Can PyScript lazy loading defer startup until first interaction?

Direct Answer

PyScript lazy loading defers module imports, not interpreter startup. The WebAssembly runtime (pyodide.asm.wasm) loads and initializes during page load whenever a <py-script> element exists or pyodide.load() is called, regardless of lazy import statements.

Confirmed Behavior

In PyScript 0.5+, lazy imports via import statements postpone the download and compilation of specific Python modules, but the core interpreter must still be instantiated first. This means:

• WASM initialization occurs during initial page load (not deferred)
• Module imports can be lazy-loaded when first referenced
• WebWorker execution is a separate option that moves Python code off the main thread but doesn't defer WASM loading itself

Key Metrics for Evaluation

For a low-traffic page, compare these measurements:

1. Total transferred bytes: pyodide.asm.wasm (~10-15 MB) + lazy modules
2. Main-thread blocking time: Time from page load to first callable Python code
3. Time-to-first-Python response: User interaction to first Python execution
4. Post-use heap size: Memory retained after initial Python call

Verification Steps

Run these checks to confirm actual behavior on your page:

# 1. Check network waterfall
Open DevTools > Network, reload page
Verify pyodide.asm.wasm downloads before any .py files

# 2. Log interpreter initialization
console.log('Before load');
await pyodide.load();
console.log('After load');

# 3. Measure main-thread blocking
const start = performance.now();
await pyodide.load();
console.log('Blocking:', performance.now() - start, 'ms');

Recommendation

If your page has "occasional user interactions," WebWorker execution may provide better perceived performance than lazy imports alone, since it moves the heavy WASM initialization off the critical path. However, you'll need to measure the actual bytes transferred and blocking time for your specific use case—browser caching and network conditions significantly affect the trade-off.

One missing detail: Which PyScript version are you targeting? Behavior differs between 0.5.x and later releases.