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

"""
fun1() works. All features are selected.
Usage: fun1(1) 
"""
@qgsfunction(args='auto', group='Custom')
def fun1(value1, feature, parent):
	return value1

"""
fun2() works. All features are selected.
Usage: fun2(2)
"""
@qgsfunction(args='auto', group='Custom')
def fun2(value1, feature, parent):
	return value1 + 45

"""
No features are selected on executing fun3().
Usage: fun3()
"""
@qgsfunction(args=0, group='Custom')
def fun3(value1, feature, parent):
	if feature['osm_id']:
		return True
	else:
		return False
		
""" 
fun4() is a workaround I tried to select features.
It is slow and not a good solution to the problem.
Usage: fun4()
"""
@qgsfunction(args=0, group='Custom')
def fun4(value1, feature, parent):
	layer=iface.activeLayer()
	iter=layer.getFeatures()
	for feature in iter:
		return feature['osm_id']