I have a question to add to this point. Does anyone know if you can set the extension lines to a certain point as default. for example. I do kitchen plans all the time and would like my dimensions outside the wall which in general is 300mm thick and therefore would like the extensions to start from the outside of the wall as standard rather than the object just inside the wall. I do find myself laboriously chnging individual extensions all the time.

Let's assume your 'box' is a component.
Let's assume there is a value 'x' thet you want it to 'remember' for another time...
You have a reference to the definition - let's say it's ' defn'...
Then this code remembers it:
defn.set_attribute("Cheez_Data","x",x)
Provided that you save the model the next time you want to access the SKP and that definition - perhaps via a selected instance... like defn=model.selection[0].definition - after you test for it being a suitable instance etc...
You could use:
x=defn.get_attribute("Cheez_Data","x",0)
Here it's set to return 0 if it's not been set before - otherwise it's 'nil'.

You can store loads of different data in an object's attribute-dictionary.
The data types can be integer, float, string, boolean, array...
A hash has to be stored as an array for the 'set'... and reconstructed into a hash after the 'get'.

You could attach data to the the model itself, BUT since you are going to change a specific definition you might be best storing it there...
You can attached attribute data to most 'types', but some like geometry are more 'transient'...

Yes, it'd return an error or if @ip1 is nil then it might be seen as equal to an empty @pts[] ?
If you are testing something that might be nil it's a good idea to check it exists first...