pn.cache not working with async code: RuntimeError: cannot reuse already awaited coroutine

[MarcSkovMadsen]

I'm trying to contribute a guide on Panel+Dask in #4234.

I assume its best to use Dask asynchronously, because then you can off load computation to the dask cluster while still keeping your application responsive.

Then I need a way to instantiate and share the async Dask client that work both in a notebook when people are experimenting and on the server. I assume its best to instantiate the client once and then share it across sessions.

In the notebook you can just do client = await Client(asynchronously) in a cell. But in a script running on the server you cannot use await outside of a function. If I do I get I get an error saying this is not possible.

So instead I think I can just use pn.cache to instantiate once and then share. But this does not work.

import param

from dask.distributed import Client

import panel as pn

pn.extension()

@pn.cache
async def get_client():
    return await Client(asynchronous=True)

submit_button = pn.widgets.Button(name="Submit").servable()

@pn.depends(submit_button, watch=True)
async def run(_):
    print("submit")
    client = await get_client()

panel serve script.py

The second time you click the Submit button you get

$ panel serve script.py
2022-12-24 05:59:38,361 Starting Bokeh server version 2.4.3 (running on Tornado 6.1)
2022-12-24 05:59:38,362 User authentication hooks NOT provided (default user enabled)
2022-12-24 05:59:38,366 Bokeh app running at: http://localhost:5006/script
2022-12-24 05:59:38,366 Starting Bokeh server with process id: 2564
submit
submit
2022-12-24 05:59:47,087 - tornado.application - ERROR - Exception in callback functools.partial(<bound method IOLoop._discard_future_result of <tornado.platform.asyncio.AsyncIOMainLoop object at 0x7f5e6f6b0070>>, <Task finished name='Task-1415' coro=<async_execute.<locals>.wrapper() done, defined at /opt/conda/lib/python3.10/site-packages/panel/io/server.py:155> exception=RuntimeError('cannot reuse already awaited coroutine')>)
Traceback (most recent call last):
  File "/opt/conda/lib/python3.10/site-packages/tornado/ioloop.py", line 741, in _run_callback
    ret = callback()
  File "/opt/conda/lib/python3.10/site-packages/tornado/ioloop.py", line 765, in _discard_future_result
    future.result()
  File "/opt/conda/lib/python3.10/site-packages/panel/io/server.py", line 161, in wrapper
    state._handle_exception(e)
  File "/opt/conda/lib/python3.10/site-packages/panel/io/state.py", line 408, in _handle_exception
    raise exception
  File "/opt/conda/lib/python3.10/site-packages/panel/io/server.py", line 159, in wrapper
    return await func(*args, **kw)
  File "/opt/conda/lib/python3.10/site-packages/param/_async.py", line 28, in cb
    yield from func(*args, **dep_kwargs) # noqa: E999
  File "/home/jovyan/repos/private/panel/script.py", line 18, in run
    client = await get_client()
RuntimeError: cannot reuse already awaited coroutine

Please make pn.cache work with async too or provide a seperate pn.async_cache function.

[MarcSkovMadsen]

Workaround

Instead of using @pn.cache I can use pn.state.cache directly.

async def get_client():
    if not "dask-client" in pn.state.cache:
        pn.state.cache["dask-client"]=await Client(asynchronous=True)
    return pn.state.cache["dask-client"]

[MarcSkovMadsen]

Just experienced this again when working on panel-chat-examples and trying to make llama_and_mistral.py example more efficient.

[philippjfr]

The cache presumably should cache the awaitable but the return value of the awaitable.