RUN01

A zero-install, browser-native Python IDE — edit, execute, and visualise data science workloads entirely client-side via WebAssembly, with server-side proxies for market data and compiled languages.

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Overview
Architecture
Feature Matrix
Tech Stack
Project Structure
Data Flow
Runtime Execution Model
Proxy API Reference
Package Support
Service Worker & Caching
Theme System
Keyboard Shortcuts
Local Development
Vercel Deployment
Environment Variables
Security Considerations
Performance Tuning
Extending Run01
Troubleshooting
Roadmap
Contributing

Overview

Run01 is a premium browser-based Python sandbox that merges a production-grade code editor with a full scientific Python stack — all executing inside your browser via WebAssembly. There is no backend compute, no container spin-up, and no per-execution billing. The Python runtime boots once per session and stays warm.

The server layer (Flask / Vercel) is intentionally thin: it serves static assets, proxies financial data from Yahoo Finance (bypassing browser CORS), and routes compiled-language jobs (C++, C#, Rust) to the Piston execution engine.

Design philosophy

Principle	Implementation
Zero cold-start	Pyodide + packages cached via Service Worker on first visit
Privacy-first	Code never leaves the browser; only `yf_download()` and Piston routes touch the network
Streaming output	`stdout` lines render immediately as they arrive — no waiting for the run to complete
Visual-first data science	Matplotlib PNGs and interactive Plotly charts render inline in the console
Monochrome glassmorphic	Single coherent design language across light and dark modes

Architecture

┌──────────────────────────────────────────────────────────────────────┐
│                          BROWSER (CLIENT)                            │
│                                                                      │
│  ┌─────────────────────┐    ┌──────────────────────────────────────┐ │
│  │   Monaco Editor     │    │          Output Console              │ │
│  │  (AMD, CDN-loaded)  │    │  ┌─────────┐ ┌────────┐ ┌────────┐   │ │
│  │                     │    │  │ stdout  │ │  PNG   │ │Plotly  │   │ │
│  │  Python / C++ / C#  │    │  │ stream  │ │ chart  │ │ chart  │   │ │
│  │  / Rust source      │    │  └─────────┘ └────────┘ └────────┘   │ │
│  └──────────┬──────────┘    └──────────────────────────────────────┘ │
│             │ getValue()                         ▲                   │
│             ▼                                    │                   │
│  ┌──────────────────────────────────────────┐    │ render            │
│  │         Pyodide (WebAssembly)            │────┘                   │
│  │                                          │                        │
│  │  runPythonAsync(code)                    │                        │
│  │  ├─ stdout → batched callback            │                        │
│  │  │   ├─ __RUN01_IMG__:<b64>  → PNG       │                        │
│  │  │   └─ __RUN01_PLOTLY__:<b64> → JSON    │                        │
│  │  └─ stderr → batched callback            │                        │
│  │                                          │                        │
│  │  Packages (WASM):                        │                        │
│  │  numpy · pandas · scipy · scikit-learn   │                        │
│  │  matplotlib · statsmodels · seaborn      │                        │
│  │  plotly                                  │                        │
│  └────────────┬─────────────────────────────┘                        │
│               │ pyfetch("/api/yf/...")                               │
│               │ (yfinance CORS bypass)                               │
└───────────────┼──────────────────────────────────────────────────────┘
                │ HTTP
┌───────────────┼──────────────────────────────────────────────────────┐
│               ▼            FLASK / VERCEL                            │
│  ┌─────────────────────────────────────────────────────────────────┐ │
│  │  GET  /api/yf/<ticker>?period=&interval=   →  yfinance (PyPI)   │ │
│  │  POST /api/run  {language, code}           →  Piston API        │ │
│  │  GET  /sw.js                               →  Service Worker    │ │
│  │  GET  /                                    →  index.html        │ │
│  └─────────────────────────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────────────────────────┘
                │
                ▼ external
    ┌───────────────────────┐    ┌──────────────────────────┐
    │  Yahoo Finance API    │    │  Piston API (emkc.org)   │
    │  finance.yahoo.com    │    │  C++ · C# · Rust · more  │
    └───────────────────────┘    └──────────────────────────┘

