gistfile1.py

    def _socket(self):
        """Create a listening socket.
        
        This handles IPv6 and allows socket re-use by spawned processes.
        """
        
        host, port = self.address
        families = set()
        for family, kind, protocol, cname, sa in socket.getaddrinfo(host or None, port, flags=socket.AI_PASSIVE):
            families.add(family)
        
        family = socket.AF_INET6 if socket.AF_INET6 in families else socket.AF_INET
        sock = socket.socket(family, socket.SOCK_STREAM)
        
        flags = fcntl.fcntl(sock.fileno(), fcntl.F_GETFD)
        flags |= fcntl.FD_CLOEXEC
        fcntl.fcntl(sock.fileno(), fcntl.F_SETFD, flags)
        
        sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
        
        sock.setblocking(0)
        
        if socket.AF_INET not in families:
            try:
                sock.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 1)
            except (AttributeError, socket.error):
                pass
        
        return sock