I would try out asyncio to see what it is like.

asyncio is select based on *nix systems under the hood, so you won't be able to do non-blocking file I/O without the use of threads. On Windows, asyncio can use IOCP, which supports non-blocking file I/O, but this isn't supported by asyncio.

Your code is fine, except you should do blocking I/O calls in threads, so that you don't block the event loop if the I/O is slow. Fortunately, it's really simple to off load work to threads using the loop.run_in_executor function.

First, setup a dedicated thread-pool for your I/O:

from concurrent.futures import ThreadPoolExecutor
io_pool_exc = ThreadPoolExecutor()

And then simply offload any blocking I/O calls to the executor:

...
line = yield from loop.run_in_executor(io_pool_exc, f.readline)
...