如何检查类是否已在Python中实例化？

class MyClass:
    pass

obj = MyClass()

if isinstance(obj, MyClass):
    print("obj是MyClass的实例")
else:
    print("obj不是MyClass的实例")

class MyClass1:
    pass

class MyClass2:
    pass

obj = MyClass1()

if isinstance(obj, (MyClass1, MyClass2)):
    print("obj是MyClass1或MyClass2的实例")
else:
    print("obj不是MyClass1或MyClass2的实例")

class MyClass:
    pass

obj = MyClass()

if type(obj) == MyClass:
    print("obj是MyClass的实例")
else:
    print("obj不是MyClass的实例")