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。

 >>> class A:
     pass

 >>> A.__name__ # the name of class, function, method, descriptor, generator
 'A'
 >>> A.__qualname__ # qualified name (path from global scope to here)
 'A'
 >>> A.__mro__       # look for base classed during method resolution
 (<class '__main__.A'>, <class 'object'>)
 >>> A.mro()            # result is stored in __mro__
 [<class '__main__.A'>, <class 'object'>]
 >>> A.__subclasses__()  # immediate subclasses
 []