Feature Matrix

Feature	Status	Notes
Python (Pyodide WASM)	✅	No server round-trip for execution
NumPy / Pandas / SciPy	✅	Pre-loaded at startup
Scikit-Learn	✅	Pre-loaded
Statsmodels	✅	Pre-loaded
Matplotlib inline PNG	✅	`plt.show()` patched to emit base64
Seaborn	✅	Installed via micropip
Plotly interactive charts	✅	`fig.show()` patched to emit JSON → rendered with Plotly.js
Yahoo Finance (live data)	✅	`await yf_download(ticker)` via server proxy
Streaming stdout	✅	Each `print()` renders immediately
C++ / C# / Rust	✅	Via `/api/run` → Piston
Dark / Light theme	✅	Persisted in `localStorage`
Service Worker caching	✅	Pyodide WASM, Monaco, Plotly cached after first load
Draggable split pane	✅	Mouse and keyboard (Arrow keys)
Download output `.txt`	✅	`Ctrl+D`
Reset to starter code	✅	`Ctrl+R`
Cursor position in status bar	✅	`ln N, col N`
`stdin` support	🔜	Roadmap Q3
Multi-file editor	🔜	Roadmap Q4
Share snippet via URL	🔜	Roadmap Q4

Tech Stack

Layer            Library / Service           Version       Where it runs
──────────────── ─────────────────────────── ───────────── ──────────────
Python runtime   Pyodide                     0.26.4        Browser (WASM)
Code editor      Monaco Editor               0.52.0        Browser (CDN)
Charts (static)  Matplotlib + Seaborn        latest        Browser (WASM)
Charts (interac) Plotly.js                   2.35.2        Browser (CDN)
ML               Scikit-Learn                latest        Browser (WASM)
Stats            Statsmodels                 latest        Browser (WASM)
Market data      yfinance                    ≥1.4.1        Server (Flask)
Compiled langs   Piston API (emkc.org)       v2            External API
Web server       Flask                       3.0.3         Server
Deployment       Vercel (@vercel/python)     latest        Edge / Lambda
Fonts            Outfit, JetBrains Mono,     variable      Google Fonts CDN
                 Pixelify Sans
SW caching       Service Worker (Cache API)  —             Browser

Project Structure

run01/
├── api/
│   └── index.py                  # Vercel WSGI entrypoint — imports pyrunner.app
│
├── pyrunner/
│   ├── __init__.py               # Package marker
│   ├── app.py                    # Flask app: routes, yf proxy, Piston proxy
│   ├── requirements.txt          # flask, yfinance
│   │
│   ├── static/
│   │   ├── app.js                # Monaco + Pyodide bootstrap, run loop, output renderer
│   │   ├── style.css             # Monochrome glassmorphic design system
│   │   └── sw.js                 # Service Worker: cache-first CDN strategy
│   │
│   └── templates/
│       └── index.html            # Shell HTML: nav, split panes, init overlay
│
├── vercel.json                   # Build + routing config for Vercel
├── requirements.txt              # Root-level (mirrors pyrunner/requirements.txt)
├── .gitignore
└── README.md                     ← you are here

Key file responsibilities

`pyrunner/app.py`

Three distinct responsibilities on a single Flask app object:

Static serving — GET / renders index.html; GET /sw.js serves the Service Worker with correct Service-Worker-Allowed: / header (required for full-origin scope)
Yahoo Finance proxy — GET /api/yf/<ticker> calls yfinance.Ticker.history() server-side and returns clean OHLCV JSON. Strips timezone info so strftime works across yfinance versions.
Piston proxy — POST /api/run forwards {language, code} to https://emkc.org/api/v2/piston/execute and relays the result. Timeout: 30 s.

`pyrunner/static/app.js`

The heaviest file. Key sections:

PYODIDE_SETUP — Python bootstrap string injected into the WASM runtime at startup. Patches plt.show() and fig.show() to emit __RUN01_IMG__: and __RUN01_PLOTLY__: sentinel lines on stdout rather than attempting to open a GUI window.
monacoReady — Promise that resolves when the AMD loader has finished and the editor is mounted. 30 s timeout (extended from the original 5 s to handle CDN cold-starts).
pyodideReady — Calls loadPyodide(), then a single pyodide.loadPackage([...]) to batch-download all stdlib packages in parallel, then micropip.install(['seaborn', 'plotly']).
processOutput() — Per-line stdout router: detects sentinels and delegates to renderImage() or renderPlotly(); everything else goes to appendLine().
getPlotly() — Polls window.Plotly with exponential back-off, falling back to a dynamic <script> inject if the synchronous CDN load raced ahead.

