将子进程输出显示到stdout并将其重定向

Question

我正在通过Python的子流程模块运行脚本。目前我使用：

p = subprocess.Popen('/path/to/script', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
result = p.communicate()

然后将结果打印到标准输出。一切都很好，但是由于脚本需要很长时间才能完成，因此我也希望将脚本实时输出到stdout。我通过管道传输输出的原因是因为我想解析它。

Answer 1

将子流程的标准输出保存到变量以进行进一步处理，并保存到display it while the child process is running as it arrives：

#!/usr/bin/env python3
from io import StringIO
from subprocess import Popen, PIPE

with Popen('/path/to/script', stdout=PIPE, bufsize=1,
           universal_newlines=True) as p, StringIO() as buf:
    for line in p.stdout:
        print(line, end='')
        buf.write(line)
    output = buf.getvalue()
rc = p.returncode

保存子进程的stdout和stderr都比较复杂，因为您应该consume both streams concurrently to avoid a deadlock：

stdout_buf, stderr_buf = StringIO(), StringIO()
rc =  teed_call('/path/to/script', stdout=stdout_buf, stderr=stderr_buf,
                universal_newlines=True)
output = stdout_buf.getvalue()
...

其中teed_call() is define here。

---

更新：这是teed_call()。

---

^旧版本：

这里是基于a simpler asyncio version的单线程解决方案：

asyncio

脚本运行child_process.py example from tulip命令并逐行同时读取其stdout＆stderr。这些行将相应地打印到父级的stdout / stderr，并另存为字节串以供将来处理。要运行child_process.py协程，我们需要一个事件循环：

tulip

Answer 2

import asyncio
import sys
from asyncio.subprocess import PIPE

@asyncio.coroutine
def read_and_display(*cmd):
    """Read cmd's stdout, stderr while displaying them as they arrive."""
    # start process
    process = yield from asyncio.create_subprocess_exec(*cmd,
            stdout=PIPE, stderr=PIPE)

    # read child's stdout/stderr concurrently
    stdout, stderr = [], [] # stderr, stdout buffers
    tasks = {
        asyncio.Task(process.stdout.readline()): (
            stdout, process.stdout, sys.stdout.buffer),
        asyncio.Task(process.stderr.readline()): (
            stderr, process.stderr, sys.stderr.buffer)}
    while tasks:
        done, pending = yield from asyncio.wait(tasks,
                return_when=asyncio.FIRST_COMPLETED)
        assert done
        for future in done:
            buf, stream, display = tasks.pop(future)
            line = future.result()
            if line: # not EOF
                buf.append(line)    # save for later
                display.write(line) # display in terminal
                # schedule to read the next line
                tasks[asyncio.Task(stream.readline())] = buf, stream, display

    # wait for the process to exit
    rc = yield from process.wait()
    return rc, b''.join(stdout), b''.join(stderr)

，然后立即返回其全部输出。

您是否尝试过类似的方法，逐行读取子流程的输出？

'/path/to/script

Answer 3

Popen.communicate文档明确指出：

read_and_display()

import os

if os.name == 'nt':
    loop = asyncio.ProactorEventLoop() # for subprocess' pipes on Windows
    asyncio.set_event_loop(loop)
else:
    loop = asyncio.get_event_loop()
try:
    rc, *output = loop.run_until_complete(read_and_display("/path/to/script"))
    if rc:
        sys.exit("child failed with '{}' exit code".format(rc))
finally:
    loop.close()

因此，如果需要实时输出，则需要使用类似以下的内容：

p.communicate() waits for the subprocess to complete

Answer 4

这会将stdout和stderr都打印到终端，并将stdout和stderr都保存到变量中：

p.communicate()

但是，根据您的实际工作，可能需要注意：通过使用

p = subprocess.Popen('/path/to/script', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
for line in p.stdout:
  # do something with this individual line
  print line

，我们无法再区分stdout和stderr，并且通过调用Note: The data read is buffered in memory, so do not use this method if the data size is large or unlimited. ，您的输出将始终打印到stdout，不管它来自stdout还是stderr。

对于Python <3.7，您将需要使用https://docs.python.org/2/library/subprocess.html#subprocess.Popen.communicate而不是

stream_p = subprocess.Popen('/path/to/script', stdout=subprocess.PIPE, stderr=subprocess.PIPE)

while stream_line in stream_p:
    #Parse it the way you want
    print stream_line

。

版本3.7中的新增功能：已添加文本作为Universal_newlines的易读别名。

来源：

from subprocess import Popen, PIPE, STDOUT

with Popen(command, stdout=PIPE, stderr=STDOUT, text=True, bufsize=1) as p:
    output = "".join([print(buf, end="") or buf for buf in p.stdout])