Add QGSExpression to QGraphicsItem in Layout using PyQGIS

Question

I would like to add and modify and expression for Opacity in a QGraphicsLineItem

when I have the item QGraphicsLineItem, it not have a variable to modify the opacity expression

Code to get the item:

QgsProject.instance().layoutManager().layouts()[0].items()[0]

Answer 1

I found the solution to read and modify the opacity expression for any item of the Layout in the following way:

items = QgsProject.instance().layoutManager().layouts()[0].items()

for item in items:
    # Visualize the current expression
    expression = item.dataDefinedProperties().property(QgsLayoutObject.Opacity).asExpression()
    print(expression)

    # Modify the previous expression of Opacity
    item.dataDefinedProperties().property(QgsLayoutObject.Opacity).setExpressionString('if(@atlas_pagename = 6, 100, 0)')

    # Visualize the modified expression
    print(item.dataDefinedProperties().property(QgsLayoutObject.Opacity).asExpression())

Answer 2

You could try this:

from PyQt6.QtGui import QPen, QColor
from PyQt6.QtWidgets import QGraphicsLineItem

line_item = QgsProject.instance().layoutManager().layouts()[0].items()[0]

pen = line_item.pen()

color = pen.color()
color.setAlpha(128)  # Set to 50% opacity

pen.setColor(color)
line_item.setPen(pen)