从multiprocessing.Process内部调用线程时，is_alive（）始终返回False。

Question

I'm trying to use a thread inside a process, I would also like to check if the thread is still running from within the process. The problem I'm facing is that when I run the same code inside a process is_alive() called on the thread always returns False .

from threading import Thread
from multiprocessing import Process
import time

class WorkerThread(Thread):
    def run(self):
        i = 0 
        while i < 10:
            print ("worker running..")
            time.sleep(1)
            i += 1

class ProcessClass:
    def run_worker(self, worker):
        self.worker = worker
        self.worker.daemon = True
        self.worker.start()
        i = 0
        while i < 12:
            print (F"Is worker thread alive? {self.worker.is_alive()}")
            time.sleep(1)
            i += 1

worker = WorkerThread()
processclass = ProcessClass()

# Calling directly works as expected
processclass.run_worker(worker)

When I call the method directly is_alive() works as expected, but when calling the method inside a process, it always returns False

# Calling inside a process always returns False
parentProcess = Process(target = processclass.run_worker, args = (worker,))
parentProcess.start()

Answer 1

You should pass the thread class, not an instance, and have run_worker create the instance.

class ProcessClass:
    def run_worker(self, worker):
        self.worker = worker()
        ...

worker = WorkerThread

This way the thread is created in the child process rather than in the parent with a reference passed to the child.

Answer 2

Process es are separate processes ; threads are local to a single process. Even when you fork (the default mechanism behind Process on UNIX-like systems), only the thread that fork ed continues to exist in the new process, no other thread survives the fork (this is why mixing threads and fork is dangerous; if one of those other threads holds a lock at the time of the fork , that lock remains held by a non-existent thread in the child).

If you want the thread to run in the child, create it there (within the function passed as the target to Process ), not in the parent process.