Async file write with tornado

app.py

import os
from uuid import uuid4

from tornado import web
from tornado.httpserver import HTTPServer
from tornado.ioloop import IOLoop
from tornado.iostream import PipeIOStream

IOLoop.configure('tornado.platform.asyncio.AsyncIOMainLoop')


class SafeFileIOStream:
    def __init__(self, fname):
        self.fname = fname

    def __enter__(self):
        # Create file
        os.open(self.fname, os.O_CREAT)
        # Create stream
        fd = os.open(self.fname, os.O_WRONLY)
        self.stream = PipeIOStream(fd)
        return self.stream

    def __exit__(self, exc_type, exc_val, exc_tb):
        # Close stream
        self.stream.close()


class TestHandler(web.RequestHandler):
    async def get(self):
        fname = '/tmp/' + uuid4().hex
        with SafeFileIOStream(fname) as stream:
            await stream.write(b'hello stranger')
        self.write('success')


app_handlers = [
    (r'/test/?', TestHandler),
]

application = web.Application(app_handlers)

# application.listen(8000, '0.0.0.0')

if __name__ == '__main__':
    server = HTTPServer(application)
    server.bind(8000)
    server.start(0)

    loop = IOLoop.current()
    loop.start()

This may seem async, because the server is using multiple processes, thus it executes requests in parallel. If you limit the processes to one (server.start(1)), the server will be sync and not asyc. Also, try using bigger files.