Blender Game Runtime scaling

My Script failed to work in game, The blender terminal shows:
AttributeError: KX_GameObject.worldScale is read-only

the part of my code that is troublesome:
obj.worldScale.x += 0.01212

also, i run that code trought the eval() function if that has anything to do with this

[im using blender 2.79’s game engine]

obj.scaling = [x,y,z]