问题

How can I use Asynchronous Widgets on jupyter lab?

I'm trying to reproduce the official Asynchronous Widgets-Example on jupyter lab, but the await never continues.

Setup / reproduction

docker run --rm -p 8888:8888 -e JUPYTER_ENABLE_LAB=yes jupyter/datascience-notebook start-notebook.sh --NotebookApp.token=''
firefox 0.0.0.0:8888
create a new python3 notebook
create a cell and enter the code below
run cell
move slider

code for the cell

%gui asyncio

import asyncio
def wait_for_change(widget, value):
    future = asyncio.Future()
    def getvalue(change):
        # make the new value available
        future.set_result(change.new)
        widget.unobserve(getvalue, value)
    widget.observe(getvalue, value)
    return future

from ipywidgets import IntSlider
slider = IntSlider()

async def f():
    for i in range(10):
        print('did work %s'%i)
        #x = await asyncio.sleep(1)
        x = await wait_for_change(slider, 'value')
        print('async function continued with value %s'%x)
asyncio.ensure_future(f())
#task = asyncio.create_task(f())
slider

Expected result

The cell outputs

did work 0
async function continued with value 1
did work 1
async function continued with value 2
[...]

Actual output

nothing after the first did work 0

Notes

I'm specifically talking about jupyter lab and not about regular jupyter notebooks
There is no error-message or anything. The expected output just doesn't happen
The minimal asyncio-example does work in jupyter lab:

import asyncio
async def main():
    print('hello')
    await asyncio.sleep(1)
    print('world')
await main()

when you leave out the -e JUPYTER_ENABLE_LAB=yes, then you get a regular jupyter notebook without jupyter lab and the expected result happens.
this is not a duplicate of ipywidgets widgets values not changing or Jupyter Interactive Widget not executing properly, because these questions nether include jupyter lab nor asyncio

回答1:

Actually it works, but jupyter lose print output. Try this code:

from IPython.display import display
import ipywidgets as widgets

out = widgets.Output()

import asyncio
def wait_for_change(widget, value):
    future = asyncio.Future()
    def getvalue(change):
        # make the new value available
        future.set_result(change.new)
        widget.unobserve(getvalue, value)
    widget.observe(getvalue, value)
    return future



from ipywidgets import IntSlider
slider = IntSlider()

# Now the key: the container is displayed (while empty) in the main thread
async def f():
    for i in range(10):
        out.append_stdout('did work %s'%i)
        x = await wait_for_change(slider, 'value')
        out.append_stdout('async function continued with value %s'%x)
asyncio.ensure_future(f())

display(slider)
display(out)

You can find more details here: https://github.com/jupyter-widgets/ipywidgets/issues/2567#issuecomment-535971252