Fix render.download not working in Express

[jcheng5]

@gadenbuie Did I put the Express example in the right place?

[gadenbuie]

@gadenbuie Did I put the Express example in the right place?

Yup, the example's in the right place. 👍

[jcheng5]

The Playwright failures appear to be real!? api-examples/Progress/app-express.py is throwing an error shiny.express.input can only be used inside of a Shiny Express app upon startup.

Yes, my adding the __bool__ operator to MockSession caused this problem. @cpsievert @wch are either of you available to take a look at this? I'm in and out of meetings all afternoon.

[gadenbuie]

The issue specifically depends on using a reactive.effect(input.___) or reactive.event(). Here's a minimal reprex -- uncommenting the @reactive.event() introduces the error:

from shiny import reactive
from shiny.express import input, render, ui

ui.input_action_button("btn", "Press me!")


@render.text
# @reactive.event(input.btn)
def text():
    return f"Number of clicks: {input.btn()}"

[wch]

Shouldn't it be reactive.event, not reactive.effect?

[gadenbuie]

Shouldn't it be reactive.event, not reactive.effect?

Yes it should, but reactive.effect() introduces the bug. I'll update the example. Updated: I can reproduce the issue when either reactive.effect() or reactive.event() are used. The example now uses reactive.event()

[gadenbuie]

In case it matters to anyone, I found the source of the bug in get_current_session():

py-shiny/shiny/session/_utils.py

Line 57 in fc55acc

return _current_session.get() or _default_session

should be

    cs = _current_session.get()
    return cs if cs is not None else _default_session

[wch]

@gadenbuie good find! I feel like falsiness in general causes more problems than it helps with. I think we should fix that in another PR.

[wch]

Closing because this was fixed by #1085.