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Assuming that you don't want to just loop over a busdata2 function, you could make a bus subsystem and run zonm on it.
# Define a bus subsystem
psspy.bsys(0,0,[0,500],0,[],3,[20304,20305,20306],0,[],0,[])
# Change the zone for the buses in the subsystem to 3
psspy.zonm(0,0,[1,1],3)
# Restore the subsystem view to show all buses.
psspy.bsysdef(0,0)
Remember to change the "3" being passed to bsys if you're not modifying three buses.