s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)

any_connection = False

while True:
    try:
        conn, addr = s.accept()
        data = conn.recv(1024)
        any_connection = True

        # keep looking
        if not data: continue

        pid = os.fork()

        if pid == 0:
            server_process(data, conn)
    except KeyboardInterrupt:
        break

if any_connection:
    print("Closing connection")
    conn.close()

I'm catching the KeyboardInterruptsignal here on an infinite-running TCP server I wrote in Python. However, even though I know it is closing the connection because it prints Closing Connection, when I try to re-run the server I get:

OSError: [Errno 48] Address already in use

I have no idea what's happening because I know for sure I'm calling conn.close().

And running killall python3doesn't fix it, I keep getting the error unless I wait for a long time or change port. Also I tried to grep all python3 processes but I get nothing.

I'm running OS X Yosemite.

As per the docs the error OSError: [Errno 48] Address already in use occurs because the previous execution of your script has left the socket in a TIME_WAIT state, and can’t be immediately reused. This can be resolved by using the socket.SO_REUSEADDR flag.

For eg:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((HOST, PORT))