Getting my expression to process the text in field

I found a solution worked for me.

• Add this script to the "Function Editor":

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

@qgsfunction(args='auto', group='Custom')
def isSelected(feat, field, feature, parent):
    layer = QgsProject.instance().mapLayersByName(field)[0]
    for f in layer.getSelectedFeatures():
        if f['fid'] == feat['fid']:
            return 1 # True
    return 0 # False

• Run this expression for layer A (see the picture below):

isSelected(
  get_feature('C', 'fid', "fid"),
  lyr_field
)

Explanation:

• get_feature('C', 'fid', "fid"):

  Return the feature of C matching (fid in C == "fid" in A)

• QgsProject.instance().mapLayersByName(field)[0]:

  Return the layer matching the lyr_field value (B here) of the feature in A.

• layer.getSelectedFeatures():

  Get selected features in B.

• if f['fid'] == feat['fid']:

  If fid value of one of the selected features in B exists in C then return 1, else return 0

If get_feature('C', 'fid', "fid") returns None, the expression returns nothing.

I used aa field to populate.

See below the changed code: you will not need the parameter "entidad" you can use the default parameter "feature" instead:

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args='auto', group='Custom')
def isSelected(capa, feature, parent):
    proy= QgsProject.instance()
    capap= proy.mapLayersByName(capa)[0]
    les= list(capap.getSelectedFeatures())
    boolean=False
    for f in les:
        if f.id() == feature.id():
            boolean= True
            break
        else:
            continue
    return boolean