在Python 3.2的文档中，它说;
如果省略第二个参数，则返回的超级对象是未绑定的
根据我的理解，未绑定（如'unbound-to-instance'）对象是从super（class，class）返回的对象。那么，超级（类）中的'未绑定'是什么意思？你怎么绑它？
class Base:
def printme(self): print("I'm base")
class Derived(Base):
def printme(self): print("I'm derived")
class Derived2(Derived):
def printme(self):
super().printme()
# next-in-mro bound-to-self method
super(Derived, self).printme()
# beyond-Derived-in-mro bound-to-self method
super(Derived, Derived2).printme(self)
# beyond-Derived-in-mro-of-Derived2 unbound method
super(Derived2).printme
# WTF is this? There's not even a printme attribute in it
Derived2().printme()