Need stronger ui.hold()-like context manager for non-UI objects

[jcheng5]

We need another context manager, maybe ui.discard(), to prevent Shiny Express from trying to interpret non-UI objects as UI. Or, perhaps Shiny Express should not be so strict about what kinds of objects are allowed to be displayhooked (no-op instead of error).

from shiny.express import ui

class A:
    def __init__(self):
        self.options = {}

    def set_option(self, name, value):
        self.options[name] = value
        
        # for method chaining purposes
        return self

a = A()

# This next line causes the error.
# Enclosing the line in `with ui.hold()` does not help.
a.set_option("host", "localhost")

with ui.card():
    "Hello"

For now the best workaround I can think of is to turn such lines into dummy assignment statements, like _ = a.set_option("host", "localhost"), but that is not very satisfying.

[jcheng5]

We could also have a more imperative "echo on/off" type of function.

ui.echo(False)

# Do lots of non-UI stuff

ui.echo(True)

You wouldn't even have to wrap it in try/finally, instead you could say that if there is an enclosing UI context manager (like ui.card()), the effect of ui.echo(False) does not survive beyond the end of that context manager.

The nice thing about this approach is not having to indent the non-UI stuff.

There's a good chance this is a terrible idea and I'm not thinking clearly about it, it's pretty late.

[wch]

The specific problem with ui.hold() is that it's a RecallContextManager that wraps a TagList, and the TagList cares about what goes into it. This is so all the things in the with ui.hold() as x: block can be referred to as a single object x. But now that I think about it, I think it could be changed to a regular list without breaking anything.

Or another alternative: the objects get put in a subclass of list, like HoldList. If a function consumes that object and doesn't know about HoldLists, then it gets treated like a list. But if function consumes it and and does know about HoldLists (like a RecallContextManager) then it can use the spread operator on it.

[wch]

Another thought that I had was that perhaps RecallContextManager could add a filter argument, so we could have something like:

def page_auto_cm(
    *,
    filter: Callable[[object], bool] | None = None,
) -> RecallContextManager[Tag]:
    return RecallContextManager(
        ui.page_auto,
        filter=filter,
    )

Where the filter returns True if the displayhooked objects should be stored by the RecallContextManager, False otherwise.

And then the default filter function could be one that returns True only for TagChild objects.

(Even with this, there would still be the issue of ui.hold() not accepting non-TagChild objects, because it doesn't use a RecallContextManager. But that could be solved by the proposal in my previous comment.)

[wch]

• Create subclass of RecallContextManager, RecallContextManagerUi, with a default filter function.
• Use a filter function that throws as soon as the displayhook is called, print error message with line number and text
  • Need to add contextvar state to track current line number.
• ui.hold() should store contents with a list instead of TagList