blocks|key|567499|text|该文件确实被垃圾回收，因此被关闭。GC决定它何时关闭，而不是您。显然，这不是推荐的做法，因为如果您不在使用完文件后立即关闭文件，则可能会达到打开文件句柄限制。如果在您的for循环中打开更多的文件，并让它们继续存在，会发生什么呢？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|567500|entityMap^0|2C|3|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

The file does get garbage collected, and hence closed. The GC determines when it gets closed, not you. Obviously, this is not a recommended practice because you might hit open file handle limit if you do not close files as soon as you finish using them. What if within that <code>for</code> loop of yours, you open more files and leave them lingering?

blocks|key|567537|text|一些Python会在文件不再被引用时自动关闭文件，而另一些则不会，当Python解释器退出时，由操作系统来关闭文件。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|567538|即使对于将为您关闭文件的Python，时间也不能保证:可能是立即关闭，也可能是几秒/分钟/小时/天后。|567539|因此，尽管您使用的Python可能不会遇到问题，但让您的文件保持打开状态肯定不是一种好的做法。事实上，在cpython3中，如果你不这样做，你会得到警告，系统必须为你关闭文件。|567540|寓意:把自己清理干净。:)|567541|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Some Pythons will close files automatically when they are no longer referenced, while others will not and it's up to the O/S to close files when the Python interpreter exits.

Even for the Pythons that will close files for you, the timing is not guaranteed: it could be immediately, or it could be seconds/minutes/hours/days later.

So, while you may not experience problems with the Python you are using, it is definitely not good practice to leave your files open. In fact, in cpython 3 you will now get warnings that the system had to close files for you if you didn't do it.

Moral: Clean up after yourself. :)

blocks|key|567628|text|在您的示例中，不能保证在解释器退出之前关闭该文件。在当前版本的CPython中，该文件将在for循环结束时关闭，因为CPython使用引用计数作为其主要的垃圾收集机制，但这是实现细节，而不是该语言的特性。Python的其他实现不能保证以这种方式工作。例如，IronPython、PyPy和Jython不使用引用计数，因此不会在循环结束时关闭文件。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|567629|依赖CPython的垃圾收集实现是一种糟糕的做法，因为它会降低代码的可移植性。如果您使用CPython，您可能不会有资源泄漏，但如果您切换到不使用引用计数的Python实现，您将需要检查所有代码，并确保所有文件都已正确关闭。|567630|对于您的示例，请使用：|567631|with+open("filename")+as+f:
+++++for+line+in+f:
++++++++#+...+do+stuff+...|code-block|syntax|javascript|567632|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

In your example the file isn't guaranteed to be closed before the interpreter exits. In current versions of CPython the file will be closed at the end of the for loop because CPython uses reference counting as its primary garbage collection mechanism but that's an implementation detail, not a feature of the language. Other implementations of Python aren't guaranteed to work this way. For example IronPython, PyPy, and Jython don't use reference counting and therefore won't close the file at the end of the loop.

It's bad practice to rely on CPython's garbage collection implementation because it makes your code less portable. You might not have resource leaks if you use CPython, but if you ever switch to a Python implementation which doesn't use reference counting you'll need to go through all your code and make sure all your files are closed properly.

For your example use:

<pre><code>with open("filename") as f:
 for line in f:
 # ... do stuff ...
</code></pre>

blocks|key|783835|text|当你要在同一个python脚本中使用它的内容时，关闭你的文件描述符是非常重要的。经过这么长时间的调试，今天我自己意识到了。原因是只有在您关闭文件描述符后，内容才会被编辑/删除/保存，文件的更改才会受到影响！|type|unstyled|depth|inlineStyleRanges|entityRanges|data|783836|因此，假设您将内容写入一个新文件，然后没有关闭fd，而是在读取其内容的另一个shell命令中使用该文件(而不是fd)。在这种情况下，您将不会像预期的那样获得shell命令的内容，并且如果您尝试进行调试，您将很难找到bug。您也可以在我的博客http://magnificentzps.blogspot.in/2014/04/importance-of-closing-file-descriptor.html中阅读更多内容|offset|length|783837|entityMap|0|LINK|mutability|MUTABLE|url|http://magnificentzps.blogspot.in/2014/04/importance-of-closing-file-descriptor.html^0|0|3C|2C|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

Hi It is very important to close your file descriptor in situation when you are going to use it's content in the same python script. I today itself realize after so long hecting debugging. The reason is content will be edited/removed/saved only after you close you file descriptor and changes are affected to file!

So suppose you have situation that you write content to a new file and then without closing fd you are using that file(not fd) in another shell command which reads its content. In this situation you will not get you contents for shell command as expected and if you try to debug you can't find the bug easily. you can also read more in my blog entry <a href="http://magnificentzps.blogspot.in/2014/04/importance-of-closing-file-descriptor.html" rel="nofollow noreferrer">http://magnificentzps.blogspot.in/2014/04/importance-of-closing-file-descriptor.html</a>

blocks|key|567753|text|在I/O过程中，数据被缓冲:这意味着数据在写入文件之前被保存在一个临时位置。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|567754|在确定完成写入之前，Python不会刷新缓冲区，也就是说，将数据写入文件。一种方法是关闭该文件。|567755|如果在未关闭的情况下写入文件，数据将无法到达目标文件。|567756|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

During the I/O process, data is buffered: this means that it is held in a temporary location before being written to the file.

Python doesn't flush the buffer—that is, write data to the file—until it's sure you're done writing. One way to do this is to close the file. 

If you write to a file without closing, the data won't make it to the target file.

In Python, if you either open a file without calling <code>close()</code>, or close the file but not using <code>try</code>-<code>finally</code> or the "<code>with</code>" statement, is this a problem? Or does it suffice as a coding practice to rely on the Python garbage-collection to close all files? For example, if one does this:

<pre><code>for line in open("filename"):
 # ... do stuff ...
</code></pre>

... is this a problem because the file can never be closed and an exception could occur that prevents it from being closed? Or will it definitely be closed at the conclusion of the <code>for</code> statement because the file goes out of scope?

Is explicitly closing files important?

在Python语言中，如果不调用close()就打开文件，或者不使用try-finally或"with“语句关闭文件，这会有问题吗？或者，依赖Python垃圾收集来关闭所有文件作为编码实践就足够了吗？例如，如果有人这样做：for line in open("filename"):    # ... do stuff ....

问显式关闭文件重要吗？
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