python tips:类的专有属性
实例通常能够调用类的属性，但是有些属性是类专有的，实例无法调用。
实例调用方法时查找属性时，首先在自己的__dict__中找，找不到去类中找，在类中能够找到的属性都位于dir(cls)中，如果类的某些属性不在dir(cls)中，那么实例无法访问。
1 >>> class A: 2 pass 3 4 >>> A.__name__ # the name of class, function, method, descriptor, generator 5 'A' 6 >>> A.__qualname__ # qualified name (path from global scope to here) 7 'A' 8 >>> A.__mro__ # look for base classed during method resolution 9 (<class '__main__.A'>, <class 'object'>) 10 >>> A.mro() # result is stored in __mro__ 11 [<class '__main__.A'>, <class 'object'>] 12 >>> A.__subclasses__() # immediate subclasses 13 []
