将stdout重定向到Python中的文件？

本文翻译自：Redirect stdout to a file in Python?

How do I redirect stdout to an arbitrary file in Python? 如何在Python中将stdout重定向到任意文件？

When a long-running Python script (eg, web application) is started from within the ssh session and backgounded, and the ssh session is closed, the application will raise IOError and fail the moment it tries to write to stdout. 当从ssh会话中启动运行了长时间的Python脚本（例如，Web应用程序）并进行背景调整，并且ssh会话关闭时，该应用程序将在尝试写入stdout时引发IOError并失败。 I needed to find a way to make the application and modules output to a file rather than stdout to prevent failure due to IOError. 我需要找到一种方法来使应用程序和模块输出到文件而不是stdout，以防止由于IOError而导致失败。 Currently, I employ nohup to redirect output to a file, and that gets the job done, but I was wondering if there was a way to do it without using nohup, out of curiosity. 当前，我使用nohup将输出重定向到文件，并且可以完成工作，但是我想知道是否有一种出于好奇而无需使用nohup的方法。

I have already tried sys.stdout = open('somefile', 'w') , but this does not seem to prevent some external modules from still outputting to terminal (or maybe the sys.stdout = ... line did not fire at all). 我已经尝试过sys.stdout = open('somefile', 'w') ，但这似乎并不能阻止某些外部模块仍然输出到终端（或者sys.stdout = ...行未在所有）。 I know it should work from simpler scripts I've tested on, but I also didn't have time yet to test on a web application yet. 我知道它应该可以通过我测试过的简单脚本工作，但是我还没有时间在Web应用程序上进行测试。

#1楼

参考：https://stackoom.com/question/jCmY/将stdout重定向到Python中的文件

#2楼

The other answers didn't cover the case where you want forked processes to share your new stdout. 其他答案没有涉及您希望分叉的进程共享新标准输出的情况。

To do that: 要做到这一点：

from os import open, close, dup, O_WRONLYold = dup(1)
close(1)
open("file", O_WRONLY) # should open on 1..... do stuff and then restoreclose(1)
dup(old) # should dup to 1
close(old) # get rid of left overs

#3楼

Quoted from PEP 343 -- The "with" Statement (added import statement): 引用自PEP 343-“ with”语句（添加的导入语句）：

Redirect stdout temporarily: 暂时重定向标准输出：

import sys
from contextlib import contextmanager
@contextmanager
def stdout_redirected(new_stdout):save_stdout = sys.stdoutsys.stdout = new_stdouttry:yield Nonefinally:sys.stdout = save_stdout

Used as follows: 用法如下：

with open(filename, "w") as f:with stdout_redirected(f):print "Hello world"

This isn't thread-safe, of course, but neither is doing this same dance manually. 当然，这不是线程安全的，但是也没有手动进行相同的舞蹈。 In single-threaded programs (for example in scripts) it is a popular way of doing things. 在单线程程序中（例如在脚本中），这是一种流行的处理方式。

#4楼

Programs written in other languages (eg C) have to do special magic (called double-forking) expressly to detach from the terminal (and to prevent zombie processes). 用其他语言（例如C）编写的程序必须做特别的魔术（称为双叉）才能与终端分离（并防止僵尸进程）。 So, I think the best solution is to emulate them. 因此，我认为最好的解决方案是模仿它们。

A plus of re-executing your program is, you can choose redirections on the command-line, eg /usr/bin/python mycoolscript.py 2>&1 1>/dev/null 重新执行程序的一个/usr/bin/python mycoolscript.py 2>&1 1>/dev/null是，您可以在命令行上选择重定向，例如/usr/bin/python mycoolscript.py 2>&1 1>/dev/null

See this post for more info: What is the reason for performing a double fork when creating a daemon? 有关更多信息，请参见此帖子：创建守护程序时执行双叉的原因是什么？

#5楼

There is contextlib.redirect_stdout() function in Python 3.4: Python 3.4中有contextlib.redirect_stdout()函数：

from contextlib import redirect_stdoutwith open('help.txt', 'w') as f:with redirect_stdout(f):print('it now prints to `help.text`')

It is similar to: 它类似于：

import sys
from contextlib import contextmanager@contextmanager
def redirect_stdout(new_target):old_target, sys.stdout = sys.stdout, new_target # replace sys.stdouttry:yield new_target # run some code with the replaced stdoutfinally:sys.stdout = old_target # restore to the previous value

that can be used on earlier Python versions. 可以在早期的Python版本中使用。 The latter version is not reusable . 后一版本不可重用。 It can be made one if desired. 如果需要，可以将其制成一个。

It doesn't redirect the stdout at the file descriptors level eg: 它不会在文件描述符级别重定向标准输出，例如：

import os
from contextlib import redirect_stdoutstdout_fd = sys.stdout.fileno()
with open('output.txt', 'w') as f, redirect_stdout(f):print('redirected to a file')os.write(stdout_fd, b'not redirected')os.system('echo this also is not redirected')

b'not redirected' and 'echo this also is not redirected' are not redirected to the output.txt file. b'not redirected'和'echo this also is not redirected'未重定向到output.txt文件。

