previousValue from change Scripts Issue

Ignition
1

so I am trying to pull the previous value from a change script but I unable to use the methods or properties associated with QualifiedValues datatype I can see if I:
prevV = previousValue system.perspective.print(prevV)
it returns QV[[Lcom.inductiveautomation.ignition.common.model.values.QualifiedValue;@5db0174b, Good, Thu Oct 24 12:49:52 EDT 2024]

But if i try to reference or use any of the qualified values methods it gives me this
AttributeError("'NoneType' object has no attribute 'getValue'",)

This is the code that throws the error
prevV = previousValue.getValue() system.perspective.print(prevV)

any ideas on why this would be happening?

2

Are you checking the initialChange flag? There's no previous value at all when that flag is true.

3

Are these available when you are doing a change script on a component?
I have a change script on an array of objects, sorry should have clarified earlier

I know they are available on tag changes, but I cannot see/find where you pass the flag for component property change scripts.

4

Interesting. I hadn't noticed that there's no initialChange flag. But previousValue being null on first change makes sense. You should check for that:

	if previousValue is None:
		# handle first update
	else:
		# handle change
5

Okay that makes sense, I have a property that is binded to a sql query and data being pulled is default data in my databse so it should never be null.

After testing aswell your theory was correct, on the first change i get the error but the second change it returns the data.

should I have this configured differently for my use case?

6

You haven't explained your use-case, so I don't know.

7

oh sorry, so I have a tables components that renders data depending what on what data is being passed, I also want the option for the user to change the data via change script. So I use an embedded view and referenced my table component. The data param for the embedded view I bind to an sql query(this is also where the change script is)

so the data will theoretically never be null initially

8

I would make the property non-persistent so it always does start with a null.

9

Okay I will try that, thank you for the suggestion