执行打印到标准输出的子进程，等待文本，然后继续执行主脚本

Question

在Python 3中，我如何：

1. 执行一个过程，并将其标准输出打印到标准输出，
2. 监视标准输出以等待一些文本，然后
3. 继续执行我的Python程序，而不会在找到文本后终止进程，也不会停止将其打印到stdout。

[live output from subprocess command不是重复的，因为那里的所有答案都不允许您的Python代码继续运行，并且仍然打印该进程的标准输出。

我想解决方案将涉及某种线程，协程或任务。

Answer 1

一种相对简单的方法是，在完成处理后，启动一个新线程将进程的标准输出复制到Python的标准输出。这是运行webpack-dev-server并从中抓取URL的示例（未实际实现）：

#!/usr/bin/env python3

import subprocess
import sys
import os
from threading import Thread

from typing import IO

def pipe(a: IO[bytes], b: IO[bytes]):
  for x in a:
    b.write(x)
    b.flush()


process = subprocess.Popen(
  [
    "node_modules/.bin/webpack-dev-server",
    # Note that I had to add --colour because for some reason
    # webpack-dev-server detects stdout as supporting colour,
    # but not stderr.
    "--color",
    # This prints a progress bar using ANSI escape characters
    # which works too!
    "--progress",
  ],
  cwd=".",
  env=dict(os.environ, NODE_ENV="development"),
  stdout=subprocess.PIPE,
)

for line in process.stdout:
  sys.stdout.buffer.write(line)
  # Flush is necessary otherwise stdout will buffer more
  # than one line.
  sys.stdout.buffer.flush()

  # Process the text however you want.
  if b"Project is running at" in line:
    print("Server started at address ??")
    break

# Start a thread to do the pipe copying.
thread = Thread(target=pipe, args=(process.stdout, sys.stdout.buffer))
thread.start()

print("Now we can do other stuff and the process will continue to print to stdout")

请注意，我尚未考虑适当的清理操作-退出线程，关闭文件等。