`pyrunner/static/sw.js`

Cache-first Service Worker with a versioned cache (run01-v2). Only caches GET responses from four CDN origins: cdn.jsdelivr.net, cdn.plot.ly, fonts.googleapis.com, fonts.gstatic.com. All other requests (including the Flask API routes) pass through uncached.

Data Flow

Python execution (happy path)

User clicks ▶ Run
     │
     ▼
monacoEditor.getValue()  →  raw source string
     │
     ▼
pyodide.setStdout({ batched: processOutput })
pyodide.setStderr({ batched: processOutput })
     │
     ▼
pyodide.runPythonAsync(code)          ← awaited; non-blocking
     │
     ├─ print("hello")
     │       └─→ processOutput("hello", false, block)
     │                └─→ appendLine(...)
     │
     ├─ plt.show()
     │       └─→ _mpl_capture() in WASM
     │                └─→ savefig → base64 PNG
     │                └─→ print("__RUN01_IMG__:<b64>")
     │                         └─→ processOutput detects prefix
     │                                  └─→ renderImage(b64, block)
     │
     ├─ fig.show()
     │       └─→ _plotly_capture() in WASM
     │                └─→ pio.to_json(fig) → base64 UTF-8
     │                └─→ print("__RUN01_PLOTLY__:<b64>")
     │                         └─→ processOutput detects prefix
     │                                  └─→ renderPlotly(b64, block)
     │
     └─ return / raise
              └─→ finishOutputBlock(block, success)

Yahoo Finance proxy flow

Python code:  df = await yf_download("AAPL", period="3mo")
                    │
                    ▼ (inside Pyodide)
              pyodide.http.pyfetch("/api/yf/AAPL?period=3mo")
                    │
                    ▼ HTTP GET (same origin — no CORS issue)
              Flask /api/yf/AAPL
                    │
                    ▼
              yfinance.Ticker("AAPL").history(period="3mo", interval="1d")
                    │
                    ▼
              pd.DataFrame → JSON records → jsonify(records)
                    │
                    ▼ HTTP 200 JSON
              resp.json() inside pyfetch
                    │
                    ▼
              pd.DataFrame(data).set_index("Date")   ← returned to caller

Runtime Execution Model

Parallel initialisation

Monaco and Pyodide initialise in parallel via Promise.all([monacoReady, pyodideReady]). Neither blocks the other. The UI is unlocked only after both resolve.

t=0ms ──► loadPyodide()          ─────────────────────────────────► ✓ (~4-8s cold)
t=0ms ──► require(['vs/editor.main']) ──────────────► ✓ (~1-2s cold)
                                                               │
                                                     Promise.all resolves
                                                               │
                                                    hideOverlay() + btnRun.enable()

Package loading strategy

# Single call — Pyodide resolves the full dependency graph internally
# and downloads all wheels in parallel. Much faster than sequential awaits.
await pyodide.loadPackage([
    'numpy', 'pandas', 'scipy', 'scikit-learn',
    'matplotlib', 'statsmodels', 'micropip',
])

# Pure-Python packages not in Pyodide's package index
micropip = pyodide.pyimport('micropip')
await micropip.install(['seaborn', 'plotly'], keep_going=True)

keep_going=True means a single package failure (e.g. a transient CDN error) doesn't abort the entire install.

