子进程未在 Windows 上继承 Python 全局变量

Question

I know that child processes won't see changes made after a fork/spawn, and Windows processes don't inherit globals not using shared memory. But what I have is a situation where the children can't see changes to a global variable in shared memory made before the fork/spawn.

Simple demonstration:

from multiprocessing import Process, Value
global foo
foo = Value('i',1)
def printfoo():
  global foo 
  with foo.get_lock():
    print(foo.value)
if __name__ == '__main__':
  with foo.get_lock():
    foo.value = 2
  Process(target=printfoo).start()

On Linux and MacOS, this displays the expected 2. On Windows, it displays 1, even though the modification to the global Value is made before the call to Process . How can I make the change visible to the child process on Windows, too?

Answer 1

The problem here is that your child process creates a new shared value, rather than using the one the parent created. Your parent process needs to explicitly send the Value to the child, for example, as an argument to the target function:

from multiprocessing import Process, Value

def use_shared_value(val):
    val.value = 2

if __name__ == '__main__':
    val = Value('i', 1)
    p = Process(target=use_shared_value, args=(val,))
    p.start()
    p.join()
    print(val.value)

(Unfortunately, I don't have a Windows Python install to test this on.)

---

Child processes cannot inherit globals on Windows, regardless of whether those globals are initialized to multiprocessing.Value instances. multiprocessing.Value does not change the fact that the child re-executes your file, and re-executing the Value construction doesn't use the shared resources the parent allocated.