[Bug]: Using "native" @shiny.express rendering for Plotly charts causes a chart to be not rendered on initial app load

[pfalcon]

Component

UI (ui.*)

Severity

P2 - Medium (workaround exists)

Shiny Version

1.4.0

Python Version

3.10.12

Minimal Reproducible Example

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


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

@render.express
def render_func():
    _ = input.n()
    ui.h1("Heading1")
    fig = px.line(x=[1, 2, 3], y=[4, 1, 2])
    fig

Behavior

Please see #2007 for the motivation to work out such issues.

What I see with the above code is that on initial load, "Heading1" is rendered, but the chart isn't. Dragging slider causes the chart to be rendered.

Note the code

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

has the chart rendered on the initial load.

Mind that shinywidgets (or more specifically IPython) should not be install for anything of that to work at all: posit-dev/py-shinywidgets#201

Error Messages (if any)

Environment

Ubuntu Linux 22.04

Chromium 126


$ pip freeze
anyio==4.9.0
appdirs==1.4.4
asgiref==3.8.1
click==8.2.1
exceptiongroup==1.3.0
h11==0.16.0
htmltools==0.6.0
idna==3.10
linkify-it-py==2.0.3
markdown-it-py==3.0.0
mdit-py-plugins==0.4.2
mdurl==0.1.2
narwhals==1.41.1
numpy==2.2.6
orjson==3.10.18
packaging==25.0
pandas==2.3.0
plotly==6.1.2
prompt_toolkit==3.0.51
python-dateutil==2.9.0.post0
python-multipart==0.0.20
pytz==2025.2
questionary==2.1.0
shiny==1.4.0
six==1.17.0
sniffio==1.3.1
starlette==0.47.0
typing_extensions==4.14.0
tzdata==2025.2
uc-micro-py==1.0.3
uvicorn==0.34.3
watchfiles==1.0.5
wcwidth==0.2.13
websockets==15.0.1

[pfalcon]

Whoa, replacing fig with ui.HTML(fig.to_html()) (re: posit-dev/py-shinywidgets#156 (comment)) in "Minimal Reproducible Example" resolves this issue! Including the case when there're 2 charts, only 1st case needs to be replaced, next fig2 is just rendered.

So, this bug is definitely bustable.

[cpsievert]

See posit-dev/py-shinywidgets#201 (comment) and #2008 for some context on why I'd avoid this "last resort" approach to rendering plotly graphs (or any other ipywidget for that matter). Here's how I would approach this problem instead:

import plotly.express as px
from shiny.express import input, ui, render
from shinywidgets import render_plotly

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

with ui.card():
    ui.card_header("Heading 1")
    @render_plotly
    def plot():
        _ = input.n()
        return px.line(x=[1, 2, 3], y=[4, 1, 2])

Also, I'm pretty sure this specific behavior you're seeing is plotly specific, so I'll be closing this issue.