OSError: Address Already in Use

Why This Happens

When a server binds to a port, the OS reserves it exclusively. If a previous instance is still running or the port is in TIME_WAIT state, binding fails.

The Problem

import socket
s = socket.socket()
s.bind(('0.0.0.0', 8000))

The Fix

import socket
s = socket.socket()
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind(('0.0.0.0', 8000))

Step-by-Step Fix

1. 1

   Find the process

   Run lsof -i :PORT to find what is using the port.

2. 2

   Set SO_REUSEADDR

   Allow port reuse with setsockopt before binding.

3. 3

   Use a different port

   Change to a different port number.