the for loop is correctly defined, So the questions did you add mgmt_description, mgmt_ip and cable_test variables to your python?

In nso development doc there is steps & sample for action handler where you could create action package where you could use it in $ ncs-make-package --service-skeleton python pyaction --component-class action.Action --action-example where you could...

Could you try to increase your Juniper QFX read/write timeout   devices device <device-name>  read-timeout 60 write-timeout 60  disconnect connect