Nu — A Simple Python Code Editor

Nu is a maintained, modernized continuation of the Mu editor — a simple code editor for beginner programmers, shaped by extensive feedback from teachers and learners. It is also a pleasant "no frills" editor for anyone who wants one. Most importantly: have fun!

Why Nu?

The original Mu was archived by its maintainers and is no longer developed. Nu exists so that beginners — and the wider community — can keep using the editor safely into the future. The goal is simple: maintain and modernize.

Nu is a friendly fork. It builds directly on the work of Nicholas Tollervey and the many Mu contributors (see AUTHORS.md) and remains free software under the GPLv3.

What Nu is

Like Mu, Nu is a modal editor written in Python and built on the Qt toolkit: a small editor whose behaviour changes depending on the selected mode. The text area is a Scintilla-based widget with syntax highlighting, and there is a built-in Jupyter REPL and a data plotter. Nu runs on Windows, macOS, Linux and Raspberry Pi.

Nu currently ships these modes:

• Python 3 — write and run desktop Python, with a built-in REPL and plotter.
• Pygame Zero — make games the beginner-friendly way.
• CircuitPython — work with Adafruit CircuitPython boards via the CIRCUITPY drive and serial REPL.
• Debugger — a visual debugger for stepping through Python code.

To keep Nu nimble and easy to maintain, the more niche, hardware-specific modes inherited from upstream Mu (micro:bit, ESP, Pyboard, Lego, Pico, Snek and the Flask/web deploy mode) — along with their vendored, unmaintained device tooling — have been removed.

Project status

Nu is under active modernization. It now runs on PyQt6 and Python 3.11+ (verified through 3.14), with development managed by uv. The remaining work — uv-driven user environments and consolidated Briefcase installers — is tracked in TODO.md. Guidance for working in this repository lives in CLAUDE.md.

Running from source

Nu uses uv for development. With uv installed:

git clone https://github.com/kovirlabs/nu.git
cd nu
uv sync                  # downloads a managed CPython and the pinned deps
uv run python -m mu      # launch Nu

Development

Run the test suite (add QT_QPA_PLATFORM=offscreen to run the Qt tests headlessly, without a display):

uv run pytest

Lint and format with ruff (config lives in pyproject.toml):

uv run --no-sync python -m ruff check     # lint
uv run --no-sync python -m ruff format    # reformat in place

Documentation

The developer documentation lives in docs/, is built with MkDocs and is published with GitHub Pages at https://kovirlabs.github.io/nu/. Build it locally with make docs (or make docs-serve for a live preview).

Note that much of this documentation is inherited from upstream Mu and predates this fork — it is being modernized incrementally.

Code of Conduct

We want this to be a friendly place. Contributors and collaborators are expected to follow the Code of Conduct in CODE_OF_CONDUCT.md.

License

Nu is free software, licensed under the GNU General Public License v3 (see LICENSE). It derives from Mu, Copyright (c) Nicholas H. Tollervey and contributors.