为什么我的实例变量不在__dict__中？

class MyClass:
    my_attribute = "Hello, world!"

    def __init__(self):
        self.dynamic_attribute = "I am dynamic!"

my_instance = MyClass()

print(MyClass.__dict__)
print(my_instance.__dict__)

{'my_attribute': 'Hello, world!', '__init__':<function MyClass.__init__ at 0x7f9a8c034e60>, '__module__': '__main__', '__dict__':<attribute '__dict__' of 'MyClass' objects>, '__weakref__':<attribute '__weakref__' of 'MyClass' objects>, '__doc__': None}
{'dynamic_attribute': 'I am dynamic!'}