Four things that surprised me running Python in the browser

I built a debugging-practice site where student code runs entirely in the browser . Python via Pyodide , JavaScript in a worker. No server executes anything. No execution bill, no queue, no sandbox to maintain. But four things bit me hard. 1. Your arguments aren't Python objects Pass a JS object into Python and you get this: TypeError: 'pyodide.ffi.JsProxy' object is not subscriptable It's not a dict . It's a live view of the JS object, and it supports neither obj[key] nor .get() . Convert explicitly: const pyArgs = input . map (( arg ) => pyodide . toPy ( arg )); const result = fn (... pyArgs ); 2. null is not None This one passed my entire test suite while being broken in production. pyodide . toPy ( null ) check result type(v) JsNull bool(v) False ✅ falsy, as expected v is None False ❌ the surprise It's falsy, so truthiness checks work fine. But is None fails — which was exactly what my code was checking. Why my tests missed it: the harness used json.loads . The app used toPy . Different conversion paths, different answers. If you need a real None , create it in Python. Don't pass one across. 3. sys.settrace is a free step debugger Want to show users their code running line by line? Python basically hands it to you: def _tracer ( frame , event , arg ): if frame . f_code . co_name != target : return None # skip library frames if event == " line " : steps . append ({ " line " : frame . f_lineno , " locals " : dict ( frame . f_locals ), }) return _tracer Two things this naive version gets wrong: Add a step cap. A tight loop generates steps faster than it burns a 5-second timeout. You need both guards. Handle exception . During unwinding, the return event still fires with arg=None . Miss it and your trace says "returned None" for code that crashed. 4. Your snapshots are lying A user screenshot exposed this one. Every step in the trace showed the final state of a list. Step 1 included mutations that hadn't happened yet. tracing: nums = []; nums.append(1); nums.append(