[Bug]: update_action_button() has no effect on download button

[nvelden]

Component

UI (ui.*)

Severity

P3 - Low (minor inconvenience)

Shiny Version

1.4.0

Python Version

3.13.2

Minimal Reproducible Example

from shiny.express import input, render, ui
from shiny import reactive
import pandas as pd

# UI
ui.input_file(
    "csv_file",
    "Upload CSV file:",
    multiple=False,
    accept=".csv"
)

@render.download(filename="download.csv")
def download_csv():
    file = input.csv_file()
    if not file:
        yield "No CSV file uploaded."
        return

    # Read uploaded file
    df = pd.read_csv(file[0]["datapath"])

    # Yield the CSV content as a string
    yield df.to_csv(index=False)



@reactive.effect
def _():
    ui.update_action_button("download_csv", disabled=not input.csv_file())

Behavior

I am building a shiny.express app where users can upload a file, make some modifications, and then download the result. I’d like the download button to be disabled by default and only become enabled after a file has been uploaded.

However, using update_action_button() to control the button’s state doesn’t seem to have any effect.

Do I miss something or is there an alternative method I should use?

Error Messages (if any)

Environment

MacOS Sequuoia 15.4.1