Normally, applying look at constraints require applying the constraint to an object, clicking on the target object, assigning the aiming axis, and optionally adding an upnode object then choosing it's axis.

This tool uses the selection order to automatically assigns the target, the upnode, and the aim axis based on the object's world positions to the constrained object.

To apply without an upnode:  select the target first, the slave second, and apply the script.

To apply with an upnode:  select the target first, the upnode second, the slave third, and apply the script.