Popen waiting for child process even when the immediate child has terminated

Question

I'm working with Python 2.7 on Windows 8/XP.

I have a program A that runs another program B using the following code:

p = Popen(["B"], stdout=PIPE, stderr=PIPE)
stdout, stderr = p.communicate()
return

B runs a batch script C. C is a long running script and I want B to exit even though C has not finished. I have done it using the following code (in B):

p = Popen(["C"])
return

When I run B, it works as expected. When I run A however, I expected it to exit when B exits. But A waits until C exits even though B has already exitted. Any ideas on what's happening and what possible solutions could be?

Unfortunately, the obvious solution of changing A to look like B is not an option.

Here is a functional sample code to illustrate this issue: https://www.dropbox.com/s/cbplwjpmydogvu2/popen.zip?dl=1

The zip file consists of the following files with the following contents:

A.py

from subprocess import PIPE, Popen
import sys

def log(line):
    with open("log.txt", "a") as logfile:
        logfile.write(line)

log("\r\n\r\nA: I'll wait for B\r\n")

p = Popen(["C:\\Python27\\python.exe", "B.py"], stdout=PIPE, stderr=PIPE)
stdout, stderr = p.communicate()

log("A: Done.\r\n")
sys.exit(0)

B.py

from subprocess import Popen, PIPE
import sys

def log(line):
    with open("log.txt", "a") as logfile:
        logfile.write(line)

log("B: launching C\r\n")

p = Popen(["C.bat"])

log("B: Not waiting for C at all. bye!\r\n")
sys.exit(0)

C.bat

@echo off
echo C: Start long running task : %time% >>  "log.txt"
ping -n 10 127.0.0.1>nul
echo C: Stop long running task : %time% >>  "log.txt"

Any input is much appreciated.

Answer 1

You could provide start_new_session analog for the C subprocess:

#!/usr/bin/env python import os import sys import platform from subprocess import Popen, PIPE  # set system/version dependent "start_new_session" analogs kwargs = {} if platform.system() == 'Windows':     # from msdn [1]     CREATE_NEW_PROCESS_GROUP = 0x00000200  # note: could get it from subprocess     DETACHED_PROCESS = 0x00000008          # 0x8 | 0x200 == 0x208     kwargs.update(creationflags=DETACHED_PROCESS | CREATE_NEW_PROCESS_GROUP)   elif sys.version_info < (3, 2):  # assume posix     kwargs.update(preexec_fn=os.setsid) else:  # Python 3.2+ and Unix     kwargs.update(start_new_session=True)  p = Popen(["C"], stdin=PIPE, stdout=PIPE, stderr=PIPE, **kwargs) assert not p.poll() 

[1]: Process Creation Flags for CreateProcess()