`plt.show()` interception

# Injected at startup — replaces Matplotlib's GUI backend
def _mpl_capture(*args, **kwargs):
    buf = io.BytesIO()
    plt.savefig(buf, format='png', dpi=150, bbox_inches='tight',
                facecolor='#0a0a0a', edgecolor='none')
    b64 = base64.b64encode(buf.getvalue()).decode('ascii')
    buf.close()
    plt.close('all')
    print(f'__RUN01_IMG__:{b64}', flush=True)   # ← sentinel on stdout

plt.show = _mpl_capture

`fig.show()` interception

import plotly.io as _pio

def _plotly_capture(fig, *args, **kwargs):
    fig_json = _pio.to_json(fig)                           # full Plotly JSON spec
    encoded  = base64.b64encode(fig_json.encode()).decode()
    print(f'__RUN01_PLOTLY__:{encoded}', flush=True)       # ← sentinel on stdout

_pio.show = _plotly_capture
go.Figure.show = lambda self, *a, **kw: _plotly_capture(self, *a, **kw)

On the JS side, renderPlotly() decodes the base64, parses the JSON, and calls Plotly.newPlot() with layout overrides to match the current theme.

Proxy API Reference

`GET /api/yf/<ticker>`

Fetches OHLCV price history for a given ticker from Yahoo Finance.

Path parameters

Parameter	Type	Description
`ticker`	string	Case-insensitive. Uppercased internally. E.g. `AAPL`, `TSLA`, `BTC-USD`

Query parameters

Parameter	Default	Valid values
`period`	`1mo`	`1d` `5d` `1mo` `3mo` `6mo` `1y` `2y` `5y` `10y` `ytd` `max`
`interval`	`1d`	`1m` `2m` `5m` `15m` `30m` `60m` `90m` `1h` `1d` `5d` `1wk` `1mo` `3mo`

Response (200)

[
  {
    "Date": "2024-01-02",
    "Open": 185.25,
    "High": 186.10,
    "Low": 183.92,
    "Close": 185.64,
    "Volume": 71628400
  }
]

Response (404) — symbol not found or delisted

{ "error": "No price data found for 'XYZ'. Symbol may be delisted or invalid." }

Response (500) — upstream failure

{ "error": "<exception message>" }

Usage in Python (inside Run01)

# Async — must be awaited at top level or inside an async function
df = await yf_download("AAPL", period="6mo", interval="1d")

# df is a pd.DataFrame with DatetimeIndex and columns:
# Open  High  Low  Close  Volume
print(df.tail())

`POST /api/run`

Compiles and executes code in a compiled language via the Piston API.

Request body (JSON)

{
  "language": "cpp",
  "code": "#include <iostream>\nint main() { std::cout << \"hello\"; }"
}

Field	Type	Valid values
`language`	string	`cpp` · `csharp` · `rust`

Response — passes through the Piston response shape:

{
  "run": {
    "stdout": "hello",
    "stderr": "",
    "code": 0,
    "signal": null,
    "output": "hello"
  },
  "language": "c++",
  "version": "10.2.0"
}

Package Support

Pre-loaded at startup (Pyodide native wheels)

Package	Use case
`numpy`	Numerical arrays, linear algebra, FFT
`pandas`	DataFrames, time-series, CSV/JSON I/O
`scipy`	Stats, signal processing, optimization
`scikit-learn`	ML: regression, classification, clustering, preprocessing
`statsmodels`	OLS, GLM, ARIMA, VAR, hypothesis tests
`matplotlib`	Static charts (rendered as inline PNG)
`micropip`	Pure-Python package installer

Installed via micropip at startup

Package	Use case
`seaborn`	Statistical visualisation built on Matplotlib
`plotly`	Interactive charts (rendered inline via Plotly.js)

Available via `micropip.install()` at runtime

Any pure-Python package on PyPI that has no compiled C/Fortran extensions. Examples:

import micropip
await micropip.install('sympy')       # symbolic mathematics
await micropip.install('networkx')    # graph algorithms
await micropip.install('faker')       # test data generation

Not available in WASM

Packages with native extensions that haven't been compiled for Emscripten/WASM:

yfinance — use await yf_download() instead
tensorflow, torch — too large / not WASM-compiled
psycopg2, pymysql — no socket access in browser

Service Worker & Caching

The Service Worker (/sw.js) implements a cache-first strategy for CDN assets. This means repeat visits load in under 500 ms regardless of network speed — Pyodide's ~12 MB WASM bundle is served from the browser cache.