To redirect at the file descriptor level, os.dup2() could be used: 要在文件描述符级别重定向，可以使用os.dup2() ：

import os
import sys
from contextlib import contextmanagerdef fileno(file_or_fd):fd = getattr(file_or_fd, 'fileno', lambda: file_or_fd)()if not isinstance(fd, int):raise ValueError("Expected a file (`.fileno()`) or a file descriptor")return fd@contextmanager
def stdout_redirected(to=os.devnull, stdout=None):if stdout is None:stdout = sys.stdoutstdout_fd = fileno(stdout)# copy stdout_fd before it is overwritten#NOTE: `copied` is inheritable on Windows when duplicating a standard streamwith os.fdopen(os.dup(stdout_fd), 'wb') as copied: stdout.flush()  # flush library buffers that dup2 knows nothing abouttry:os.dup2(fileno(to), stdout_fd)  # $ exec >&toexcept ValueError:  # filenamewith open(to, 'wb') as to_file:os.dup2(to_file.fileno(), stdout_fd)  # $ exec > totry:yield stdout # allow code to be run with the redirected stdoutfinally:# restore stdout to its previous value#NOTE: dup2 makes stdout_fd inheritable unconditionallystdout.flush()os.dup2(copied.fileno(), stdout_fd)  # $ exec >&copied

The same example works now if stdout_redirected() is used instead of redirect_stdout() : 如果使用stdout_redirected()而不是redirect_stdout()则现在可以使用相同的示例：

import os
import sysstdout_fd = sys.stdout.fileno()
with open('output.txt', 'w') as f, stdout_redirected(f):print('redirected to a file')os.write(stdout_fd, b'it is redirected now\n')os.system('echo this is also redirected')
print('this is goes back to stdout')

The output that previously was printed on stdout now goes to output.txt as long as stdout_redirected() context manager is active. 只要上下文管理器stdout_redirected()处于活动状态，以前打印在stdout上的输出现在将进入output.txt 。

Note: stdout.flush() does not flush C stdio buffers on Python 3 where I/O is implemented directly on read() / write() system calls. 注意： stdout.flush()不会刷新Python 3上的C stdio缓冲区，在该Python 3上，I / O是直接在read() / write()系统调用上实现的。 To flush all open C stdio output streams, you could call libc.fflush(None) explicitly if some C extension uses stdio-based I/O: 要刷新所有打开的C stdio输出流，如果某些C扩展使用基于stdio的I / O，则可以显式调用libc.fflush(None) ：

try:import ctypesfrom ctypes.util import find_library
except ImportError:libc = None
else:try:libc = ctypes.cdll.msvcrt # Windowsexcept OSError:libc = ctypes.cdll.LoadLibrary(find_library('c'))def flush(stream):try:libc.fflush(None)stream.flush()except (AttributeError, ValueError, IOError):pass # unsupported

You could use stdout parameter to redirect other streams, not only sys.stdout eg, to merge sys.stderr and sys.stdout : 您可以使用stdout参数重定向其他流，而不仅仅是sys.stdout例如，合并sys.stderr和sys.stdout ：

def merged_stderr_stdout():  # $ exec 2>&1return stdout_redirected(to=sys.stdout, stdout=sys.stderr)

Example: 例：

from __future__ import print_function
import syswith merged_stderr_stdout():print('this is printed on stdout')print('this is also printed on stdout', file=sys.stderr)

Note: stdout_redirected() mixes buffered I/O ( sys.stdout usually) and unbuffered I/O (operations on file descriptors directly). 注意： stdout_redirected()将缓冲的I / O（通常为sys.stdout ）和无缓冲的I / O（直接对文件描述符的操作）混合使用。 Beware, there could be buffering issues . 当心，可能存在缓冲问题。

To answer, your edit: you could use python-daemon to daemonize your script and use logging module (as @erikb85 suggested ) instead of print statements and merely redirecting stdout for your long-running Python script that you run using nohup now. 要回答，请进行编辑：您可以使用python-daemon并使用logging模块（如@ erikb85建议）代替print语句，而仅将stdout重定向为您现在使用nohup运行的长期运行的Python脚本。

#6楼

Based on this answer: https://stackoverflow.com/a/5916874/1060344 , here is another way I figured out which I use in one of my projects. 基于以下答案： https : //stackoverflow.com/a/5916874/1060344 ，这是我想出在我的一个项目中使用的另一种方法。 For whatever you replace sys.stderr or sys.stdout with, you have to make sure that the replacement complies with file interface, especially if this is something you are doing because stderr/stdout are used in some other library that is not under your control. 对于用sys.stderr或sys.stdout替换的任何内容，都必须确保替换符合file接口，尤其是在执行此操作时，因为stderr / stdout用于其他不受您控制的库中。 That library may be using other methods of file object. 该库可能正在使用文件对象的其他方法。

Check out this way where I still let everything go do stderr/stdout (or any file for that matter) and also send the message to a log file using Python's logging facility (but you can really do anything with this): 看看这种方式，我仍然可以让所有事情继续进行stderr / stdout（或与此有关的任何文件），并使用Python的日志记录工具将消息发送到日志文件中（但您实际上可以执行任何操作）：

class FileToLogInterface(file):'''Interface to make sure that everytime anything is written to stderr, it isalso forwarded to a file.'''def __init__(self, *args, **kwargs):if 'cfg' not in kwargs:raise TypeError('argument cfg is required.')else:if not isinstance(kwargs['cfg'], config.Config):raise TypeError('argument cfg should be a valid ''PostSegmentation configuration object i.e. ''postsegmentation.config.Config')self._cfg = kwargs['cfg']kwargs.pop('cfg')self._logger = logging.getlogger('access_log')super(FileToLogInterface, self).__init__(*args, **kwargs)def write(self, msg):super(FileToLogInterface, self).write(msg)self._logger.info(msg)

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