[BUG] Long callbacks create processes

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Describe your context
Please provide us your environment, so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                 2.18.2
dash-core-components 2.0.0
dash-html-components 2.0.0
dash-table           5.0.0

• if frontend related, tell us your Browser, Version and OS

  • OS: Win 10
  • Browser Edge

Describe the bug

This basic code inspired from the documentation fails:

import time
from dash import Dash, html, Output, Input
from dash.long_callback import DiskcacheLongCallbackManager
import diskcache

cache = diskcache.Cache("./cache")
long_callback_manager = DiskcacheLongCallbackManager(cache)

app = Dash()

app.layout = html.Div([
    html.Div([
        html.P(id="paragraph_id", children=["Button not clicked"]),
        html.Progress(id="progress_bar"),
    ]),
    html.Button(id='button_id', children="Run Job!")
])

@app.callback(
    output=[Output("paragraph_id", "children")],
    inputs=[Input("button_id", "n_clicks")],
    progress=[Output("progress_bar", ("value", "max"))],
    prevent_initial_call=True,
    background=True,
    manager=long_callback_manager
)
def callback(set_progress, n_clicks):
    total = 10
    for i in range(total):
        time.sleep(0.5)
        set_progress((str(i + 1), str(total)))

    return [f"Clicked {n_clicks} times"]

if __name__ == "__main__":
    app.run_server(debug=True)

with this error:

NameError: name 'time' is not defined

It seems that Dash creates a new python process in the background to execute the long callback without importing all the packages.

Expected behavior

Dash should handle long callbacks with threading or other mechanisms to avoid these issues, or explain in the documentation how to overcome them.

Screenshots

If applicable, add screenshots or screen recording to help explain your problem.

Start by reproducing the provided Dash example with DiskcacheLongCallbackManager and the callback that calls time.sleep and set_progress. Trace how the background callback process is started and how imports are made available; done means the example runs without the reported NameError or the required import behavior is clearly documented.