PyScript loader: py-script blocks remain inert when Pyodide core files fail to load

Answer

No. Pinning only the loader URL is not sufficient for a safe zero-downtime static swap, and py-config alone cannot guarantee a safe cutover. You can declare runtime dependencies in py-config before the loader initializes, but you must also version and cache-control both the loader and the Pyodide core assets it fetches. Without that, a returning visitor can load a new loader with an old Pyodide core or vice versa, leaving py-script blocks inert with no server-side rollback signal.

Confirmed facts

• PyScript loader initializes Pyodide in the browser and parses py-config before executing py-script blocks.
• If Pyodide core files fail to fetch, py-script blocks remain inert and the page renders without Python execution.
• py-config can declare packages for the loader to preload or install via micropip. Declaration happens at loader init time.

Likely explanation

• Package availability and micropip behavior vary across Pyodide releases. Package names or wheels can change between releases, so a py-config list valid for one Pyodide version can be invalid for the next.
• The loader itself fetches Pyodide core and runtime assets from its own base URL. Pinning only the loader script URL does not pin those downstream assets.
• Zero-downtime cannot be fully guaranteed client-side. Preloading via py-config reduces runtime installs but does not eliminate silent failures when core files are missing or mismatched.

Steps needed for this case

1. Stage the new release under a distinct versioned path and reference an exact loader URL. Example pattern: https://cdn.jsdelivr.net/npm/pyscript@2026.1.0/dist/core.js with a matching Pyodide base.
2. List all runtime dependencies in py-config packages. This is parsed before first py-script execution.

   <py-config>
   packages = ["numpy", "pandas"]
   </py-config>

3. Serve PyScript and Pyodide assets with immutable caching. Use long max-age and Cache-Control: public, max-age=31536000, immutable for versioned files.
4. Serve the HTML entry point with a short max-age or cache-busting so the new loader URL is picked up immediately on reload.
5. Verify the cutover in a fresh incognito session: Network tab shows 200 for loader and Pyodide core from the pinned versioned path, console has no PyScript loader errors, and a known py-script output executes.

One missing diagnostic detail that changes the recommendation: what exact PyScript version and Pyodide version pair are you migrating from and to? Package names and loader-core coupling differ between releases, so the pinning and py-config list must match that specific pair.