def __str__(self) :
#Check the outfile, filter, or stdout option has been set.
#We can't simply do this via the required flag for the outfile
#output - this seems the simplest solution.
if not (self.outfile or self.filter or self.stdout) :
raise ValueError("You must either set outfile (output filename), "
"or enable filter or stdout (output to stdout).")
return AbstractCommandline.__str__(self)
