0kb backup using mysqldump
Some troubleshooting:


Run the command (dictated by dumpcmd) from the command-line and see if there are any errors
Does the password have any special characters in it that could mess up in a shell
Try changing mysqldump to the full path, eg: /usr/local/bin/mysqldump
Check the return value from os.system() in your script


Changing to the full path appears to be what you were looking for.