How does the Python runner execute code without a server?

It loads Pyodide, a CPython interpreter compiled to WebAssembly, inside a background Web Worker completely within your browser tab.

What happens if my Python code loops forever?

Press Stop to terminate the worker instantly. Your tab never hangs because execution happens off the main thread.

Can I install packages like numpy or pandas?

Yes. List package names separated by commas and they load from the Pyodide CDN before your script runs. Graphical plotting libraries are not supported.

Why does the first run download ~12 MB?

That is the CPython runtime itself. Browsers cache it, so later visits and reruns start almost instantly.

Python Code Runner: Run real Python 3

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