IPython dependency mess leads to drastic make-or-break behavior changes in simple Shiny Express apps

[pfalcon]

Very simple Shiny Express code can either magically work, or magically not work, depending on whether shinywidgets package is installed. Steps to reproduce:

1. Create 2 virtual environments, venv1, venv2. Let's assume 2 terminal windows. In 1st:

cd tmp
python3 -m venv venv1
source venv1/bin/activate

Similarly for for venv2.
2. In venv1, install:

pip install shiny plotly[express] pandas

3. In venv2, install:

pip install shiny plotly[express] pandas shinywidgets

4. Use following simple app:

import plotly.express as px
from shiny.express import input, ui

ui.input_slider("n", "Number of sth", 1, 100, 20)

fig = px.line(x=[1, 2, 3], y=[4, 1, 2])
fig

5. Run in venv1, and observe that a plot is properly rendered:

shiny run -r -b plot_ex.py

6. Now run in venv2, and observe that a plot isn't rendered:

shiny run -r -b plot_ex.py

[pfalcon]

The culprit here is actually Plotly, and the issue was reported there, with an initial workaround-level : plotly/plotly.py#5222 . Specifically, shinywidgets has a dependency on IPython, and mere fact that IPython is installed cause Plotly to make over-reaching decisions (like assuming that IPython is actually used).

However, the above is again just a workaround, and the issue was apparently present in Plotly for years, so it may be a prudent thing to think about some Plan B on the Shiny/shinywidgets side, if it keeps being so upstream.

[cpsievert]

As it seems you've discovered, the example works because Shiny will (attempt to) render any objects that have a ._repr_html() method. Rendering objects this way is sort of a "last resort" that won't always work. When more explicit rendering options exist (i.e., shinywidgets' render_plotly/render_widget), it should be used over the "last resort" option:

import plotly.express as px
from shiny.express import input, ui

ui.input_slider("n", "Number of sth", 1, 100, 20)

@render_plotly
def plot():
    return px.line(x=[1, 2, 3], y=[4, 1, 2])

This issue, as well as the other issues you've filed (posit-dev/py-shiny#2008, posit-dev/py-shiny#2007) are all using the "last resort" option over @render_plotly. I think if you switch to this approach, then all of your issues can be addressed?

[cpsievert]

Closing since I don't think there's anything concrete here that shinywidgets can do to improve the situation.