如何在Python中使用子进程重定向输出?

问题:如何在Python中使用子进程重定向输出?

我在命令行中执行的操作:

cat file1 file2 file3 > myfile

我想用python做什么:

import subprocess, shlex
my_cmd = 'cat file1 file2 file3 > myfile'
args = shlex.split(my_cmd)
subprocess.call(args) # spits the output in the window i call my python program

What I do in the command line:

cat file1 file2 file3 > myfile

What I want to do with python:

import subprocess, shlex
my_cmd = 'cat file1 file2 file3 > myfile'
args = shlex.split(my_cmd)
subprocess.call(args) # spits the output in the window i call my python program

回答 0

更新:不鼓励使用os.system,尽管在Python 3中仍然可用。


用途os.system

os.system(my_cmd)

如果您确实要使用子流程,请使用以下解决方案(大部分从子流程的文档中删除):

p = subprocess.Popen(my_cmd, shell=True)
os.waitpid(p.pid, 0)

OTOH,您可以完全避免系统调用:

import shutil

with open('myfile', 'w') as outfile:
    for infile in ('file1', 'file2', 'file3'):
        shutil.copyfileobj(open(infile), outfile)

UPDATE: os.system is discouraged, albeit still available in Python 3.


Use os.system:

os.system(my_cmd)

If you really want to use subprocess, here’s the solution (mostly lifted from the documentation for subprocess):

p = subprocess.Popen(my_cmd, shell=True)
os.waitpid(p.pid, 0)

OTOH, you can avoid system calls entirely:

import shutil

with open('myfile', 'w') as outfile:
    for infile in ('file1', 'file2', 'file3'):
        shutil.copyfileobj(open(infile), outfile)

回答 1

Python 3.5+中,要重定向输出,只需将参数的打开文件句柄传递stdoutsubprocess.run

# Use a list of args instead of a string
input_files = ['file1', 'file2', 'file3']
my_cmd = ['cat'] + input_files
with open('myfile', "w") as outfile:
    subprocess.run(my_cmd, stdout=outfile)

正如其他人指出的那样,cat为此完全不需要外部命令。

In Python 3.5+ to redirect the output, just pass an open file handle for the stdout argument to subprocess.run:

# Use a list of args instead of a string
input_files = ['file1', 'file2', 'file3']
my_cmd = ['cat'] + input_files
with open('myfile', "w") as outfile:
    subprocess.run(my_cmd, stdout=outfile)

As others have pointed out, the use of an external command like cat for this purpose is completely extraneous.


回答 2

@PoltoS我想加入一些文件,然后处理生成的文件。我以为使用猫是最简单的选择。有更好的/ pythonic的方法吗?

当然:

with open('myfile', 'w') as outfile:
    for infilename in ['file1', 'file2', 'file3']:
        with open(infilename) as infile:
            outfile.write(infile.read())

@PoltoS I want to join some files and then process the resulting file. I thought using cat was the easiest alternative. Is there a better/pythonic way to do it?

Of course:

with open('myfile', 'w') as outfile:
    for infilename in ['file1', 'file2', 'file3']:
        with open(infilename) as infile:
            outfile.write(infile.read())

回答 3

size = 'ffprobe -v error -show_entries format=size -of default=noprint_wrappers=1:nokey=1 dump.mp4 > file'
proc = subprocess.Popen(shlex.split(size), shell=True)
time.sleep(1)
proc.terminate() #proc.kill() modify it by a suggestion
size = ""
with open('file', 'r') as infile:
    for line in infile.readlines():
        size += line.strip()

print(size)
os.remove('file')

当您使用进程时,必须终止该进程。这是一个示例。如果不终止该进程,则文件将为空,并且无法读取任何内容。它可以在Windows上运行。我无法确保它可以在Unix上运行。

size = 'ffprobe -v error -show_entries format=size -of default=noprint_wrappers=1:nokey=1 dump.mp4 > file'
proc = subprocess.Popen(shlex.split(size), shell=True)
time.sleep(1)
proc.terminate() #proc.kill() modify it by a suggestion
size = ""
with open('file', 'r') as infile:
    for line in infile.readlines():
        size += line.strip()

print(size)
os.remove('file')

When you use subprocess , the process must be killed.This is an example.If you don’t kill the process , file will be empty and you can read nothing.It can run on Windows.I can`t make sure that it can run on Unix.


回答 4

一种有趣的情况是通过将类似文件附加到文件来更新文件。这样就不必在此过程中创建一个新文件。在需要附加大文件的情况下,此功能特别有用。这是直接从python使用终端命令行的一种可能性。

import subprocess32 as sub

with open("A.csv","a") as f:
    f.flush()
    sub.Popen(["cat","temp.csv"],stdout=f)

One interesting case would be to update a file by appending similar file to it. Then one would not have to create a new file in the process. It is particularly useful in the case where a large file need to be appended. Here is one possibility using teminal command line directly from python.

import subprocess32 as sub

with open("A.csv","a") as f:
    f.flush()
    sub.Popen(["cat","temp.csv"],stdout=f)