如何在Python中的不同类之间共享实例变量

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

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

a = ClassA(10)
b = ClassB(a)
print(b.instance.var)  # 输出 10

class ClassA:
    var = 10

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

print(ClassB.instance.var)  # 输出 10

var = 10

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

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

print(ClassB.instance.var)  # 输出 10