Python Start Local Server

Overview

Starts a Flask server locally on your development machine for testing and development. Can run in foreground (interactive) or background (persistent) mode.

When to Use

Invoke this skill when the user:

• Asks to "start the server"
• Wants to "run the Flask app"
• Says "start local server"
• Mentions running the Python server locally
• Wants to test server changes locally

Prerequisites

• Python 3 installed
• Flask installed: pip3 install flask
• Other dependencies installed: pip3 install -r requirements.txt
• Server code (app.py) in current directory

Instructions

Option 1: Foreground Mode (Development)

1. Navigate to the server directory:

   bashCopy

   cd path/to/server/imp/py
   

2. Run the Flask app directly:

   bashCopy

   python3 app.py
   

3. Inform the user:

   • Server running on http://localhost:8080
   • Auto-reload on code changes (if debug=True)
   • Detailed error messages in terminal
   • Press Ctrl+C to stop
   • Terminal must stay open

Option 2: Background Mode (Persistent)

1. Navigate to the server directory:

   bashCopy

   cd path/to/server/imp/py
   

2. Run the start script:

   bashCopy

   ./start.sh
   

3. The script will:

   • Kill any existing server on port 8080
   • Start server with nohup python3 app.py
   • Save process ID to server.pid
   • Log output to server.log
   • Run in background (terminal can close)

4. Inform the user:

   • Server started in background
   • Access at http://localhost:8080 or http://0.0.0.0:8080
   • View logs: tail -f server.log
   • Stop with: ./stop.sh or py-stop-local skill
   • Process ID saved in server.pid

Expected Output

Foreground mode:

 * Serving Flask app 'app'
 * Debug mode: on
WARNING: This is a development server. Do not use it in a production deployment.
 * Running on http://0.0.0.0:8080
Press CTRL+C to quit
 * Restarting with stat
 * Debugger is active!

Background mode:

Firefly server started on port 8080
PID: 12345
Logs: tail -f server.log
Stop: ./stop.sh
Access at: http://localhost:8080

Modes Comparison

Foreground (python3 app.py):

• ✅ Auto-reload on code changes
• ✅ Detailed error messages
• ✅ Interactive debugger
• ❌ Stops when terminal closes
• ❌ Blocks terminal
• Use for: Active development

Background (./start.sh):

• ✅ Persists after terminal close
• ✅ Logs to file
• ✅ Easy stop/restart
• ❌ No auto-reload
• ❌ Less visible errors
• Use for: Local testing, demos

Accessing the Server

Once started, access via:

• http://localhost:8080
• http://0.0.0.0:8080
• http://127.0.0.1:8080

From other devices on same network:

• http://YOUR_LOCAL_IP:8080
• Find your IP: ifconfig | grep "inet " | grep -v 127.0.0.1

Flask Configuration

The server typically runs with:

• Host: 0.0.0.0 (accepts external connections)
• Port: 8080
• Debug mode: True (development) or False (background)

Verification

Test the server is running:

curl http://localhost:8080/api/ping

Expected response:

{"message":"Firefly server is running","status":"ok"}

Common Issues

"Address already in use":

OSError: [Errno 48] Address already in use

• Port 8080 already occupied
• Kill existing process: lsof -ti:8080 | xargs kill
• Or use py-stop-local skill first

"Module not found":

ModuleNotFoundError: No module named 'flask'

• Install Flask: pip3 install flask
• Install all deps: pip3 install -r requirements.txt

"Permission denied":

• Don't use ports < 1024 without sudo
• Port 8080 should work fine
• Check file permissions on app.py

Can't access from other devices:

• Ensure Flask uses host='0.0.0.0' not '127.0.0.1'
• Check macOS firewall allows Python incoming connections
• Verify devices on same network

Monitoring Logs

Foreground mode:

• Logs appear directly in terminal
• Error stack traces visible immediately

Background mode:

• View logs: tail -f server.log
• Last 20 lines: tail -20 server.log
• Search logs: grep "ERROR" server.log

Stopping the Server

Foreground mode:

• Press Ctrl+C in terminal

Background mode:

• Use ./stop.sh script
• Or use py-stop-local skill
• Or kill manually: lsof -ti:8080 | xargs kill

Development Workflow

Typical workflow:

# Start in foreground for development
python3 app.py

# Make changes to app.py
# Flask auto-reloads (if debug=True)

# Test changes
curl http://localhost:8080/api/ping

# When done, Ctrl+C to stop

start.sh Script Details

The start.sh script:

# Kill any existing server on port 8080
lsof -ti:8080 | xargs kill -9 2>/dev/null

# Start server in background with nohup
nohup python3 app.py > server.log 2>&1 &

# Save process ID
echo $! > server.pid

This ensures clean startup and process tracking.

Flask Debug Mode

Debug mode (in app.py):

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8080, debug=True)

• debug=True: Auto-reload, detailed errors (development)
• debug=False: Stable, minimal output (production/background)

Notes

• Flask development server is single-threaded
• For production, use gunicorn or similar WSGI server
• localhost vs 0.0.0.0: use 0.0.0.0 to accept external connections
• Port 8080 is standard for development (no sudo required)