Python
A real CPython 3 interpreter compiled to WebAssembly, running in your browser — write Python, run it, and read stdout, tracebacks and the last value.
This is not a transpiler or a sandbox that fakes a subset of the language: it is CPython itself, built to WebAssembly by the Pyodide project, with the full standard library along for the ride. Comprehensions, generators, decorators, `dataclasses`, `itertools`, `json`, `re`, `datetime` — the things that make Python Python — all work, and a real traceback comes back when your code raises. The value of the last expression is shown the way a REPL would show it.
The interpreter is about 12MB and downloads the first time you open the app, then the browser caches it; after that it starts quickly and runs offline. Your code and its output never leave the page, which makes it a genuine scratch interpreter rather than a form that posts your snippet to a server.
What people open it for
- Trying a Python snippet without a local install
- Checking exactly what a piece of Python does, traceback and all
- Teaching or learning Python with nothing to set up
- Running standard-library code entirely in the browser