Read value from current (and unsaved) form

Question

I would like to implement a function to check that there are no features already inserted with the same value in a certain field.

This is the function that I implemented as Action, but during editing [%id_lavoro%] always seems to be empty

from qgis.utils import iface
from qgis.PyQt.QtWidgets import *
msg = QMessageBox()
l = iface.activeLayer()

expr = QgsExpression( "\"id_lavoro\"=[%id_lavoro%]")
it = l.getFeatures(QgsFeatureRequest(expr))
ids = str([i.id() for i in it])
msg.setText(ids)
msg.show()

QGIS has a function (current_value), but I don't know how to use it in Python code.

Is there a way to return value of a field in an unsaved form?