WASM Compatibility Checker for marimo Notebooks Check whether a marimo notebook can run in a WebAssembly (WASM) environment — the marimo playground, community cloud, or exported WASM HTML. Instructions 1. Read the notebook Read the target notebook file. If the user doesn't specify one, ask which notebook to check. 2. Extract dependencies Collect every package the notebook depends on from both sources: PEP 723 metadata — the

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block at the top:

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dependencies = [

"marimo",

"torch>=2.0.0",

]

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Import statements — scan all cells for import foo and from foo import bar . Map import names to their PyPI distribution name using this table: Import name Distribution name sklearn scikit-learn skimage scikit-image cv2 opencv-python PIL Pillow bs4 beautifulsoup4 yaml pyyaml dateutil python-dateutil attr / attrs attrs gi PyGObject serial pyserial usb pyusb wx wxPython For most other packages, the import name matches the distribution name. 3. Check each package against Pyodide For each dependency, determine if it can run in WASM: Is it in the Python standard library? Most stdlib modules work, but these do not : multiprocessing — browser sandbox has no process spawning subprocess — same reason threading — emulated, no real parallelism (WARN, not a hard fail) sqlite3 — use apsw instead (available in Pyodide) pdb — not supported tkinter — no GUI toolkit in browser readline — no terminal in browser Is it a Pyodide built-in package? See pyodide-packages.md for the full list. These work out of the box. Is it a pure-Python package? Packages with only .py files (no compiled C/Rust extensions) can be installed at runtime via micropip and will work. To check: look for a py3-none-any.whl wheel on PyPI (e.g. visit https://pypi.org/project//#files ). If the only wheels are platform-specific (e.g. cp312-cp312-manylinux ), the package has native extensions and likely won't work. Common pure-Python packages that work (not in Pyodide built-ins but installable via micropip): plotly , seaborn , humanize , pendulum , arrow , tabulate dataclasses-json , marshmallow , cattrs , pydantic (built-in) httpx (built-in), tenacity , backoff , wrapt (built-in) Does it have C/native extensions not built for Pyodide? These will not work. Common culprits: torch / pytorch tensorflow jax / jaxlib psycopg2 (suggest psycopg with pure-Python mode) mysqlclient (suggest pymysql ) uvloop grpcio psutil 4. Check for WASM-incompatible patterns Scan the notebook code for patterns that won't work in WASM: Pattern Why it fails Suggestion subprocess.run(...) , os.system(...) , os.popen(...) No process spawning in browser Remove or gate behind a non-WASM check multiprocessing.Pool(...) , ProcessPoolExecutor No process forking Use single-threaded approach threading.Thread(...) , ThreadPoolExecutor Emulated threads, no real parallelism WARN only — works but no speedup; use asyncio for I/O open("/absolute/path/...") , hard-coded local file paths No real filesystem; only in-memory fs Fetch data via URL ( httpx , urllib ) or embed in notebook sqlite3.connect(...) stdlib sqlite3 unavailable Use apsw or duckdb pdb.set_trace() , breakpoint() No debugger in WASM Remove breakpoints Reading env vars ( os.environ[...] , os.getenv(...) ) Environment variables not available in browser Use mo.ui.text for user input or hardcode defaults Path.home() , Path.cwd() with real file expectations Virtual filesystem only Use URLs or embedded data Large dataset loads (>100 MB) 2 GB total memory cap Use smaller samples or remote APIs 5. Check PEP 723 metadata WASM notebooks should list all dependencies in the PEP 723

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block so they are automatically installed when the notebook starts. Check for these issues: Missing metadata: If the notebook has no

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block, emit a WARN recommending one. Listing dependencies ensures they are auto-installed when the notebook starts in WASM — without it, users may see import errors. Missing packages: If a package is imported but not listed in the dependencies, emit a WARN suggesting it be added. Note: version pins and lower bounds in PEP 723 metadata are fine — marimo strips version constraints when running in WASM. 6. Produce the report Output a clear, actionable report with these sections: Compatibility: PASS / FAIL / WARN Use these verdicts: PASS — all packages and patterns are WASM-compatible WARN — likely compatible, but some packages could not be verified as pure-Python (list them so the user can check) FAIL — one or more packages or patterns are definitely incompatible Package Report — table with columns: Package, Status (OK / WARN / FAIL), Notes Example: Package Status Notes marimo OK Available in WASM runtime numpy OK Pyodide built-in pandas OK Pyodide built-in torch FAIL No WASM build — requires native C++/CUDA extensions my-niche-lib WARN Not in Pyodide; verify it is pure-Python Code Issues — list each problematic code pattern found, with the cell or line and a suggested fix. Recommendations — if the notebook fails, suggest concrete fixes: Replace incompatible packages with WASM-friendly alternatives Rewrite incompatible code patterns Suggest moving heavy computation to a hosted API and fetching results Additional context WASM notebooks run via Pyodide in the browser Memory is capped at 2 GB Network requests work but may need CORS-compatible endpoints Chrome has the best WASM performance; Firefox, Edge, Safari also supported micropip can install any pure-Python wheel from PyPI at runtime For the full Pyodide built-in package list, see pyodide-packages.md