How to copy the file from source to destination only once at a time?

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http://askubuntu.com – I have to copy the file from my desktop to my mounted directory. I was using the following command to copy the file from my desktop to mounted directory. os.system("cp -f /home/Desktop/filename /media/folder_1"). It works fine. But the problem is while copying the file from my source to mounted directory(folder_1) if any interruption is happens like network down, then the system continuously keep on trying. It couldn't skip that process. Finally, when the network comes the files are again copy to my mounted directory. Due to this continuous trying, next time i try to move the content it (HowTos)