app.py

def download(q: Q, loop: asyncio.AbstractEventLoop):
    bucket = 'h2o-wave'
    file_key = 'releases/h2o_wave-0.22.0-py3-none-win_amd64.whl'
    # Get the total size of the file.
    total = boto3.client('s3').head_object(Bucket=bucket, Key=file_key)['ContentLength']
    already_downloaded = 0

    # Nested update function that is called periodically from within boto.
    def update(val):
        nonlocal already_downloaded
        already_downloaded += val
        # Update the Wave UI.
        asyncio.ensure_future(update_ui(q, already_downloaded / total), loop=loop)

    # Since boto3 is synchronous, this download is blocking.
    boto3.resource('s3').Bucket(bucket).download_file(
        file_key, 'hello.whl', Callback=update
    )
    asyncio.ensure_future(show_notification(q), loop=loop)

async def show_notification(q: Q):
    q.page['meta'].dialog = None
    q.page['meta'].notification_bar = ui.notification_bar(
        name='success_notification',
        text='Job done!',
        type='success',
        events=['dismissed']
    )
    await q.page.save()

async def update_ui(q: Q, value: int):
    q.page['meta'].dialog.items[0].progress.value = value
    await q.page.save()