aysncio cannot read stdin on Windows

Question

I'm trying to read stdin asynchronously on Windows 7 64-bit and Python 3.4.3

I tried this inspired by an SO answer:

import asyncio
import sys


def reader():
    print('Received:', sys.stdin.readline())


loop = asyncio.get_event_loop()
task = loop.add_reader(sys.stdin.fileno(), reader)
loop.run_forever()
loop.close()

However, it raises an OSError: [WInError 100381] An operation was attempted on something that is not a socket.

Could a file-like object like stdin be wrapped in a class to give it the API of a socket? I have asked this question separately, but if the solution is simple please answer here.

Assuming that I cannot wrap a file-like object to make it a socket, I tried using streams as inspired by this gist:

import asyncio
import sys


@asyncio.coroutine
def stdio(loop):
    reader = asyncio.StreamReader(loop=loop)
    reader_protocol = asyncio.StreamReaderProtocol(reader)
    yield from loop.connect_read_pipe(lambda: reader_protocol, sys.stdin)


@asyncio.coroutine
def async_input(loop):
    reader = yield from stdio(loop)
    line = yield from reader.readline()
    return line.decode().replace('\r', '').replace('\n', '')


@asyncio.coroutine
def main(loop):
    name = yield from async_input(loop)
    print('Hello ', name)


loop = asyncio.get_event_loop()
loop.run_until_complete(main(loop))
loop.close()

And that raises a NotImplementedError in asyncio.base_events._make_read_pipe_transport

Please advise how to read stdin using asyncio on Windows...

Answer 1

The NotImplementedError exception is raised because the connect pipes coroutines are not supported by the SelectorEventLoop, which is the default event loop set on asyncio. You need to use a ProactorEventLoop to support pipes on Windows. However, it would still not work because apparently the connect_read_pipe and connect_write_pipe functions doesn't support stdin/stdout/stderr or files in Windows as Python 3.5.1.

One way to read from stdin with an asynchronous behavior is using a thread with the loop's run_in_executor method. Here is a simple example for reference:

import asyncio
import sys

async def aio_readline(loop):
    while True:
        line = await loop.run_in_executor(None, sys.stdin.readline)
        print('Got line:', line, end='')

loop = asyncio.get_event_loop()
loop.run_until_complete(aio_readline(loop))
loop.close()

In the example the function sys.stdin.readline() is called within another thread by the loop.run_in_executor method. The thread remains blocked until stdin receives a linefeed, in the mean time the loop is free to execute others coroutines if they existed.