for open in paradigm python

Question

我总是用Python读取文件：

for line in open('filename'):
    pass

最近，python开始喷出这个警告：

ResourceWarning: unclosed file <_io.TextIOWrapper name='filename' mode='r' encoding='UTF-8'>

如果没有这种警告，什么是pythonic方式读取文件中的每一行？

with open('filename') as f:
    for line in f:
        pass

感觉像线条（和缩进级别）太多。