Python subprocess.stdout.readline() 挂起 [英] Python subprocess.stdout.readline() hang

问题描述

我在 python 上使用 subprocess 尝试简单的代码，同时调用 process.stdout.readline() 它会挂在它上面.如何解决这个问题呢?在输出中尝试 fcntl 用于非阻塞时，我得到了空白输出.

I try simple code using subprocess on python, while calling process.stdout.readline() it will hang on it. How to solve this problem? While trying fcntl for nonblock in the output I got blank output.

我的源代码:

from subprocess import Popen,PIPE 
import fcntl
import os

proc = Popen(['./test'],stdin=PIPE,stdout=PIPE) 

fcntl.fcntl(proc.stdout.fileno(), fcntl.F_SETFL, os.O_NONBLOCK)

print "Started!"

print proc.stdout.readline()

proc.stdin.write('5\n')
proc.stdin.flush()

print proc.stdout.readline()

proc.stdin.write('5\n')
proc.stdin.flush()


print proc.stdout.readline()
proc.stdout.close()
proc.stdin.close()

输出应该是这样的:

First number: 5
Second number: 5
Answer: 10

推荐答案

您想要的输出表明 First number: 之后没有换行符，因此阻塞 proc.stdout.readline() 调用(在 proc.stdin.write('5\n') 行之前)挂起并且非阻塞变体不返回任何内容(空字节串)——没有换行符/EOF还没回来.

Your desired output suggests that there is no newline after First number: and therefore the blocking proc.stdout.readline() call (before proc.stdin.write('5\n') line) hangs and non-blocking variant returns nothing (empty bytestring) -- there is no newline/EOF to return yet.

要修复它，您可以在提示的末尾(在冒号 ':' 处)停止阅读.请参阅子进程 readline 挂起等待 EOF

To fix it, you could stop reading at the end of the prompt (at the colon ':'). See subprocess readline hangs waiting for EOF

即使提示后有换行符；readline() 可能由于缓冲问题而挂起.参见 Python C 程序子进程在for line in iter"处挂起

Even if there were a newline after the prompt; readline() may hang due to buffering issues. See Python C program subprocess hangs at "for line in iter"

例如，如果 test 等价于:

For example, if test is equivalent to:

#!/usr/bin/env python3
a = int(input('First number: '))
b = int(input('Second number: '))
print('Answer:', a + b)

然后要得到答案，您可以使用pexpect 模块::>

then to get the answer, you could use pexpect module:

#!/usr/bin/env python
import pexpect # $ pip install pexpect

def print_result(locals):
    result = int(locals['child'].after.rpartition(b' ')[2])
    print(result)

pexpect.run('./test', events={'First number': '5\n', 'Second number': '5\n',
                              r'Answer:\s*\d+': print_result})

如果你想同时看到输入和输出；您可以设置 logfile=sys.stdout 参数.

If you want to see both input and output; you could set logfile=sys.stdout parameter.

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