def someComputedProperty(self):
    if not self.someCondition:
        return []
src = self.someReferenceProperty
list = src.list1 + src.list2 + src.list3 + src.list4 + [src.str1, src.str2]
return map(lambda x: '' if not x else x.lower(), list)
Workaround to return a list from a ComputedProperty function in NDB