multiprocessing.Process.is_alive() returns True although process has finished, why?

Question

I use multiprocess.Process to create a child process and then call os.wait4 until child exists. When the actual child process finishes, multiprocess.Process.is_alive() still returns True. That's contradicting. Why?

Code:

from multiprocessing import Process
import os, sys

proc = Process(target=os.system, args= ("sleep 2", ))
proc.start()

print "is_alive()", proc.is_alive()
ret = os.wait4(proc.pid, 0)
procPid, procStatus, procRes = ret
print "wait4 = ", ret

## Puzzled!
print "----Puzzled below----"
print "is_alive()", proc.is_alive()
if os.WIFEXITED(procStatus):
    print "exit with status", os.WEXITSTATUS(procStatus)
print "is_alive()", proc.is_alive()
sys.exit(1)

Output:

is_alive() True
wait4 =  (11137, 0, resource.struct_rusage(ru_utime=0.0028959999999999997, ru_stime=0.003189, ru_maxrss=1363968, ru_ixrss=0, ru_idrss=0, ru_isrss=0, ru_minflt=818, ru_majflt=0, ru_nswap=0, ru_inblock=0, ru_oublock=0, ru_msgsnd=0, ru_msgrcv=0, ru_nsignals=0, ru_nvcsw=1, ru_nivcsw=9))
----Puzzled below----
is_alive() True
exit with status 0
is_alive() True

My question is about the last three output lines. Why is_alive() return True when the actual process is finished. How can that happen?

Answer 1

You should use Process.join instead of os.wait4.

• Process.is_alive calls waitpid internally through multiprocessing.forking.Popen.poll.
• If you call os.wait4, waitpid raises os.error which cause the poll() to return None. (http://hg.python.org/cpython/file/c167ab1c49c9/Lib/multiprocessing/forking.py#l141)
• is_alive() use that return value to determine whether the process is alive. (http://hg.python.org/cpython/file/c167ab1c49c9/Lib/multiprocessing/process.py#l159)
  • => return True.

http://asciinema.org/a/5901

---

Replace following line:

ret = os.wait4(proc.pid, 0)

with:

proc.join()