In GDScript in Godot 3, you use a .() to call a method on the parent class. You can use that to choose if you want to completely override a method existing on the parent class or extend it. For example:
var _contents = 
.add_child(node) # This calls the Node class's add_child() implementation
_contents.append(node) # But in this script, I want to store extra children in an array named _contents
In the case of _init(), the class constructor, the notation (or syntax if you prefer) is unique because you have to add the .() inside the function definition, on the first line:
func _init(data: AttackActionData, actor, targets: Array).(data, actor, targets) -> void:
This will trigger a call to the parent class's _init() with the arguments data, actor, and targets. Only _init() requires this as far as I know.
Note I believe this inconsistency is meant to be fixed in Godot 4 with GDScript 2.0.