Workflow best practices

Overview

Agenda

• Overview

• UI Functions

• Question & Answer

Goals

• Learn some Shiny best practices
• Get a sense of how large complicated apps are structured
• Know when you need to refactor your application

Avoid premature refactoring

• Start by just getting your app to work
• Take small steps to avoid repetition
• Ultimately you’re the one who needs to work with the code

When should you refactor?

• You’re trying to hold too much in your head
• Changing the app is difficult
• Other people don’t understand your code
• You don’t understand your code!

Three main techniques

• Use functions to generate UI
• Separate reactive and non-reactive code
• Modules

We’re only going to cover two

• Use functions to generate UI
• Separate reactive and non-reactive code
• Modules

UI Functions

Agenda

• Overview

• UI Functions

• Question & Answer

Background on functions

• In Python, functions are the best way to improve code quality
  • Don’t Repeat Yourself (DRY) principle
  • Can define variables within the function scope
  • Can write tests against them
  • Easy to use in list comprehensions
• If your code is getting crazy, start refactoring into functions

Data Science blind spot

Data scientists forget functions when writing Shiny UIs

UI elements are just values

• Recall that Shiny UI elements are just values
  • They can be saved as variables
  • They can be stored in lists
  • They can be passed into functions
  • They can be returned by functions

Using functions

from shiny import App, render, ui

app_ui = ui.page_fluid(
    ui.input_slider("n1", "N", 0, 100, 20),
    ui.input_slider("n2", "N", 0, 100, 20),
    ui.input_slider("n3", "N", 0, 100, 20),
    ui.input_slider("n4", "N", 0, 100, 20),
    ui.input_slider("n5", "N", 0, 100, 20),
    ui.input_slider("n6", "N", 0, 100, 20),
)

app = App(app_ui, None)

Using functions

from shiny import App, render, ui

def my_slider(id, label):
    return ui.input_slider(id, "N", 0, 100, 20)

app_ui = ui.page_fluid(
    my_slider("n1"),
    my_slider("n2"),
    my_slider("n3"),
    my_slider("n4"),
    my_slider("n5"),
)

app = App(app_ui, None)

Applying a function over a list

from shiny import App, render, ui

def my_slider(id):
    return ui.input_slider(id, "N", 0, 100, 20)

ids = ["n1", "n2", "n3", "n4", "n5"]

app_ui = ui.page_fluid(
    [my_slider(x) for x in ids]
)

app = App(app_ui, None)

Iterating across two lists

from shiny import App, render, ui

def my_slider(id, label):
    return ui.input_slider(id, label + "Number", 0, 100, 20)

numbers = ["n1", "n2", "n3", "n4", "n5", "n6"]
labels = ["First", "Second", "Third", "Fourth", "Fifth", "Sixth"]

app_ui = ui.page_fluid(
    [my_slider(x, y) for x, y in zip(numbers, labels)]
)

app = App(app_ui, None)

Your turn

Go to exercises/13-ui-functions or run problems 5.1 and 5.2 in apps/problem-sets/exercises/5-workflows.

Separate Reactive and Non-reactive logic

• Most Shiny app code is non-reactive
  • Drawing plots
  • Summarizing data
  • Interacting with databases
  • (Really, everything except the actual reading of reactive inputs and calcs)
• It’s fine to include this code inside reactive functions
• As your app grows you should separate them

Reactivity makes everything harder

• Reactive context makes them inherently more difficult
  • Harder to debug
  • Harder to test
  • Harder to document
  • Harder to reason about

Pull logic out of the reactive context

• Non-reactive functions are familliar and predictable
• You can call them in a notebook
• You can write unit tests
• Makes your reactive code much clearer
• You can reuse them in other contexts

Your turn

Go to exercises/14-extract-functions or run problem 5.3 in apps/problem-sets/exercises/5-workflows.

Modules

• Not going to teach them, but you should know about them
• Used when you want to encapsulate both UI and server logic together
• Allows you to package and re-use parts of your application
• Great for working with teams of developers
• Essential for large applications

Question & Answer

Agenda

• Overview

• UI Functions

• Question & Answer

Also, please fill out the survey:
https://pos.it/conf-workshop-survey