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In Python, if you either open a file without calling
close(), or close the file but not using
finally or the "
with" 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:
for line in open("filename"): # ... do stuff ...
... 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
for statement because the file goes out of scope?
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:
with open("filename") as f: for line in f: # ... do stuff ...
What happens if I don't call fclose() in a C program?, is automatically closed. Trying to read from f after we have exited from the “with” block will result in the same ValueError exception that we saw above. Hidden inside the file object is a reference to a "file descriptor", the actual OS resource associated with the open file. Closing the file tells the system to release this resource. As an OS resource, the number of files a process can keep open is limited: Long ago the per-process limit was about 20 on Unix. Right now my OS X box imposes a
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. :)
Why should I close files in Python?, The simplest answer is that we can explicitly close our file by invoking f.close(). Once we have done that, the object continues to exist — but we The file does not go out of scope at the end of the for block. Its reference count will go to zero, causing it to be closed automatically, but only functions, classes, and modules define scopes in Python, not other compound statements. It's not a problem unless it's a problem.
If you don't use "with", when does Python close files? The answer is , You ask: Why is it so important to close files that have been opened in Python? If your program is short-lived, it's not so important, as the operating system will You should be able explicitly close the file by calling qq.close(). Also, python does not close a file right when it is done with it, similar to how it handles its garbage collection. You may need to look into how to get python to release all of its unused file descriptors.
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
for loop of yours, you open more files and leave them lingering?
Why is it so important to close files that have been opened in Python , When do files get closed? As we can learn from Is explicitly closing files important? (StackOverflow), the Python interpreter closes the file in the Without the #Close, you can not be sure when the underlying file handle will be properly closed. Sometimes, this can be at app shutdown. Thanks, that helps clear up some confusion. In my case, the file handle was never closed even at app shutdown.
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 http://magnificentzps.blogspot.in/2014/04/importance-of-closing-file-descriptor.html
Is closing a file after having opened it with `open()` required in , How to safely open/close files in python has a built-in function open() to open a It is important to note that always make sure you explicitly close each open file, Close is always necessary when dealing with files, it is not a good idea to leave open file handles all over the place. They will eventually be closed when the file object is garbage collected but you do not know when that will be and in the mean time you will be wasting system resources by holding to file handles you no longer need.
How to open and close a file in Python, I guess a system without open and close wouldn't be inconceivable, The concept of the file handle is important because of UNIX's design WRT code, i want to explicitly "save" the file without calling close(). i know there is no need to call close() as fsteam will call the destructor and save the file when fstream object goes out of scope. But i want to "explictly" save the file without waiting for fstream object to go out of scope. Is there a way to do this in C++ ?
On Unix systems, why do we have to explicitly `open()` and `close , Now, we will see how to use actual data files.Python Python provides basic functions and methods necessary to manipulate files by default. You can do Returns false if space explicitly required with print, true otherwise. Just as an added note, there are still situations where it is necessary to explicitly close a file. Say you have a vector fileNames of files you need to process. The reason you need to call close () at the end of the loop is that trying to open a new file without closing the first file will fail.
Opening and Closing Files in Python, Python has a built-in function, open, for opening a file on disk. open returns a file object, It's important to close files as soon as you're finished with them. That's perfectly legal; “handling” an exception can mean explicitly doing nothing. You can close files with a CLOSE command, with or without the FORCE option. Closing files normally. You can close a file explicitly with one of the following commands . CEMT SET FILE(filename) CLOSED EXEC CICS SET FILE(filename) CLOSED. The file is closed immediately if there are no transactions using the file at the time of the request.
- The file does not go out of scope at the end of the
forblock. Its reference count will go to zero, causing it to be closed automatically, but only functions, classes, and modules define scopes in Python, not other compound statements.
- It's not a problem unless it's a problem. At the OS level, any files opened by the script will be closed when the script exits, so you needn't worry about closing files in throwaway tool scripts. However, processes have a limit on the number of open files they can maintain, so long-lived or complex scripts may need to be more careful. In any case, it's a good habit to close your files.
- @agf: You are right that the file doesn't go out of scope, but it's not related to the distinction between
forblocks and functions/classes/modules. It's much simpler than that: objects don't have scopes, only names do. There is no name that refers to this object, so there is nothing here to stay in scope or go out of scope.
- @max My comment is correcting his assumption that there is a scope associated with the
forloop, and mentioning that the file gets closed for an entirely different reason. It doesn't get into what scopes are in Python, as it's not relevant here.
- @max there's an implicit reference scoped to that for loop... this is an argument of semantics
- Does using
with open() as fautomatically close the file after it is done?
- @Rohan yes, that is the little magic that the
withstatement provide, but of course for this magic to work the object must have the especial methods
__exit__, in the latter the object do the
closeand any other cleanup stuff that need to be done at the end of the
- FYI: This answer only explains "when it would be closed" but does not explain "what if it stays open". For the latter, please read the "What would happen if a file stays open?" part in this answer (askubuntu.com/questions/701491/…)
- Moreover, not closing files can result in truncated files as file contents have not been flushed.
- So if I don't close the file, will I get my memory back for sure once the program stops running? Or do I actually have to quit out of the entire interpreter?
- Files get closed when they're no longer referenced in CPython, but that's not a language feature. If it was you could quite happily rely on it.
- But if you opened other files within that for loop, it would still be the case that there would be more than one file open simultaneously whether you explicitly close any of them or not. Are you saying that the file isn't necessarily garbage-collected as soon as the file goes out of scope, thus it would be closed sooner if done explicitly? What about when an exception happens (when you use with/try-finally vs. not doing so)?
- In CPython, reference counting will cause it to be collected after the
forstatement -- you won't have to wait for the next garbage collection run.