[StackOverflow/python] Heroku error H10 status 503 Python flask app

### ROOT CAUSE The Heroku H10 error (App Crashed) typically occurs when the application fails to start properly. Common causes include: 1. **Missing dependencies**: Required packages not installed in the virtual environment or not listed in `requirements.txt`. 2. **Incorrect Procfile**: The Procfile might be misnamed (e.g., `Procfile.txt` instead of `Procfile`) or contain an invalid command. 3. **Syntax errors**: Uncaught exceptions in the code during startup (e.g., missing imports, invalid syntax). 4. **Port configuration**: The app isn't listening on the correct port (Heroku requires port 5000 for Python apps). 5. **WSGI server issues**: Using the built-in Flask server (`flask run`) instead of a production-ready server like Gunicorn. ### CODE FIX 1. **Check `requirements.txt`**: Ensure all dependencies are listed. Rebuild the virtual environment and redeploy: ```bash pip install -r requirements.txt git commit -am "Update dependencies" git push heroku main ``` 2. **Verify Procfile**: Ensure the file is named `Procfile` (no `.txt` extension) and contains the correct command (e.g., for Gunicorn): ``` web: gunicorn app:app ``` 3. **Debug startup errors**: Check Heroku logs for detailed errors: ```bash heroku logs --tail ``` 4. **Use Gunicorn**: Replace the default server with Gunicorn. Install it via `requirements.txt` and update the Procfile. 5. **Fix port binding**: Ensure the Flask app listens on port `5000` (Heroku injects the port dynamically): ```python if __name__ == '__main__': app.run(host='0.0.0.0', port=5000) ``` 6. **Test locally**: Simulate Heroku's environment locally: ```bash PORT=5000 flask run ``` If the issue persists, share the full error logs and `Procfile`/`requirements.txt` for further diagnosis.