多进程传递类实例后类属性重置为默认值的问题

我创建了一个用于全局存储项目数据的Data类，通过类方法设置root_path，但在多进程的Runner子进程中访问时，root_path始终为空字符串。

Data类代码 #

class Data:
    """This class stores project-related constants as class attributes for easy access throughout the project."""
    ORG_URL: str = "https://www.myurl.com/"
    PROJECT_NAME: str = "My Project"
    root_path: str = ""

    @classmethod
    def set_root_path(cls, new_path):
        cls.root_path = new_path

Main类代码 #

from multiprocessing import Process, Manager
import sys, os

class Main:
    def __init__(self) -> None:
        # determine if application is a script file or frozen exe
        if getattr(sys, 'frozen', False):
            application_path = os.path.dirname(sys.executable)
        elif __file__:
            application_path = os.path.dirname(__file__)

        data = Data()
        data.set_root_path(f"{application_path}")
        

        with Manager() as manager:
            q = manager.Queue()
            args = manager.Queue()
            print(data.root_path)                          ## prints correct path
            gui = Process(target=GUI,args=(q,args,data))
            runner = Process(target=Runner,args=(q,args,data))
            gui.start()
            runner.start()
            gui.join()
            runner.join()

if __name__ == "__main__":
    Main()

Runner类代码 #

class Runner:
    def __init__(self,q,args,data) -> None:
        print("Initiating Runner")
        self.q = q
        self.args = args
        self.data = data
        print(self.data.root_path)  # prints ''

Python多进程采用复制内存空间的方式创建子进程，复制操作在子进程启动时完成。主进程中修改Data类的root_path类属性后才启动子进程，但子进程内的Data类是复制的原始版本（root_path为空），类属性的修改无法跨进程同步。同时，传递给子进程的data实例也是一个独立副本，和主进程的实例没有关联。

提供三种可行的解决方式：

方法1：使用Manager共享命名空间 #

利用multiprocessing.Manager创建可跨进程共享的命名空间，存储需要共享的属性：

from multiprocessing import Process, Manager
import sys, os

class Data:
    """This class stores project-related constants as class attributes for easy access throughout the project."""
    ORG_URL: str = "https://www.myurl.com/"
    PROJECT_NAME: str = "My Project"

    @classmethod
    def init_shared_data(cls, shared_data):
        shared_data.ORG_URL = cls.ORG_URL
        shared_data.PROJECT_NAME = cls.PROJECT_NAME

class Main:
    def __init__(self) -> None:
        # determine if application is a script file or frozen exe
        if getattr(sys, 'frozen', False):
            application_path = os.path.dirname(sys.executable)
        elif __file__:
            application_path = os.path.dirname(__file__)

        with Manager() as manager:
            # 创建共享命名空间
            shared_data = manager.Namespace()
            Data.init_shared_data(shared_data)
            shared_data.root_path = application_path

            q = manager.Queue()
            args = manager.Queue()
            print(shared_data.root_path)
            gui = Process(target=GUI, args=(q, args, shared_data))
            runner = Process(target=Runner, args=(q, args, shared_data))
            gui.start()
            runner.start()
            gui.join()
            runner.join()

class Runner:
    def __init__(self,q,args,shared_data) -> None:
        print("Initiating Runner")
        self.q = q
        self.args = args
        self.shared_data = shared_data
        print(self.shared_data.root_path)  # 输出正确路径

方法2：直接传递目标参数 #

如果仅需共享root_path，可以直接将路径作为参数传入子进程，无需传递整个Data实例：

# Main类中修改子进程启动代码
gui = Process(target=GUI, args=(q, args, application_path))
runner = Process(target=Runner, args=(q, args, application_path))

# Runner类修改
class Runner:
    def __init__(self,q,args,root_path) -> None:
        print("Initiating Runner")
        self.q = q
        self.args = args
        self.root_path = root_path
        print(self.root_path)  # 输出正确路径

方法3：通过环境变量传递 #

将root_path设置为主进程的环境变量，子进程会自动继承主进程的环境变量：

# Main类中添加环境变量设置
import os
os.environ["PROJECT_ROOT_PATH"] = application_path

# Runner类中读取环境变量
class Runner:
    def __init__(self,q,args,data) -> None:
        import os
        print("Initiating Runner")
        self.q = q
        self.args = args
        self.root_path = os.environ.get("PROJECT_ROOT_PATH", "")
        print(self.root_path)  # 输出正确路径

内容的提问来源于stack exchange，提问作者Tester