Cache scope

Origin	What's cached
`cdn.jsdelivr.net`	Pyodide WASM + wheels, Monaco editor
`cdn.plot.ly`	Plotly.js dist
`fonts.googleapis.com`	Font CSS
`fonts.gstatic.com`	Font files (woff2)

All Flask routes (/, /api/*, /sw.js) bypass the cache intentionally.

Cache versioning

The cache is keyed by CACHE_VERSION = 'run01-v2'. On SW activation, all caches with a different key are deleted. To bust the cache on a new release, increment this constant in sw.js.

const CACHE_VERSION = 'run01-v3';  // ← bump on CDN library upgrades

Registering the SW

The SW is registered at the end of index.html:

navigator.serviceWorker.register('/sw.js')

The /sw.js route sets Service-Worker-Allowed: / so the worker can control the full origin (not just /static/*).

Theme System

Run01 ships with two complete themes driven by a single data-theme attribute on <html>.

CSS custom properties

Variable	Dark	Light	Role
`--bg`	`#050505`	`#F2F2F7`	Page background
`--glass-bg`	`rgba(255,255,255,0.02)`	`rgba(255,255,255,0.75)`	Panel fill
`--glass-border`	`rgba(255,255,255,0.07)`	`rgba(0,0,0,0.08)`	Panel stroke
`--text`	`#F5F5F7`	`#1D1D1F`	Primary text
`--text-muted`	`#8E8E93`	`#6E6E73`	Secondary text
`--text-dim`	`#48484A`	`#A1A1A6`	Tertiary / disabled
`--white`	`#FFFFFF`	`#000000`	Accent / CTA fill
`--black`	`#000000`	`#FFFFFF`	Accent / CTA text

Monaco theme mapping

App theme	Monaco theme
`dark`	`run01-dark` (base: `vs-dark`, transparent background)
`light`	`run01-light` (base: `vs`, transparent background)

Both themes use editor.background: #00000000 so the editor's glass panel shows through.

Persistence

localStorage.setItem('run01-theme', 'dark' | 'light')

The saved theme is applied before first paint to avoid flash-of-wrong-theme (FOWT).

Keyboard Shortcuts

Shortcut	Action
`Ctrl/⌘ + Enter`	Run code
`Ctrl/⌘ + D`	Download output as `.txt`
`Ctrl/⌘ + R`	Reset editor to starter code
`Ctrl/⌘ + Z`	Undo (Monaco native)
`Ctrl/⌘ + Shift + Z`	Redo (Monaco native)
`Ctrl/⌘ + /`	Toggle line comment (Monaco native)
`Alt + ←/→`	Navigate word (Monaco native)
`Tab` on resize handle	Focus the resize handle
`←/→` on resize handle	Move divider 20 px
`Shift + ←/→` on handle	Move divider 50 px

Local Development

Prerequisites

Tool	Minimum version
Python	3.10
pip	23.x
A modern browser	Chrome 115+, Firefox 116+, Safari 16.4+

Setup

# 1. Clone the repository
git clone https://github.com/your-org/run01.git
cd run01

# 2. Create and activate a virtual environment
python -m venv .venv
source .venv/bin/activate       # Windows: .venv\Scripts\activate

# 3. Install dependencies
pip install -r requirements.txt

# 4. Run the development server
python pyrunner/app.py

Open http://localhost:5000. Flask runs in debug mode (debug=True) so the server reloads on file changes.

First-run experience

On first load, the browser fetches ~15 MB of CDN assets (Pyodide WASM, Monaco, Plotly). This takes 5–20 s depending on connection speed. All assets are cached by the Service Worker; subsequent loads take under 500 ms.

Vercel Deployment

How it works

vercel.json routes all traffic to api/index.py, which imports the Flask app object from pyrunner.app. Vercel's @vercel/python builder wraps it as a serverless WSGI function.

{
  "builds": [{ "src": "api/index.py", "use": "@vercel/python" }],
  "routes": [{ "src": "/(.*)", "dest": "api/index.py" }]
}

# api/index.py — Vercel detects the `app` name automatically
from pyrunner.app import app

Deploy steps

# Install Vercel CLI (one-time)
npm i -g vercel

# Link and deploy
vercel --prod

Or push to GitHub and connect the repository in the Vercel dashboard — Vercel will auto-deploy on every push to main.

