How to create a simple application using Dash

Key takeaways:

  • Dash is a powerful tool for building interactive web applications with Python. It combines Flask, Plotly, and React to create dynamic user experiences.

  • Layouts and callbacks are key concepts in Dash. Layouts define how your app looks, and callbacks make it interactive by responding to the user’s input.

  • Dash applications are easily scalable. They can be used for simple projects and can even expand into complex dashboards that manage large datasets.

What is Dash in Python?

Dash is an open-source Python framework for building interactive web applications. It allows developers to create rich data visualizations, dashboards, and analytical tools using Python. Whether it is a business dashboard or a complex data-driven application, Dash simplifies the process by integrating with tools like Flask, Plotly, and React.

Core technologies behind the Dash framework

Dash is built on top of the following powerful technologies that enable it to create engaging web applications:

  • Flask (Back-end server): Dash uses Flask, a lightweight web framework, to handle the back-end logic and server-side operations.

  • Plotly (Data visualization): Dash integrates with Plotly, allowing developers to create highly customizable and interactive data visualizations in Python.

  • React (Front-end framework): React manages the front-end interactions and renders components, enhancing the app’s user interface.

Dash also stands out because it’s built on Plotly, which allows you to create interactive, high-quality visualizations, and React, which makes your app fast and responsive. Plus, with Dash, you can connect your app to real-time data and see changes live without needing to reload the page!

Main packages in Dash for building Python applications

While Dash was initially introduced as a collection of smaller packages, each catering to specific areas of app development, starting with Dash v2, these packages have been consolidated into the main dash library. This integration simplifies the import process and streamlines application development. The following are some of the most utilized packages of the Dash framework, which we will use to create a simple app:

  • Dash: This is the main library of the Dash framework that creates/initializes our application and provides it with its structure.

  • Dash HTML components: This package provides all the available HTML tags (such as <div>, <h1>) as Python classes. It simply converts Python to HTML.

  • Dash core components: This package is a collection of high-level components like dropdowns, sliders, markdowns, and graphs supplied by the Dash web application framework. It, therefore, facilitates the speedy creation of visually engaging and interactive web applications.

Understanding layouts and callbacks in Dash

Two important concepts in Dash are layouts and callbacks. Together, they make your application interactive and easy to use.

  • Layouts: The layout is like the blueprint for your app. It decides where things go on the page, such as text, buttons, or graphs. You can think of it as arranging pieces (like HTML tags) to build the structure of the web page. Dash provides special components that let you create this layout using Python.

  • Callbacks: Callbacks are functions that allow your application to respond when a user interacts with it. For example, when someone types in a box or clicks a button, a callback can update the page instantly without refreshing the entire page. You set up callbacks using a special decorator called callback, which links the input (like a text box) to the output (like a display area). This helps make your application interactive and dynamic.

Want to get hands-on practice with Dash? Try out the project “Build Data-Centric Dashboards Using Dash,” where we create a visualization dashboard to see how the applications on the Google Play Store are performing.

Creating a simple Dash application

Let’s take you through the steps to create a simple web application using Dash. By the end, you’ll have an interactive application with an input field. This field will take user input and dynamically update the page to display the entered text in real time.

Step 1: Install Dash

Before we begin, you need to install the Dash framework. You can do this using pip, the Python package manager.

pip install dash

Note: This command will install Dash along with the core packages: dash, dcc, html, Input, Output etc.

Step 2: Set up your project

Once Dash is installed, create a new folder for your project, and inside that folder, create a Python file named app.py. This is where we’ll build the application.

Step 3: Initialize the application

In your app.py file, start by importing the necessary libraries from Dash and creating an instance of the app.

# Importing necessary libraries from the Dash package
from dash import Dash, html, dcc, Input, Output, callback
# Instantiating the app
app = Dash(__name__)

Explanation

Here, we import the core components of Dash, which include:

  1. dcc: The dash core components for high-level components like graphs and input boxes.

  2. html: The dash html components to add basic HTML elements like headers, divs, and text fields.

  3. Input and Output: To link the inputs to outputs in our application.

Step 4: Create the layout

Next, we define the application’s layout. The layout determines how the application looks and what components it contains. In this example, we create a simple form where the user can enter their name.

# Creating the layout of the application
app.layout = html.Div([
  html.H1('Basic Dash Application!'),
  html.Div('Please enter your name:'),
  dcc.Input(id='input', type='text'),
  html.Div(id='output')
])

Explanation

  1. We use an H1 element for the page title that contains the title of our application page.

  2. A Div element prompts the user to enter their name.

  3. A text input box dcc.Input that takes the user’s name as input.

  4. Another Div element is used to display the result of the input, which will be updated dynamically.

Step 5: Add interactivity with callbacks

To make the application interactive, we need to add a callback. A callback allows the application to respond to user input and update the page without a full reload.

# Defining the callback to update the output based on user input
@callback(
  Output('output', 'children'),
  Input('input', 'value')
)
def update_output(value):
  if value is None:
    return ' '
  else:
    return f'Your name is {value}!'

Explanation

  1. The @callback decorator links the input field (with ID input) to the output area (with ID output).

  2. The update_output function takes the user’s input and updates the display area with a personalized message.

    1. If the input is empty, the function returns a blank space.

Step 6: Run the application

Finally, we run the application server so that it’s accessible in a browser. The app.run_server() function starts the server on your local machine.

# Running the application server
if __name__ == '__main__':
  app.run_server(host='0.0.0.0', port=8050, debug=False)

Visit http://localhost:8050 in your web browser to see the application in action.

Complete code example

Here is the full code for your simple Dash app. Run the app, and you should see a simple web page where you can enter your name and have it displayed in real time.

# Importing necessary libraries from the Dash package
from dash import Dash, html, dcc, Input, Output, callback

# Instantiating the app
app = Dash(__name__)

# Creating the layout of the application
app.layout = html.Div([
  html.H1('Basic Dash Application!'),
  html.Div('Please enter your name:'),
  dcc.Input(id='input', type='text'),
  html.Div(id='output')
])
  
# Defining the callback to update the output based on user input
@callback(
  Output('output', 'children'),
  Input('input', 'value')
)
def update_output(value):
  if value is None:
    return ' '
  else:
    return f'Your name is {value}!'
  
# Running the application server
if __name__ == '__main__':
  app.run_server(host='0.0.0.0', port=8050, debug=False)
Build web application with Dash

Next Steps: Level Up with Dash

This is a basic application that gives an overview of making applications with Dash. Now that you’re comfortable with Dash, refine your skills and explore new features of Dash by creating meaningful and extensive Dash-based data visualization projects.

Dash is not only easy to start with but also incredibly scalable. This means you can use it to build simple applications like the one we just created or even large, complex web apps that handle big datasets and advanced features. Whether you’re working on a small project or a professional dashboard, Dash has the flexibility to grow with your needs.

Want to explore the concepts of data visualization with Dash in a real-world application? Try out the “Visualize Geospatial Data Using Plotly and Mapbox” project.

Frequently asked questions

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What is Dash in Python?

Dash is an open-source Python framework for building interactive web applications using Python. It helps you create data visualizations and dashboards easily.

What is a Dash application?

A Dash application is a web app built using the Dash framework that allows users to interact with data and see real-time updates based on their input.

What is Dash Plotly used for?

Dash Plotly is used for creating interactive, high-quality graphs and data visualizations in web applications, making it easy to analyze and present data visually.

Which is better, Streamlit or Dash?

Both Streamlit and Dash are great for building web apps, but Dash offers more customization and is better for complex applications, while Streamlit is simpler and faster for quick prototypes.

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