fork will share the current state, spawn will reimport and rebuild everything

python_local_patch_process.py

import os
import multiprocessing as mp


data = {
    "msg": "hello world",
}


class Demo:
    def __repr__(self):
        return f"<{self.__class__.__name__}>"


def foo(obj):
    print(os.getpid(), obj)


def bar(cls):
    print(os.getpid(), cls())


demo = Demo()


def patch_cls():
    Demo.__repr__ = lambda _: "<>"


def patch_instance():
    demo.__repr__ = lambda _: "<>"


def patch():
    data["local"] = "patch"


if __name__ == "__main__":
    ctx = mp.get_context("fork")
    # ctx = mp.get_context("spawn")

    patch()
    print(data)
    p = ctx.Process(target=foo, args=(data,))
    p.start()
    p.join()

    patch_cls()
    print(Demo())
    p = ctx.Process(target=bar, args=(Demo,))
    p.start()
    p.join()

    patch_instance()
    print(demo)
    p = ctx.Process(target=foo, args=(demo,))
    p.start()
    p.join()