python在两个导入的类之间传递变量

class ClassA:
    def __init__(self, var):
        self.var = var

class ClassB:
    def __init__(self, instance):
        self.instance = instance

var = "Hello, World!"
instance_a = ClassA(var)
instance_b = ClassB(instance_a)

# module_a.py
var = "Hello, World!"

# module_b.py
from module_a import var

class ClassA:
    def __init__(self):
        self.var = var

class ClassB:
    def __init__(self):
        self.var = var

# module_a.py
class ClassA:
    var = "Hello, World!"

# module_b.py
from module_a import ClassA

class ClassB:
    def __init__(self):
        self.var = ClassA.var