How to use the aiofiles.os.sendfile function in aiofiles

def serve_file(_, writer):
        out_fd = writer.transport.get_extra_info('socket').fileno()
        size = (yield from aiofiles.os.stat(filename)).st_size
        in_file = yield from aiofiles.open(filename)
        try:
            in_fd = in_file.fileno()
            yield from aiofiles.os.sendfile(out_fd, in_fd, 0, size)
        finally:
            yield from in_file.close()
            yield from writer.drain()
            writer.close()

"""Test the sendfile functionality, file-to-file."""
    filename = join(dirname(__file__), 'resources', 'test_file1.txt')
    tmp_filename = tmpdir.join('tmp.bin')

    with open(filename) as f:
        contents = f.read()

    input_file = yield from aiofiles.open(filename)
    output_file = yield from aiofiles.open(str(tmp_filename), mode='w+')

    size = (yield from aiofiles.os.stat(filename)).st_size

    input_fd = input_file.fileno()
    output_fd = output_file.fileno()

    yield from aiofiles.os.sendfile(output_fd, input_fd, 0, size)

    yield from output_file.seek(0)

    actual_contents = yield from output_file.read()
    actual_size = (yield from aiofiles.os.stat(str(tmp_filename))).st_size

    assert contents == actual_contents
    assert size == actual_size