a Python VM written entirely from scratch in JavaScript with some unique features
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Jaspy is a Python VM written entirely from scratch in JavaScript with some unique features. Jaspy supports multiple threads, comes with an integrated debugger which offers remote debugging and provides a flexible preprocessor based architecture. Speed is explicitly not a main goal of this project. Jaspy aims to illustrate how web programming on the client side could be done by exploring new ways.


  • suspendable interpreter with support for threading and greenlets
  • integrated debugger and interactive remote debugging (CLI, PyCharm, …)
  • flexible preprocessor based architecture to optimize Jaspy for your needs
  • easily extensible with native JavaScript modules (time, dom, …)
  • full support for meta-classes, builtin subclassing and operator overloading
  • asynchronous imports and arbitrary-length integers based on BigInteger.js


Jaspy comes with an integrated development server and interactive debugger!

First install the dependencies, if they are not installed already:

pip3 install --user -r requirements.txt
pip3 install --user ptpython pygments

Clone the repository and build the interpreter:

git clone --recursive https://github.com/koehlma/jaspy.git; cd jaspy
python3 build.py  # build the interpreter

Switch to the example directory and start the server in interactive mode:

cd example
PYTHONPATH=../ python3 -m jaspy.cli --interactive

Visit http://localhost:8080/hello.html in your browser and click run:

Jaspy Screencast


There are already many other Python-to-JavaScript approaches out there:

Most of them are faster than Jaspy but none of them offers the unique features of Jaspy, which are the fully suspendable interpreter with threading support, the integrated debugger and the flexible, preprocessor based architecture.


Just to get an impression how slow Jaspy really is!

Jaspy (enabled Debugger, enabled Threading) 195
Jaspy (disabled Debugger, enabled Threading) 199
Jaspy (disabled Debugger, disabled Threading) 206
Brython 4184
PyPy.js (cold) 41425
PyPy.js (warm) 847457

However this is a somewhat unfair benchmark because no dom manipulation or anything else browser specific is going on. Surprisingly threading and debugging introduces nearly no overhead.


This project is still in an alpha state. The APIs are unstable, it is untested and not ready for productive use. Some of the features listed above are not yet implemented.

I started this project in my semester break and now, as the new semester started, I have much less spare time. Therefore it might take a while until I will be able to invest much more time into it. However I very welcome all sorts of contributions.


If you like the ideas of Jaspy feel free to join, there are many things to do:

  • implement all the batteries-included-builtin stuff of Python
  • implement native JS modules for the DOM, JS objects and some Web APIs
  • improve the debugger and make it fully compatible to the PyDev protocol
  • implement a parser and bytecode compiler in JavaScript
  • complete and adjust the implementation of the Python bytecode VM
  • support for Apache Cordova (Jaspy for cross platform mobile applications)
  • implement a neat UI library on top of Jaspy (using a flexbox based grid)
  • implement a just-in-time compiler to speed things up
  • add a virtual file system (consider using: BrowserFS)
  • … and, of course, your own great ideas and cool features

Do not hesitate to contribute or ask if there is anything unclear about the code or the process of contributing in general.


libs:third-party dependencies
modules:bundled native JavaScript modules
src:JavaScript source files (need to be preprocessed)
jaspy:Python server, converter and remote debugger


Many thanks to the Brython project for the inspiration for many parts of code of the builtin-classes. Many thanks also to the book “500 Lines or Less” which is a good starting point if you want to know how the interpreter works.