Python文件与关闭

Question

如果我写这样的代码，会发生什么？文件对象会自动关闭吗？

# Do something before
# Make the value = 1 when the number in file.
for line in open('input_file.txt', 'r'):
    index_list[int(line)] = 1
# Do something later

或者我必须编写这样的代码？

with open('xxx.txt', 'r') as f:
    # Do something