Python子进程在循环中调用时不会输出到文件

import subprocess

with open('output.txt', 'w') as f:
    process = subprocess.Popen(['command'], stdout=f)
    while True:
        # 循环中的其他操作
        f.flush()  # 刷新文件对象

import subprocess

with open('output.txt', 'w') as f:
    process = subprocess.Popen(['command'], stdout=f, universal_newlines=True)
    while True:
        # 循环中的其他操作

import subprocess
import sys

process = subprocess.Popen(['command'], stdout=subprocess.PIPE)
while True:
    # 循环中的其他操作
    sys.stdout.flush()  # 刷新标准输出