Avoid having to specify pack=True when using data() for components

[mtanco]

Question and Background

It seems to the developer that 1. ui.form_card with ui.visualization would be the same as 2. ui.plot_card. This is not explicitly stated in the docs, but from looking at examples it seems like 1. is the only way to replicate 2. in a card with other items, so the user assumes you could copy-past your plot and data code from one and put it in the other. This is not the case when using a dataframe, and addition option pack=True is required in 1.

Reproducible Example

n = 100
    df = pd.DataFrame({'length': np.random.rand(n),
                       'width': np.random.rand(n),
                       'data_type': np.random.choice(a=['Train', 'Test'], size=n, p=[0.8, 0.2])
                       })

    plot_marks = [ui.mark(type='point', x='=length', x_title='Length (cm)', y='=width', y_title='Width (cm)',
                          color='=data_type', shape='circle')]

    q.page['scatter_plot_card'] = ui.plot_card(
        box='1 1 4 4',
        title='Scatter Plot from Dataframe',
        data=data(
            fields=df.columns.tolist(),
            rows=df.values.tolist()
        ),
        plot=ui.plot(marks=plot_marks)
    )

    q.page['scatter_viz_form_card'] = ui.form_card(
        box='1 5 4 4',
        items=[ui.visualization(
                plot=ui.plot(marks=plot_marks
                             ),
                data=data(
                    fields=df.columns.tolist(),
                    rows=df.values.tolist(),
                    pack=True
                ),
        )]
    )

Notice the pack=True of scatter_viz_form_card which is not required for scatter_plot_card. If you do not include it you get this error:

TypeError: Object of type Data is not JSON serializable

Actual Question

You can add pack=True to scatter_plot_card and nothing changes, but I am wondering why it's not required. Is this user confusion, a possible bug in ui.visualization, or missing docs?

[lo5]

I can understand the confusion - this needs to be made clearer in the docs.

[1] is a card. Cards are backed by data buffers, which means they support incremental updates to individual rows or values in their data.

[2] is a component. It's not backed by data buffers, which means if you need to update a plot's data, you cannot use the simplified data-buffer syntax to edit individual values in the data. Instead, you'll need to do something like q.page['card_name'].items[0].visualization.data.rows[42][3] = 1024.

The pack=True indicates that you do not intend to edit individual rows or values in the data once the plot is rendered. Setting this attribute makes the server perform less work to host your page, and consequently improves performance.

We should probably make [2] work like [1] (without having to provide pack=True.