考虑一下：

class Base:
  pass

class Derived(Base):
  pass

print(Base.__dict__)
print(Derived.__dict__)

print('__dict__' in Base.__dict__)
print('__dict__' in Derived.__dict__)

输出为：

{'__module__': '__main__','__dict__': <attribute '__dict__' of 'Base' objects>,'__weakref__': <attribute '__weakref__' of 'Base' objects>,'__doc__': None}
{'__module__': '__main__','__doc__': None}
True
False

为什么'__dict__'类缺少'__weakref__'和Derived键？ 这是什么意思/影响？