Allow replacement of cell outputs by index

[seantur]

Description

I would like to be able to replace a certain index of a cell output. My use case is that I want to be able to have a progress bar at output index 0, and occasionally update a plot at output index 1. If I clear the output to show the new plot, I lose the progress bar.

I would be happy to submit a PR if there's an API for this that you like.

Suggested solution

I've been using this, which seems to work okay (modified from the functions in _output.py):

def replace_output_idx(value: object, idx: int):
    ctx = get_context()
    if ctx.execution_context is None:
        return

    if len(ctx.execution_context.output) < idx:
        raise Exception
    elif len(ctx.execution_context.output) == idx:
        ctx.execution_context.output.append(formatting.as_html(value))
    else:
        current_fig = ctx.execution_context.output[idx]
        ctx.execution_context.output[idx] = formatting.as_html(value)
    write_internal(cell_id=ctx.execution_context.cell_id, value=vstack(ctx.execution_context.output))

Alternative

Another approach that would be a little bit more work is to make mo.output behave more like a list with a __getitem__ method, and maybe an insert method:

mo.output.append(1)

mo.output[0] = 2

mo.output.insert(1, 3)

mo.output.clear()

Additional context

No response

[mscolnick]

@seantur I think a mo.output.replace_at_index. I do like the list-api on it, but mo.output[0] = 2 looks like a mutation and marimo tries to push for more more immutable practices, so that could be confusing. if we want to later, we can make it a list-like api.

A contribution would be greatly appreciated!