How to access instance variable in another process

Question

I would like to access an instance variable in another process, the second process which fetches the self.current value returns None.

This is just a sample program a session is created via the assign_value method, so I need to use that session in the check_value method in a second process.

class Check:
    def __init__(self):
        self.current = None

    def assign_value(self):
        print('Assigning value to current')
        self.current = 'hello'

    def check_value(self):
        time.sleep(1)
        print(f'check from process2 {self.current}')


if __name__ == '__main__':
    sam = Check()
    p1 = Process(target=sam.assign_value)
    p1.start()
    p2 = Process(target=sam.check_value)
    p2.start()

OUTPUT:

Assigning value to current
check from process2 None

I tried Queue, multiprocess manager but none helps

Answer 1

Each new process has its own address space. You need to serialise objects to and from the subprocess. Here's an example:

from multiprocessing import Pool

class Check:
    def __init__(self):
        self._value: str = ""
    @property
    def value(self) -> str:
        return self._value
    @value.setter
    def value(self, _value: str):
        self._value = _value
    def __str__(self) -> str:
        return self.value

def assign(check: Check, value: str) -> Check:
    check.value = value
    return check

if __name__ == "__main__":
    with Pool() as pool:
        check = Check()
        # we have to serialise the Check object as well as the value we want to assign
        check = pool.apply(assign, (check, "Hello world"))
        # the assign() subprocess has serialised the Check object upon return
        # ...so we can access it directly here
        print(check)

Output:

Hello world