Building Dynamic Web UI with Mesop: A Comprehensive Guide


In the rapidly evolving world of web development, building user interfaces efficiently and effectively is a significant challenge. Mesop, a Python-based UI framework, has emerged as a powerful tool designed to address this challenge. It facilitates the rapid development of web applications, particularly demos and internal apps, without the need for extensive knowledge of JavaScript, CSS, or HTML.

Used internally at Google, Mesop has proven its capability in handling fast iterations and providing a flexible, composable framework for UI development. This comprehensive guide will delve into the features, installation, and practical applications of Mesop, showcasing its potential to revolutionize your web development process.

Why Mesop?

Mesop offers several compelling advantages that make it an attractive choice for developers:

Fast Iteration

  • Hot Reload: Mesop supports hot reload, allowing the browser to automatically reload and preserve the state of the application during development. This feature significantly speeds up the development process by reducing the time spent on manual refreshes and state management.
  • Rich IDE Support: Mesop integrates seamlessly with modern Integrated Development Environments (IDEs), providing strong type safety and autocompletion features that enhance developer productivity.

Flexible and Composable

  • No Need for JavaScript/CSS/HTML: Mesop allows you to build custom UIs entirely in Python. This eliminates the need to switch between multiple languages and streamlines the development process.
  • Component-Based Architecture: Mesop's UI components are simply Python functions, making it easy to compose and reuse components across different parts of your application.

Getting Started with Mesop

Mesop's simplicity and ease of use are among its strongest features. You can start building applications in just a few steps. Here’s how you can get started:

See What You Can Build in Less Than 10 Lines of Code

One of the best ways to understand the power of Mesop is to see it in action. The demo gallery on Mesop's website demonstrates various applications built with Mesop in pure Python.

Check out the demo gallery and see how these applications were created using Mesop in less than 10 lines of code.

Try Mesop

You can try Mesop on Google Colab, a free hosted Jupyter notebook service provided by Google. This is particularly useful for beginners as it requires no setup and runs entirely in the cloud.

Alternatively, you can run Mesop locally on your machine.

Local Installation

If you prefer to run Mesop locally, follow these steps:

Step 1: Install Mesop

Ensure you have Python version 3.10 or later installed on your system. You can check your Python version by running:

python --version

If you don't have the required version, download and install Python from the official website.

Step 2: Create a Virtual Environment

Creating a virtual environment helps manage dependencies and avoid conflicts. Open your terminal and navigate to your project directory:

cd my-mesop-app

Create a virtual environment using venv:

python -m venv .venv

Activate the virtual environment:

  • macOS and Linux:

      source .venv/bin/activate
  • Windows Command Prompt:

  • Windows PowerShell:


Once activated, you should see ".venv" at the start of your terminal prompt.

Step 3: Install Mesop

With the virtual environment activated, install Mesop:

pip install mesop

Step 4: Run a Hello World App

Create a new Python file named and add the following code:

import mesop as me
def app():
    me.text("Hello World")

Run the application:

python -m mesop hello_world

Open the URL printed in the terminal (e.g., http://localhost:32123) in your browser to see your Mesop app loaded.

If you make changes to the code, such as changing "Hello World" to "Hi", the Mesop app will automatically hot reload to reflect the changes.

Building a Simple Interactive App

Let's build a simple interactive Mesop app to understand its components and functionality.

Text to Text App

The simplest way to get started with Mesop is to use the text_to_text component. Create a new file named and add the following code:

import mesop as me
import mesop.labs as mel
    title="Text to Text Example",
def app():
        title="Text to Text Example",

def upper_case_stream(s: str):
    return "Echo: " + s

This example demonstrates a simple text transformation app that echoes back the input text.

Hello World App

Let's start by creating a simple Hello World app in Mesop:

import mesop as me"/hello_world")
def app():
    me.text("Hello World")

This simple example demonstrates a few key concepts:

  • Importing Mesop: Every Mesop app starts with import mesop as me. This is the recommended way to import Mesop to avoid relying on internal implementation details.
  • Root Component: The decorator makes the app function a root component for a particular path. If the path parameter is omitted, it defaults to /.
  • Component Function: The app function is a Python function that creates Mesop components in its body.

Understanding Components

Components are the building blocks of a Mesop application. A Mesop application is essentially a tree of components. There are two main types of components in Mesop:

  • Native Components: These are built-in components implemented using Angular/JavaScript, often wrapping Angular Material components.
  • User-Defined Components: These are Python functions that you define, similar to the app function in the Hello World example.

Building a Counter App

To demonstrate Mesop's interactivity features, let's build a counter app. This app will allow users to click a button to increment a counter displayed on the screen.

Create a new file named and add the following code:

import mesop as me

class State:
    clicks: int

def button_click(event: me.ClickEvent):
    state = me.state(State)
    state.clicks += 1"/counter")
def main():
    state = me.state(State)
    me.text(f"Clicks: {state.clicks}")
    me.button("Increment", on_click=button_click)

State Management

The State class represents the application state for a particular browser session. Each user session has its own instance of State.

  • State Class Decorator: @me.stateclass is a class decorator similar to Python's dataclass. It sets default values based on type hints and allows Mesop to manage the state.

Event Handling

The button_click function is an event handler. It takes a single parameter, event, which contains the event details. The handler updates the state based on the incoming event.

  • Retrieving State: me.state(State) retrieves the instance of the state class for the current session.
  • Updating State: State mutations must be done within event handlers to ensure consistency.

Component Function

The main function is a component function decorated with to mark it as a root component for a path.

  • Rendering State: Use Python's f-strings for dynamic content rendering.
  • Event Binding: Connect components to event handlers (e.g., me.button("Increment", on_click=button_click)).


Mesop is a powerful and flexible tool for building dynamic web UIs in Python. Its hot reload feature, rich IDE support, and component-based architecture make it an excellent choice for rapid development of web applications. Whether you are building simple demos, internal tools, or more complex interactive applications, Mesop provides the tools you need to get the job done efficiently.

To continue your journey with Mesop, explore the official documentation, try out the examples on GitHub, and start building your own applications. Happy coding!

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