python - 为什么缓冲来自子进程的标准输出(重定向到无缓冲文件)？

Question

来自 http://docs.python.org/library/functions.html#open

The optional bufsize argument specifies the file’s desired buffer size: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of (approximately) that size. A negative bufsize means to use the system default, which is usually line buffered for tty devices and fully buffered for other files. If omitted, the system default is used.

我在下面将 0 作为 bufsize 传递，但没有使用 flush()，当我运行 main_process 时没有输出写入文件。
这是什么原因？

# --------------------------------- sub_process.py
import sys
import time

if __name__ == '__main__':
    print 'printed from redirect.py'
    # why is the following flush() needed? 'std-output' is (?) unbuffered...
    sys.stdout.flush() 
    time.sleep(6)


# --------------------------------- main_process.py
import subprocess
import time

if __name__ == '__main__':
    p = subprocess.Popen(
        ['python', 'sub_process.py'],
        stdout=open('std-output', 'w', 0))
    time.sleep(3)
    p.terminate()

Answer 1

使用带有 -u 标志的 python，例如:

if __name__ == '__main__':
    p = subprocess.Popen(
        ['python', '-u', 'sub_process.py'],
        stdout=open('std-output', 'w'))
    time.sleep(3)
    p.terminate()