Updating express inputs leads to an error

[cpsievert]

Consider the following app, which from a user perspective is functional, but does generate an error (since the 1st time it runs a session hasn't been established):

from shiny import reactive
from shiny.express import ui

ui.input_slider("n", "N", 1, 100, 50)

@reactive.effect
def _():
    ui.update_slider("n", value=10)

Traceback (most recent call last):
  File "/Users/cpsievert/github/py-shiny/shiny/reactive/_reactives.py", line 573, in _run
    await self._fn()
  File "/Users/cpsievert/github/py-shiny/shiny/_utils.py", line 243, in fn_async
    return fn(*args, **kwargs)
  File "/Users/cpsievert/github/py-shiny/sandbox/app.py", line 8, in _
    ui.update_slider("n", value=10)
  File "/Users/cpsievert/github/py-shiny/shiny/ui/_input_update.py", line 780, in update_slider
    session = require_active_session(session)
  File "/Users/cpsievert/github/py-shiny/shiny/session/_utils.py", line 127, in require_active_session
    raise RuntimeError(
RuntimeError: update_slider() must be called from within an active Shiny session.
/Users/cpsievert/github/py-shiny/shiny/reactive/_reactives.py:555: ReactiveWarning: Error in Effect: update_slider() must be called from within an active Shiny session.
  await self._run()

[wch]

This is a real issue, but I will note that in practice, most uses will only call the update_* function after something else has happened. For example, it might use reactive.event. The example below will not throw an error:

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

ui.input_slider("m", "M", 1, 100, 10)
ui.input_slider("n", "N", 1, 100, 50)


@reactive.effect
@reactive.event(input.m)
def _():
    ui.update_slider("n", value=input.m())

[cpsievert]

That's true, but it's also not great that if you try use any session methods, you get an unhelpful error. For example:

from shiny import reactive
from shiny.express import session

@reactive.effect
async def _():
    await session.send_custom_message("msg", {})

Error in Effect: '_MockSession' object has no attribute 'send_custom_message'

You also can't avoid the error with something like if session: since the mock session object is truthy

[jcheng5]

I think it's extremely important that @reactive.effect NOT run at all if it's created during the UI-generating phase of Express. We could run the UI generating pass with a MockSession in place and .close() it as soon as the UI returns, which would auto-destroy any reactive effects.

[jcheng5]

I see now that MockSession doesn't have .on_ended, it would need that too.

[wch]

Just to elaborate a bit on what @jcheng5 said about it being important that effects not run at all: I think the specific issue with session stuff being accessed is a manifestation of a more general issue, which is that a copy of these effects are created outside of the session, and they will try to execute, which the user probably will not expect.

[wch]

A problem with calling .close() is that it's async, which means that the caller, wrap_express_app, would also need to be async -- but that won't work because that function is called when you simply access the shiny.express.app.xxx, via a __getattr__ method, and that can't be async.

So I think we may need to change @reactive.effect so that it simply no-ops when running in the UI-generating phase.