Limitations on Vercel

Constraint	Value	Impact
Function timeout	60 s (Pro) / 10 s (Hobby)	Piston proxy: fine; yf proxy: fine for most tickers
Function memory	1024 MB	yfinance + pandas fit comfortably
Payload size	4.5 MB request / 4.5 MB response	Chart JSON from Piston is well under limit
Cold-start	~300 ms	Flask import is fast; no heavy ML imports server-side

Environment Variables

Run01 has no required environment variables for its core functionality. The following are optional:

Variable	Default	Purpose
`FLASK_ENV`	`production`	Set to `development` locally for debug mode
`FLASK_DEBUG`	`0`	Set to `1` to enable auto-reloader
`PISTON_URL`	`https://emkc.org/api/v2/piston/execute`	Override Piston endpoint (e.g. self-hosted)

Set these in a .env file locally (never commit) or as Vercel environment variables in the dashboard.

Security Considerations

What executes where

Code	Where it runs	Isolation
Python (user code)	Browser WASM sandbox	Full browser sandbox; no file system, no raw sockets
C++ / C# / Rust	Piston API (emkc.org)	Piston uses isolated containers with CPU/memory limits
Flask (yf proxy)	Vercel serverless	Receives only a ticker string; no user code touches the server

CORS

The yf proxy exists specifically because finance.yahoo.com blocks cross-origin requests from browsers. By routing through the same-origin Flask endpoint, Pyodide's pyfetch works without any CORS headers.

Input sanitisation

The yf proxy passes ticker through yfinance.Ticker(ticker.upper()) — yfinance validates the symbol upstream. Malformed tickers return a 404 with an error message; they never reach the shell.

The Piston proxy only forwards language (validated against an allowlist) and code (opaque string forwarded as-is). Piston handles sandboxing.

Service Worker

The SW only caches CDN GET responses. It never intercepts Flask API calls (/api/*) or the SW file itself (/sw.js), preventing stale proxy responses.

Performance Tuning

Reduce startup time

Cache hit — After first load, the SW serves Pyodide from the browser cache. No action needed.
Preload hints — index.html has <link rel="preload"> for Pyodide, Monaco loader, and Plotly. The browser starts fetching them before the parser hits the <script> tags.
Parallel package load — The single pyodide.loadPackage([...]) call downloads all wheels in parallel. Avoid splitting this into multiple sequential calls.
micropip.install last — Pure-Python packages (seaborn, plotly) via micropip run after the heavier WASM packages, so they don't block the critical path.

Reduce chart render time

Matplotlib: use dpi=100 instead of 150 for faster PNG encoding at the cost of resolution.
Plotly: use plotly.graph_objects (JSON-serialisable) rather than plotly.express figures backed by large DataFrames — pio.to_json is faster on simpler traces.

Service Worker cache eviction

Browsers cap the Cache Storage quota (typically 20–80% of available disk). If the quota is exceeded, the oldest caches are evicted. Pyodide's 12 MB WASM file is the largest single asset; on devices with very limited storage, the SW may fail to cache it. The SW handles this gracefully — failed cache writes are non-blocking.

Extending Run01

Adding a new server-side language (Piston)

Look up the language slug at Piston runtimes.

Add an entry to PISTON_LANGS in app.py:

PISTON_LANGS = {
    "cpp":    "c++",
    "csharp": "csharp",
    "rust":   "rust",
    "go":     "go",      # ← new
}

Add a <button class="lang-tab" data-lang="go"> in index.html.
Add a go entry to STARTER_CODES and LANG_META in app.js.
Wire the tab click handler in app.js.

Adding a new Python package

If it's in Pyodide's package index (compiled wheel available):

// In initPyodide() in app.js:
await pyodide.loadPackage(['numpy', 'pandas', ..., 'sympy']);  // add here

If it's pure Python (no C extensions):

// In initPyodide() in app.js:
await micropip.install(['seaborn', 'plotly', 'your-package']);

If neither works — the package requires C extensions not in Pyodide's index. Consider a server-side proxy pattern (like the yf proxy) or a separate Piston route.

Customising the theme

All design tokens live in :root and [data-theme="light"] in style.css. The entire system is driven by 7 CSS custom properties; changing them updates every component simultaneously.

