Introduction to Shiny

What is a Shiny app?

Each Shiny app involves:

1. A web page that users interact with
2. A Python process that watches the page and responds to user events

The Python process runs your code and sends results back to the browser.

Components

Each Shiny app consists of:

1. Input components — widgets the user can interact with (sliders, dropdowns, checkboxes, …)
   • https://shiny.posit.co/py/components/#inputs/
2. Output components — things Shiny renders and displays (plots, tables, text, …)
   • https://shiny.posit.co/py/components/#outputs/
3. Layout — how the components are laid out on the page (columns, sidebar, boxes, cards, …)
   • https://shiny.posit.co/py/layouts/

Example application

#| '!! shinylive warning !!': |
#|   shinylive does not work in self-contained HTML documents.
#|   Please set `embed-resources: false` in your metadata.
#| standalone: true
#| components: [viewer]
#| viewerHeight: 600

from palmerpenguins import load_penguins
from plotnine import aes, geom_point, ggplot, theme_minimal
from shiny.express import input, render, ui

dat = load_penguins().dropna()
num_cols = dat.select_dtypes("float64").columns.tolist()

ui.input_select("x", "", num_cols, selected="bill_depth_mm")
ui.input_select("y", "", num_cols, selected="body_mass_g")


@render.plot
def plot():
    return (ggplot(dat, aes(x=input.x(), y=input.y(), color="species")) +
        geom_point(alpha=0.7) +
        theme_minimal()
    )

Reactivity

When an input changes, Shiny reacts by rebuilding the outputs that depend on it — and only those outputs.

No callbacks. No manual refresh. No cache management.

Spreadsheet analogy

Think of a spreadsheet:

• Inputs = cells you type values into
• Outputs = formula cells
• Instructions = the formulas

When you change a value cell, only the formula cells that depend on it recalculate.

Shiny works the same way.

Other reactive tools

• Pluto Notebooks: https://plutojl.org//
• Marimo Notebooks: https://marimo.io/

Key features

• Easy enough for prototypes
• Efficient enough to handle complexity
• Scales from a quick script to a production-ready app

Why Shiny for Python?

Shiny lets you build reactive web apps without worrying about:

• cache
• state
• callbacks
• …or even HTML, CSS, and JavaScript

Shiny Express vs Shiny Core

Important

Shiny for Python has two syntaxes for writing the same apps. There is full feature parody between the 2 syntax modes.

1. Shiny Express — simpler syntax (think Streamlit)
2. Shiny Core — more explicit, closer to Shiny for R and Dash

Today we’ll use Shiny Express.

Running a Shiny app

shiny run app.py

Or in Positron / VS Code: click the Run App button in the editor toolbar.

Open URL:Port (e.g., http://127.0.0.1:8000) in your browser.