在python 3中子类化文件对象(以扩展打开和关闭操作) [英] subclassing file objects (to extend open and close operations) in python 3

问题描述

假设我想在 open 和 close 时间使用额外的操作来扩展内置文件抽象.在 Python 2.7 中，这有效:

class ExtFile(file):def __init__(self, *args):file.__init__(self, *args)# 额外的东西在这里定义关闭(自我):file.close(self)# 额外的东西在这里

现在我正在考虑将程序更新到 Python 3，其中 open 是一个工厂函数，它可以从 io 模块取决于它的调用方式.原则上我可以将所有这些子类化，但这很乏味，而且我必须重新实现 open 所做的分派.(在 Python 3 中，二进制文件和文本文件之间的区别比在 2.x 中更重要，我需要两者.)这些对象将被传递给可能对它们做任何事情的库代码，所以习惯用法制作一个类似文件"的鸭子类型的类来包装 open 的返回值并转发必要的方法将是最冗长的.

谁能建议一种 3.x 方法，在显示的 2.x 代码之外尽可能少地涉及额外的样板?

解决方案

您可以只使用上下文管理器.例如这个:

class SpecialFileOpener:def __init__ (self, fileName, someOtherParameter):self.f = 打开(文件名)# 做更多的事情打印(一些其他参数)def __enter__ (self):返回 self.fdef __exit__ (self, exc_type, exc_value, traceback):self.f.close()# 做更多的事情print('一切都结束了.')

然后你可以这样使用它:

<预><代码>>>>with SpecialFileOpener('C:\\test.txt', 'Hello world!') as f:打印(f.read())你好，世界！富吧全都结束了.

无论如何，对于文件对象(和其他资源)，首选使用带有 with 的上下文块.

Suppose I want to extend the built-in file abstraction with extra operations at open and close time. In Python 2.7 this works:

class ExtFile(file):
    def __init__(self, *args):
        file.__init__(self, *args)
        # extra stuff here

    def close(self):
        file.close(self)
        # extra stuff here

Now I'm looking at updating the program to Python 3, in which open is a factory function that might return an instance of any of several different classes from the io module depending on how it's called. I could in principle subclass all of them, but that's tedious, and I'd have to reimplement the dispatching that open does. (In Python 3 the distinction between binary and text files matters rather more than it does in 2.x, and I need both.) These objects are going to be passed to library code that might do just about anything with them, so the idiom of making a "file-like" duck-typed class that wraps the return value of open and forwards necessary methods will be most verbose.

Can anyone suggest a 3.x approach that involves as little additional boilerplate as possible beyond the 2.x code shown?

解决方案

You could just use a context manager instead. For example this one:

class SpecialFileOpener:
    def __init__ (self, fileName, someOtherParameter):
        self.f = open(fileName)
        # do more stuff
        print(someOtherParameter)
    def __enter__ (self):
        return self.f
    def __exit__ (self, exc_type, exc_value, traceback):
        self.f.close()
        # do more stuff
        print('Everything is over.')

Then you can use it like this:

>>> with SpecialFileOpener('C:\\test.txt', 'Hello world!') as f:
        print(f.read())

Hello world!
foo bar
Everything is over.

Using a context block with with is preferred for file objects (and other resources) anyway.

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