Create a python script to concatenate two lists.
list_1 = [1, 2, 3]
list_2 = [4, 5, 6]
def concat_lists(list_1, list_2):
    return list_1 + list_2

list_1 = [1, 2, 3]
list_2 = [4, 5, 6]

concatenated_list = concat_lists(list_1, list_2)
print(concatenated_list) # [1, 2, 3, 4, 5, 6]