Question

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？

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

注意 - 这是针对尚未拥有with语句的Python版本。

我问，因为Python文档似乎推荐这个： -

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

这似乎比必要的更冗长。

Answer 1

在处理文件时，

关闭始终，在整个地方留下打开的文件句柄并不是一个好主意。当文件对象被垃圾收集时，它们最终会被关闭，但你不知道什么时候会被收集，同时你将通过持有你不再需要的文件句柄来浪费系统资源。

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

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

这与您拥有的代码具有相同的效果：

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

with语句是对C ++中常用的Resource Acquisition Is Initialization习语的直接语言支持。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数据库连接已关闭或锁定始终如下所示。

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

Answer 2

实际上，文件将在garbage collected时关闭。有关其工作原理的详情，请参阅this question。

但仍建议您使用try / finally块或with语句。如果在使用其中一个文件对象的方法时发生异常，则引用将存储在回溯中（存储为全局变量），直到您清除它或发生另一个异常为止。

因此，依靠垃圾收集为您关闭文件是不好的。

此外，如果您已写入该文件，则无法保证更改将在关闭或刷新之前保存到文件中。

Answer 3

奇怪的是，对于本主题中关于释放系统资源的重要性的所有讨论，没有人提到在我看来确定性地关闭文件的明显更重要的原因：以便可以再次打开它。

肯定存在无关紧要的情况。如果文件对象超出范围或被删除，则基础文件将关闭。（当它关闭时取决于您正在使用的Python的具体实现。）这通常会足够好 - 如果您确切知道文件变量何时超出范围，并且 if 你知道你不关心文件是否确定性地关闭。

但是，当with陈述存在时，为什么你甚至会为这种分析烦恼呢？

Answer 4

它在所有地方都有所暗示，但为了使其最清晰，是的，你需要关闭该文件。在Python 2.5（使用 future ）和Python 2.6中，您不再需要罗嗦版本：

from __future__ import with_statement
with open("hello.txt") as f:
    for line in f:
        print line

Answer 5

退出Python解释器（或发生崩溃时的内核）将关闭文件，但是当你不需要它时关闭它们仍然是一个很好的做法。对于1或2或10个文件，它可能不是问题，但是对于更多它可能会导致整个系统崩溃。

最重要的是，这表明编写代码的人实际上关心关于他的工作。

Answer 6

是的，否则你可能会泄漏资源。

来自Python docs：

完成文件后，请致电f.close()关闭文件并释放打开文件占用的所有系统资源。

当程序退出时会发生这种情况，但是否则Python会保留它到目前为止不再需要的资源。

Answer 7

不，我不相信更长的成语是必要的，这就是为什么：

我为模式'for\s+.*\s+in\s+open\(' grepped /usr/lib/python2.6/

找到了很多例子

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

到目前为止

的零实例

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

请参阅下文，了解标准库中使用for ... in open成语的文件列表。

这自然会引出一个问题：如果Python开发人员接受了更短的习语标准库，我们如何通过使用某些东西来改进任何东西如果我们的代码依赖于标准库，我们自己的代码会有什么不同？

我认为答案是，较长的成语不能改善任何事情。

我也跑了

#!/usr/bin/env python
try:
    for i,line in enumerate(open('a')):
        print line
        raw_input()
        if i==5:
            break
except Exception:
    pass

raw_input()

