qgis custom expression to check if feature is selected

gistfile1.txt

"""
Template function file. Define new functions using @qgsfunction.
When using args="auto" you may define a new variable for each value for the function.
feature and parent must always be the last args.
To pass a any number of args into a function use args=-1 the first
variable will then be a list of values.
"""

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

@qgsfunction(1, 'Custom')
def isSelected(vectorName, feature, parent):
	if not vectorName:
		return
	
	layer = QgsMapLayerRegistry.instance().mapLayersByName(vectorName[0])
	if not layer:
		return
	
	layer = layer[0]
	selectedIds =  layer.selectedFeaturesIds()
	if feature.id() in selectedIds:
		return True
	else:
		return False