Python文件对象允许您关闭已经关闭的文件 [英] Python file object allows you to close a file that is already closed

问题描述

在2.7中创建代码(对于我正在学习的Python类)，并在3.x中交叉测试(对我来说).不需要此代码.我决定构建对将来可能有用的对象，而不是直接回答原始问题.

Creating code in 2.7 (for a Python class I am taking) and cross-testing it in 3.x (for me). This code is not required. I decided to build objects that might be useful for the future instead of simply answering the original question directly.

由于3.x没有要继承的文件对象，因此我的对象在其中使用文件实例，而不是直接从其继承.在测试下面的代码期间，我遇到了一个奇怪的问题.我可以关闭文件，然后再次关闭它，这不会触发任何类型的错误.为了合理地检查此代码是否正确构建，对此有两个问题:*为什么我可以两次关闭同一个文件*我写这个的方式，当我关闭文件时，是真的关闭它并释放文件对象占用的内存吗?

Since 3.x has no file object to inherit, my object uses file instances within it instead of inheriting from it directly. During testing of the code which follows, I encountered an oddity. I can close a file and then close it again and this does not trigger any kind of error. To sanity checking this code is being built right, two questions about this: * why can I close the same file twice * the way I have written this, when I close the file, am I really closing it and freeing up the memory the file object is taking up?

这是代码:

class FileData(object):
    def __init__(self, xfilename, xmode):
        self.filename = xfilename
        self.mode = xmode
        self.f = open(xfilename, 'r')

    def getAllFileData(self):
        self.lines = f.readlines()
        self.f.close()

    def getLineFromFile(self):
        if f.closed:
            self.f = open(xfilename, 'r')    
        self.f.readline()

    def fileHead(self, numRows):
        if self.f.closed:
            self.f = open(xfilename, 'r')

        for i in range(numRows):
            print(self.f.readline())
        self.f.close()

然后我运行了这些测试行，并意外地重新运行了该行以多次关闭文件.为了确保我没有丢失任何东西，后来我在Jupyter单元中组织了这些行并将它们放在一起.

Then I ran these test lines and accidentally re-ran the line to close the file multiple times. To make sure I wasn't missing something, I later organized these lines in a Jupyter cell and ran them together.

chatLog = FileData("script/record.txt", "r")
chatLog.fileHead(15)

chatLog.f.close()
chatLog.f.close()

请注意，fileHead()完成后也会关闭文件，因此上述代码实际上应该尝试关闭同一文件3次.

Note that fileHead() also closes the file when done, so really above code should have tried to close the same file 3 times.

为什么没有错误?并以我认为的方式关闭文件吗?学习语言，以便您进行任何输入.

Why no error? And is this closing the file the way I think it is? Learning the language so any input would be appreciated.

推荐答案

f.close()不会执行任何操作.

您可以对此进行防护，但是看到它有多么复杂，我不建议这样做:

You could protect against it but seeing how complex it is I wouldn't advise to do it:

import _io

def override_close(f):
    if f.closed:
        raise IOError("already closed error")
    else:
        _io.TextIOWrapper.close(f)

f = open("foo.c")
f.close = lambda : override_close(f)

print("closing")
f.close()
print("protected")
f.close()   # raises IOError

我已经覆盖了 f 对象的 close 方法，因此它检查已关闭的文件并引发异常.

I have overridden the close method for the f object so it checks against already closed file and raises an exception.

以您的示例为例，最好的方法是通过使其不从外部直接不可见来从外部隐藏 f (和其他数据):

For your example, the best way would be to hide f (and other data) from the outside by making them not directly invisible from the outside:

self.__f = open(...)

因此，调用者无法再(轻松地:)弄乱您的文件句柄了.

so callers cannot mess (easily :)) with your file handle anymore.

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