:root {
  --bg:           #050505;   /* page background */
  --glass-bg:     rgba(255,255,255,0.02);
  --glass-border: rgba(255,255,255,0.07);
  --text:         #F5F5F7;
  --text-muted:   #8E8E93;
  --white:        #FFFFFF;   /* primary accent */
  --black:        #000000;   /* inverse accent */
}

Troubleshooting

Pyodide fails to load

Symptoms: Overlay stays visible; status shows "Python init failed".

Causes & fixes:

CDN blocked by a corporate firewall or ad-blocker. Whitelist cdn.jsdelivr.net.
Browser too old. Pyodide requires SharedArrayBuffer. Check crossOriginIsolated in DevTools console. If false, the server is missing Cross-Origin-Opener-Policy / Cross-Origin-Embedder-Policy headers (not required on Vercel / localhost, but required on some custom servers).
Low memory device. Pyodide + packages consume ~300 MB of heap. On devices with < 2 GB RAM, the WASM instantiation may fail.

`yf_download` returns 404

Symptoms: ValueError: No price data found for 'XYZ'.

Causes: Invalid ticker, weekend/holiday (no 1d data), or intraday interval requested for a period > 60 days (Yahoo Finance limitation).

# Fine — 3 months of daily data
df = await yf_download("AAPL", period="3mo", interval="1d")

# Error — intraday data is only available for the last 60 days
df = await yf_download("AAPL", period="1y", interval="1h")  # ← will fail

`ModuleNotFoundError: No module named 'yfinance'`

You imported yfinance directly in your code. yfinance cannot run inside the browser WASM sandbox. Use the proxy helper instead:

# ✗ Wrong
import yfinance as yf
df = yf.Ticker("AAPL").history(period="3mo")

# ✓ Correct
df = await yf_download("AAPL", period="3mo")

Monaco editor blank / not loading

Symptom: Editor area is blank; no syntax highlighting.

Fix: The AMD loader (loader.js) must be available before app.js runs. The script tags are ordered: Monaco loader (sync) → Pyodide (defer) → app.js (defer). If the CDN is slow, the 30 s timeout in monacoReady will resolve gracefully without the editor, so Run01 still works (you can paste code into a textarea fallback).

Charts not rendering after `plt.show()` or `fig.show()`

Cause: PYODIDE_SETUP was not executed at startup (e.g. a previous error aborted initialisation).

Fix: Open DevTools → Console and look for errors from initPyodide(). Re-run the page.

Piston proxy 500 errors

Cause: The Piston API at emkc.org is a free community service with occasional downtime.

Fix: Self-host Piston: https://github.com/engineer-man/piston and set PISTON_URL to your instance.

Roadmap

Q3 2025

stdin support via a modal prompt interceptor
Keyboard-accessible theme toggle

Q4 2025

Multi-file editor (virtual filesystem backed by pyodide.FS)
Snippet sharing via URL (code encoded in fragment hash, no server storage)
pip install UI: type a package name, micropip installs it into the running session

2026

Collaborative editing (WebRTC or Liveblocks)
Jupyter notebook import (.ipynb → Run01 cells)
Export to Colab / Kaggle

Contributing

Contributions are welcome. Please open an issue first for non-trivial changes.

Branch conventions

Branch	Purpose
`main`	Production; auto-deploys to Vercel
`dev`	Integration branch for feature PRs
`feat/<name>`	New features
`fix/<name>`	Bug fixes

Commit style

feat: add stdin modal interceptor
fix: extend Monaco AMD timeout from 5s to 30s
docs: add Piston proxy API reference
perf: batch all pyodide.loadPackage() calls into single await

Local linting

# Python
pip install ruff
ruff check pyrunner/

# JavaScript (optional — no bundler, so ESLint runs standalone)
npx eslint pyrunner/static/app.js

License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

_{Built with Pyodide · Monaco · Flask · Vercel}

Name		Name	Last commit message	Last commit date
Latest commit History 39 Commits
api		api
components/ui		components/ui
lib		lib
pyrunner		pyrunner
.gitignore		.gitignore
README.md		README.md
requirements.txt		requirements.txt
vercel.json		vercel.json

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