[Qgis-user] Expression to read field value from composer layout?

[Patrick Dunford]

Good day

I am wishing to create a value in a field on a composer layout that 
reads the value of another field on the layout and calculates a value to 
be displayed.

For example, I have an existing field called Filename on the layout and 
in a python script running in the main window console, I have this code 
that reads the value of the Filename field as follows:

layoutTitle = "Landscape 16:9 Header Nav Copyright Filename 2021"

project = QgsProject.instance()
projectLayoutManager = project.layoutManager()
layout = projectLayoutManager.layoutByName(layoutTitle)

fileNameLabel = layout.itemById('Filename')
fileNameText = fileNameLabel.text()


Obviously I am hoping that doing this from a field within the layout 
that reads another field on the same layout will be much simpler than 
the above. How exactly would I achieve that?

Thanks in advance.