QGIS – Dynamic Height Value from a LayoutItem Using a PyQGIS Function

Question

I'm trying to make a function that allows me to get the dynamic height of a LayoutItem in the print composer. I'm working in QGIS 3.16.

The following code works well to get an item height, when I run it as a script in the Python console.

#defining variables
layout_name = "LayoutName"
item_name = "ItemName"

#getting the layout
manager = QgsProject.instance().layoutManager()
layout = manager.layoutByName(layout_name)

#geting the layoutItem --> getting item size --> getting item height
get_item = layout.itemById(item_name)
get_item_size = get_item.sizeWithUnits()
get_item_height = get_item_size.height()

print(get_item_height)

I want to change this code in a way, so I can get the item height dynamically instead and so I can use the height as a variable for defining the height of other items in my layout (like it's possible to do with the variable @layout_pageheight).

I would assume that I could just change the code in this way:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args= 'auto' , group='Custom')
def GetDynamicItemHeight (LayoutName , ItemName): 
    #variables
    layout_name = LayoutName
    item_name = ItemName

    #getting the layout
    manager = QgsProject.instance().layoutManager()
    layout = manager.layoutByName(layout_name)

    #geting the layoutItem --> getting item size --> getting item height
    get_item = layout.itemById(item_name)
    get_item_size = get_item.sizeWithUnits()
    get_item_height = get_item_size.height()

    #returning the height value
    return get_item_height

Then go to an expression editor in the layout and enter the following:

GetDynamicItemHeight(@layout_name, "ItemName") 

And I would get the dynamic height of the specified item. Only problem is – it doesn't work.

Can anyone help to figure out what needs to be changed?

Answer 1

Add feature, parent parameters to the definition line. Although not needed in the context of the layout, they are mandatory. There are some issues in GitHub.

    ...

@qgsfunction(args= 'auto' , group='Custom')
def GetDynamicItemHeight (LayoutName , ItemName, feature, parent):
    ... 
    ...

And use ' instead of " in expression for ItemName

GetDynamicItemHeight(@layout_name, 'ItemName')