Python分别从子进程stdout和stderr读取，同时保留顺序

2022-01-18 00:00:00 python subprocess stdout stderr

问题描述

我有一个 python 子进程，我正在尝试从中读取输出和错误流.目前我有它的工作，但我只能在读完 stdout 之后才能从 stderr 中读取.这是它的样子:

I have a python subprocess that I'm trying to read output and error streams from. Currently I have it working, but I'm only able to read from stderr after I've finished reading from stdout. Here's what it looks like:

process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout_iterator = iter(process.stdout.readline, b"")
stderr_iterator = iter(process.stderr.readline, b"")

for line in stdout_iterator:
    # Do stuff with line
    print line

for line in stderr_iterator:
    # Do stuff with line
    print line

如您所见，在 stdout 循环完成之前，stderr for 循环无法启动.我怎样才能修改它以便能够以正确的行进来的顺序从两者中读取?

As you can see, the stderr for loop can't start until the stdout loop completes. How can I modify this to be able to read from both in the correct order the lines come in?

澄清一下:我仍然需要能够判断一行是来自 stdout 还是 stderr 因为它们在我的代码.

To clarify: I still need to be able to tell whether a line came from stdout or stderr because they will be treated differently in my code.

解决方案

这是一个基于 selectors 的解决方案，但它保留了顺序，并且流式传输可变长度字符(甚至是单个字符).

Here's a solution based on selectors, but one that preserves order, and streams variable-length characters (even single chars).

诀窍是使用 read1()，而不是 read().

import selectors
import subprocess
import sys

p = subprocess.Popen(
    ["python", "random_out.py"], stdout=subprocess.PIPE, stderr=subprocess.PIPE
)

sel = selectors.DefaultSelector()
sel.register(p.stdout, selectors.EVENT_READ)
sel.register(p.stderr, selectors.EVENT_READ)

while True:
    for key, _ in sel.select():
        data = key.fileobj.read1().decode()
        if not data:
            exit()
        if key.fileobj is p.stdout:
            print(data, end="")
        else:
            print(data, end="", file=sys.stderr)

如果你想要一个测试程序，使用这个.

If you want a test program, use this.

import sys
from time import sleep


for i in range(10):
    print(f" x{i} ", file=sys.stderr, end="")
    sleep(0.1)
    print(f" y{i} ", end="")
    sleep(0.1)

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