Python中从管道捕获的标准输出被截断

Question

我想在Ubuntu 10.04上用Python 2.6.5来获取dpkg --list | grep linux-image的输出。

from subprocess import Popen 
from subprocess import PIPE

p1 = Popen(["dpkg", "--list"], stdout=PIPE)
p2 = Popen(["grep", "linux-image"], stdin=p1.stdout, stdout=PIPE)
stdout = p2.communicate()[0]

标准输出的内容是：

>>> print stdout
rc  linux-image-2. 2.6.31-14.48   Linux kernel image for version 2.6.31 on x86
ii  linux-image-2. 2.6.32-22.36   Linux kernel image for version 2.6.32 on x86
ii  linux-image-2. 2.6.32-23.37   Linux kernel image for version 2.6.32 on x86
ii  linux-image-2. 2.6.32-24.43   Linux kernel image for version 2.6.32 on x86
ii  linux-image-2. 2.6.32-25.44   Linux kernel image for version 2.6.32 on x86
ii  linux-image-ge 2.6.32.25.27   Generic Linux kernel image

不过，这和在命令行里运行dpkg --list | grep linux-image的结果不一样：

cschol@blabla:~$ dpkg --list | grep linux-image
rc  linux-image-2.6.31-14-generic         2.6.31-14.48                                    Linux kernel image for version 2.6.31 on x86
ii  linux-image-2.6.32-22-generic         2.6.32-22.36                                    Linux kernel image for version 2.6.32 on x86
ii  linux-image-2.6.32-23-generic         2.6.32-23.37                                    Linux kernel image for version 2.6.32 on x86
ii  linux-image-2.6.32-24-generic         2.6.32-24.43                                    Linux kernel image for version 2.6.32 on x86
ii  linux-image-2.6.32-25-generic         2.6.32-25.44                                    Linux kernel image for version 2.6.32 on x86
ii  linux-image-generic                   2.6.32.25.27                                    Generic Linux kernel image

看第一行，你会发现Python里的输出被截断了：

rc  linux-image-2. 2.6.31-14.48

而不是

rc  linux-image-2.6.31-14-generic         2.6.31-14.48

为什么会这样？有没有办法在Python中得到完全一样的输出呢？

Answer 1

其实根本不需要用grep！

import subprocess
p1 = subprocess.Popen(["dpkg", "--list"], stdout=subprocess.PIPE, env={'LANG':'C'})
out,err=p1.communicate()
for o in  out.split("\n"):
    if "linux-image" in o:
        print o

Answer 2

import subprocess
p1 = subprocess.Popen(["dpkg", "--list"], stdout=subprocess.PIPE, env={'LANG':'C'})
p2 = subprocess.Popen(["grep", "linux-image"], stdin=p1.stdout, stdout=subprocess.PIPE)
out,err=p2.communicate()
print(out)

dpkg这个命令的输出结果是跟环境变量LANG的值有关的。如果在subprocess.Popen中设置LANG=C，那么dpkg的输出就会更像你在终端看到的那样。