How to execute "gksudo umount VirtualDVD"

[Demosthenes Koptsis]

I want to execute the command "gksudo umount VirtualDVD"

My code is this but it fails:

def umount(self):
     '''unmounts VirtualDVD''' cmd ='gksudo umount VirtualDVD' proc = subprocess.Popen(str(cmd),shell=True,stdout=subprocess.PIPE).stdout.read()
     print proc

It pops up the gksudo dialog, and then fails. But i don't get any stdout 
or stderror.

I tried cmd='gksudo ls' and it succeeds.