Building a Free Online Python Interpreter with FastAPI and Monaco Editor

Building a Free Online Python Interpreter with FastAPI and Monaco Editor

Have you ever needed to quickly test a Python snippet without setting up a local environment? Or wanted to share executable code examples with others? That's why I built Python Interpreter Online - a free, browser-based Python interpreter that makes coding and sharing Python snippets easy.

The Problem

When teaching Python or collaborating with others, I often encountered these challenges:

1. Setup friction: Getting newcomers to install Python locally can be a barrier
2. Sharing executable code: Sending code snippets that others can run and modify
3. Simplicity: Existing solutions are often bloated with features or require sign-ups

The Solution: Python Interpreter Online

I created a clean, simple solution focused on the core experience of writing, running, and sharing Python code:

• Write code in a beautiful Monaco Editor (the same engine that powers VS Code)
• Run code with a single click and see results instantly
• Share code via URL - perfect for teaching or getting help on forums

Try it here: Python Interpreter Online

Technical Implementation

Frontend

The frontend is built with vanilla JavaScript, HTML, and CSS:

// API endpoint configuration
const isProduction = window.location.hostname === 'getgit789.github.io';
const API_URL = isProduction 
    ? 'https://python-interpreter-online.onrender.com' 
    : 'http://localhost:8002';

// Run code function
async function runCode() {
    const code = editor.getValue();
    try {
        const response = await axios.post(`${API_URL}/execute`, { code });
        output.textContent = response.data.output || 'No output';
        // Update status
    } catch (error) {
        handleError(error);
    }
}

Backend

The backend uses FastAPI to create a secure Python execution environment:

from fastapi import FastAPI, HTTPException
from fastapi.middleware.cors import CORSMiddleware
import os

app = FastAPI()

# Configure CORS
allowed_origins = os.getenv("ALLOWED_ORIGINS", "").split(",")
app.add_middleware(
    CORSMiddleware,
    allow_origins=allowed_origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

@app.post("/execute")
async def execute_code(request: CodeRequest):
    # Secure code execution logic
    result = execute_python_safely(request.code)
    return {"output": result}

Deployment

• Frontend: GitHub Pages with GitHub Actions for CI/CD
• Backend: Render for serverless deployment

Key Features

1. Modern Code Editor: Syntax highlighting, code completion, and error detection
2. Secure Execution: Sandboxed environment with resource limits
3. User-friendly Error Messages: Clear explanations for common Python errors
4. Code Sharing: Generate shareable URLs with your code embedded
5. Example Code: Pre-loaded examples to help users get started

Lessons Learned

Building this project taught me several valuable lessons:

1. Security is critical: Executing user code requires careful sandboxing
2. UX matters: A clean, intuitive interface makes all the difference
3. Cross-origin challenges: CORS configuration is essential for frontend-backend communication
4. Deployment separation: Using different platforms for frontend and backend offers flexibility

What's Next?

I'm planning to enhance the project with:

• Support for external libraries
• Multiple file support
• Code formatting options
• Improved mobile experience
• User accounts for saving snippets

Open Source

This project is completely open source! Check out the GitHub repository to:

• Star the project
• Report issues
• Contribute code
• Suggest features

Try It Out!

Give Python Interpreter Online a try and let me know what you think in the comments!

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Have you built similar tools for other programming languages? What features would you like to see added? I'd love to hear your thoughts and suggestions!