要读取个大文件，文件大概是3G左右，担心read会出现内存溢出的情况，网上找了个靠谱的用法：

with open(...) as f:
    for line in f:
        <do something with line>

The with statement handles opening and closing the file, including if an exception is raised in the inner block. The for line in f treats the file object f as an iterable, which automatically uses buffered IO and memory management so you don‘t have to worry about large files.

There should be one -- and preferably only one -- obvious way to do it.

python 读取大文件