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.

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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);

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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.

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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.

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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(2)

- what the trace showed        + what actually happened
- step 1   nums = [1, 2]       + step 1   nums = []
- step 2   nums = [1, 2]       + step 2   nums = [1]
- step 3   nums = [1, 2]       + step 3   nums = [1, 2]

frame.f_locals gives you references. Snapshot a list and you've stored a
pointer to something the program keeps mutating.

# copy at capture time
return json.loads(json.dumps(value))

Any time you snapshot mutable state over time, you're one reference away from
a history that rewrites itself.

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Worth it?

Yes. Zero execution cost, scales infinitely, works on a locked-down lab machine
with nothing installed.

But test against the real runtime. Every bug above was found by running the
actual thing — not by unit tests around it. The JsNull one passed a fully green
suite.

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