在 Python 文件对象上使用迭代器时是否需要 close()

问题描述

执行以下操作并且不显式处理文件对象并调用其 close() 方法是不好的做法吗?

Is it bad practice to do the following and not explicitly handle a file object and call its close() method?

for line in open('hello.txt'):
    print line

注意 - 这适用于还没有 with 语句的 Python 版本.

NB - this is for versions of Python that do not yet have the with statement.

我问，因为 Python 文档似乎建议这样做:-

I ask as the Python documentation seems to recommend this :-

f = open("hello.txt")
try:
    for line in f:
        print line
finally:
    f.close()

这似乎比必要的更冗长.

Which seems more verbose than necessary.

推荐答案

在处理文件时，总是必须关闭，不要让打开的文件句柄到处都是.当文件对象被垃圾回收时，它们最终将被关闭，但您不知道何时会关闭，同时您将通过持有不再需要的文件句柄来浪费系统资源.

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.

如果您使用的是 Python 2.5 及更高版本，则可以使用 with 语句自动为您调用 close():

If you are using Python 2.5 and higher the close() can be called for you automatically using the with statement:

from __future__ import with_statement # Only needed in Python 2.5
with open("hello.txt") as f:
    for line in f:
        print line

这与您的代码具有相同的效果:

This is has the same effect as the code you have:

f = open("hello.txt")
try:
    for line in f:
        print line
finally:
    f.close()

with 语句是对 Resource Acquisition Is Initialization C++中常用的成语.它允许安全使用和清理各种资源，例如它可以用于始终确保关闭数据库连接或始终释放锁，如下所示.

The with statement is direct language support for the Resource Acquisition Is Initialization idiom commonly used in C++. It allows the safe use and clean up of all sorts of resources, for example it can be used to always ensure that database connections are closed or locks are always released like below.

mylock = threading.Lock()
with mylock:
    pass # do some thread safe stuff

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