并检查/proc/PID/fd文件描述符何时关闭。看来，当您退出for循环时，文件将为您关闭。

在这些实验的基础上，我不相信长久 try...finally...close成语是必要的。

以下是grep：

的结果

/usr/lib/python2.6/dist-packages/NvidiaDetector/nvidiadetector.py:89:tempList = [ x.strip() for x in open(obsolete).readlines() ]
/usr/lib/python2.6/dist-packages/rpy_io.py:49:for line in open(file).readlines():
/usr/lib/python2.6/dist-packages/setuptools/command/easy_install.py:1376:for line in open(self.filename,'rt'):
/usr/lib/python2.6/dist-packages/GDebi/DscSrcPackage.py:47:for line in open(file):
/usr/lib/python2.6/dist-packages/aptsources/distinfo.py:220:[x.strip() for x in open(value)])
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeCache.py:989:for line in open("/proc/mounts"):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeAufs.py:100:for line in open("/proc/mounts"):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeAufs.py:205:for line in open("/proc/mounts"):
/usr/lib/python2.6/dist-packages/DistUpgrade/distinfo.py:220:[x.strip() for x in open(value)])
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeViewKDE.py:826:for c in open(sys.argv[2]).read():
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeConfigParser.py:45:items = [x.strip() for x in open(p)]
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:684:for line in open(cpuinfo):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:692:for line in open("/proc/mounts"):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:726:for line in open("/etc/fstab"):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:762:for line in open(fstab):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:801:for line in open("/etc/fstab"):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:874:for line in open(XORG):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeQuirks.py:939:for line in open(os.path.join(modaliasesdir,filename)):
/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeController.py:1307:for line in open(template):
/usr/lib/python2.6/dist-packages/DistUpgrade/xorg_fix_proprietary.py:23:for raw in open(xorg_source):
/usr/lib/python2.6/dist-packages/DistUpgrade/xorg_fix_proprietary.py:58:for line in open(xorg):
/usr/lib/python2.6/dist-packages/DistUpgrade/xorg_fix_proprietary.py:82:for line in open(xorg):
/usr/lib/python2.6/dist-packages/jockey/oslib.py:377:for line in open(self.apt_jockey_source):
/usr/lib/python2.6/dist-packages/jockey/oslib.py:393:for line in open(f):
/usr/lib/python2.6/dist-packages/jockey/backend.py:651:for line in open(path):
/usr/lib/python2.6/dist-packages/jockey/detection.py:277:for line in open(alias_file):
/usr/lib/python2.6/dist-packages/jockey/detection.py:597:for l in open(os.path.join(path, 'uevent')):
/usr/lib/python2.6/dist-packages/apt/cdrom.py:83:for line in open(fname):
/usr/lib/python2.6/dist-packages/problem_report.py:1119:for line in open('/proc/mounts'):
/usr/lib/python2.6/dist-packages/apport/packaging_impl.py:128:for line in open(f):
/usr/lib/python2.6/dist-packages/apport/packaging_impl.py:190:for line in open(sumfile):
/usr/lib/python2.6/dist-packages/apport/packaging_impl.py:641:for l in open('/etc/apt/sources.list'):
/usr/lib/python2.6/dist-packages/apport/hookutils.py:190:for line in open('/proc/asound/cards'):
/usr/lib/python2.6/dist-packages/apport/hookutils.py:290:for line in open('/var/log/syslog'):
/usr/lib/python2.6/dist-packages/apport/hookutils.py:493:mods = [l.split()[0] for l in open(module_list)]
/usr/lib/python2.6/dist-packages/softwareproperties/SoftwareProperties.py:597:for line in open(f):
/usr/lib/python2.6/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py:883:for x in open(tmp.name):
/usr/lib/python2.6/dist-packages/lsb_release.py:253:for line in open('/etc/lsb-release'):
/usr/lib/python2.6/dist-packages/numpy/distutils/system_info.py:815:for d in open(ld_so_conf,'r').readlines():
/usr/lib/python2.6/dist-packages/LanguageSelector/LocaleInfo.py:72:for line in open(languagelist_file):
/usr/lib/python2.6/dist-packages/LanguageSelector/LocaleInfo.py:187:for line in open(environment).readlines():
/usr/lib/python2.6/dist-packages/LanguageSelector/LocaleInfo.py:193:for line in open(environment).readlines():
/usr/lib/python2.6/dist-packages/LanguageSelector/LanguageSelector.py:125:for line in open(fname):
/usr/lib/python2.6/dist-packages/LanguageSelector/LanguageSelector.py:140:for line in open(fname):
/usr/lib/python2.6/dist-packages/LanguageSelector/LanguageSelector.py:171:for line in open(fname):
/usr/lib/python2.6/dist-packages/LanguageSelector/LanguageSelector.py:210:for line in open(fname):
/usr/lib/python2.6/dist-packages/LanguageSelector/macros.py:16:for l in open(file):
/usr/lib/python2.6/dist-packages/LanguageSelector/macros.py:37:for l in open(self.LANGCODE_TO_LOCALE):
/usr/lib/python2.6/dist-packages/LanguageSelector/LangCache.py:94:for l in open(self.BLACKLIST):
/usr/lib/python2.6/dist-packages/LanguageSelector/LangCache.py:99:for l in open(self.LANGCODE_TO_LOCALE):
/usr/lib/python2.6/dist-packages/LanguageSelector/LangCache.py:111:for l in open(self.PACKAGE_DEPENDS):
/usr/lib/python2.6/dist-packages/LanguageSelector/ImSwitch.py:78:for l in open(self.blacklist_file):

Answer 8

您需要关闭句柄以释放内存。在一次处理大量文件之前不需要。

在Python文件对象上使用迭代器时，是否需要close（）

8 个答案: