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    <p>The way Firefox does it is: the first instance creates a socket file (or a named pipe on Windows). This serves both as a way for the next instances of Firefox to detect and communicate with the first instance, and forward it the URL before dying. A socket file or named pipe being only accessible from processes running on the local system (as files are), no network client can have access to it. As they are files, firewalls will not block them either (it's like writing on a file).</p> <p>Here is a naive implementation to illustrate my point. On first launch, the socket file <code>lock.sock</code> is created. Further launches of the script will detect the lock and send the URL to it:</p> <pre><code>import socket import os SOCKET_FILENAME = 'lock.sock' def server(): print 'I\'m the server, creating the socket' s = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM) s.bind(SOCKET_FILENAME) try: while True: print 'Got a URL: %s' % s.recv(65536) except KeyboardInterrupt, exc: print 'Quitting, removing the socket file' s.close os.remove(SOCKET_FILENAME) def client(): print 'I\'m the client, opening the socket' s = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM) s.connect(SOCKET_FILENAME) s.send('http://stackoverflow.com') s.close() def main(): if os.path.exists(SOCKET_FILENAME): try: client() except (socket.error): print "Bad socket file, program closed unexpectedly?" os.remove(SOCKET_FILENAME) server() else: server() main() </code></pre> <p>You should implement a proper protocol (send proper datagrams instead of hardcoding the length for instance), maybe using <a href="http://docs.python.org/library/socketserver.html" rel="nofollow">SocketServer</a>, but this is beyond this question. The <a href="http://docs.python.org/howto/sockets.html" rel="nofollow">Python Socket Programming Howto</a> might also help you. I have no Windows machine available, so I cannot confirm that it works on that platform.</p>
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    1. POOnly one python program running (like Firefox)?
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    1. COIs that what the profile-directory/lock file does? Is there a way to do something similar in Python?
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    2. COThat's exactly what it does. I added some sample code to help you.
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    3. COls -la on the lock file ends with: .default/lock -> 192.168.0.69:+3178, does this mean the lock file is a link to a port?
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