Getting started


Being pure Python and cross platform, it should work (almost) anywhere where there’s Python and a browser. Flexx is written for Python 3.5+ and also works on Pypy. Flexx actively supports Firefox, Chrome and (with minor limitations) MS Edge.

Flexx further depends on:

All are pure Python packages, and the latter three are projects under the flexxui umbrella. Further, Flexx needs a browser. To run apps that look like desktop apps, we recommend having Firefox installed.

Developers that want to run the tests need:

  • pytest and pytest-cov (get them via conda or pip)
  • flake8 (get it via conda or pip)
  • Nodejs
  • Firefox

Current status

Flexx is in development and is in alpha status. Any part of the public API may change without notice.


  • conda install flexx -c conda-forge
  • pip install flexx
  • Old school: python install
  • Clone the repo and add the root dir to your PYTHONPATH (developer mode)


The primary motivation for Flexx is the undeniable fact that the web (i.e. browser technology) has become an increasingly popular method for delivering applications to users, also for (interactive) scientific content.

The purpose of Flexx is to provide a single application framework to create desktop applications and web apps. By making use of browser technology, the library itself can be relatively small and pure Python, making it widely and easily available.

By making use of PScript (Python to JavaScript translation), the entire library is written without (hardly) a line of JavaScript. This makes it easier to develop than if we would have a corresponding “flexx.js” to maintain. Further, it allows users to easily define callback methods that are executed in JavaScript, allowing for higher performance when needed.

Libraries written for Python, but not in Python have a much harder time to survive, because users don’t easily become contributors. This is one of the reasons of the success of e.g. scikit-image, and the demise of e.g. Mayavi. Since Flexx is written in a combination of Python and PScript, its user community is more likely to take an active role in its development.

Flexx overview


The image above outlines the structure of Flexx. The event module provides a powerful property and event system that makes it easy to connect different parts of your application. Central to the event system is the Component class. The app module runs the server to which the web runtime connects (via a websocket). Further, it extends the event.Component class into the PyComponent and JsComponent classes. Objects of these classes live in Python and JavaScript respectively, but (can) have a representation on the other side, from which properties can be accessed, and actione be invoked. The ui module defines all widgets (based on JsComponent).

The external webruntime package is used to launch a browser looking like a dektop app. The pscript library is used throughout Flexx to compile Python code to JavaScript.