逐行文件处理，for-loop vs [英] line-by-line file processing, for-loop vs with

问题描述

我试图理解这些交易的差异/
之间的差异，以逐行处理的方式打开文件

  with open（'data.txt'）as inf：
 for line in $：
 #etc 
  

 对于open（'data.txt'）中的行：
 $ etc 
  

我明白，使用和确保当
with-block（suite？）被退出（或异常被反击）时文件被关闭。所以我一直在使用和，因为我在这里了解到了它。

Re for -loop：从网上搜索到SO，似乎当​​ -loop -loop退出时，是否关闭文件
依赖于实现吗？而
我找不到有关这个构造如何处理
异常的任何事情。有人知道吗？

如果我误解了上面的任何内容，我会感谢更正
，否则有理由永远使用 for 通过
来构造？ （假设你有一个选择，即不受Python版本限制）

解决方案

 用于打开（'data.txt'）行：
＃etc 
  

是不是你打开的文件保留一个明确的引用，那么你怎样关闭它？
懒惰的方式是等待垃圾收集器清理它，但这可能意味着资源不能及时释放。

所以你可以说：

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ＃etc
inf.close（）

现在如果发生异常在for循环里面？

添加 try / finally

  inf = open（'data.txt'）
尝试：
在inf中的行：
＃etc 
 finally：
 inf.close（）
  

这是很多代码非常简单，所以Python用添加来使这个代码能够以更可读的方式写入。我们来到这里

  with open（'data.txt'）as inf：
 for line in inf： 
 #etc 
  

所以，这是打开文件的首选方式。如果你的Python对于with语句来说太老了，你应该使用 try / finally 版本来生产代码

I am trying to understand the trade offs/differences between these to ways of opening files for line-by-line processing

with open('data.txt') as inf:
    for line in inf:
       #etc

vs

for line in open('data.txt'):
   # etc

I understand that using with ensures the file is closed when the "with-block" (suite?) is exited (or an exception is countered). So I have been using with ever since I learned about it here.

Re for-loop: From searching around the net and SO, it seems that whether the file is closed when the for-loop is exited is implementation dependent? And I couldn't find anything about how this construct would deal with exceptions. Does anyone know?

If I am mistaken about anything above, I'd appreciate corrections, otherwise is there a reason to ever use the for construct over the with? (Assuming you have a choice, i.e., aren't limited by Python version)

解决方案

The problem with this

for line in open('data.txt'):
   # etc

Is that you don't keep an explicit reference to the open file, so how do you close it? The lazy way is wait for the garbage collector to clean it up, but that may mean that the resources aren't freed in a timely manner.

So you can say

inf = open('data.txt')
for line in inf:
   # etc
inf.close()

Now what happens if there is an exception while you are inside the for loop? The file won't get closed explicitly.

Add a try/finally

inf = open('data.txt')
try:
    for line in inf:
       # etc
finally:
    inf.close()

This is a lot of code to do something pretty simple, so Python added with to enable this code to be written in a more readable way. Which gets us to here

with open('data.txt') as inf:
    for line in inf:
       #etc

So, that is the preferred way to open the file. If your Python is too old for the with statement, you should use the try/finally version for production code

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