Construct a Python code to combine the elements of two given list and return a sorted union list.
input_list_1: [4, 10, 2]
input_list_2: [10, 2, 15]
def union_list(list1, list2): 
    list3 = list1 + list2 
    list3.sort() 
    return list3 
    
union_list(input_list_1, input_list_2)