如果右操作数的类型是左操作数类型的子类，并且该子类提供了操作的反射方法，则该方法将在左操作数的非反射方法之前被调用。
If the right operand’s type is a subclass of the left operand’s type and that subclass provides the reflected method for the operation, this method will be called before the left operand’s non-reflected method. This behavior allows subclasses to override their ancestors’ operations.
这样就解释了左右操作的不同，之前自己老是想着自定义的优先
>>> class Nint(int):
def __radd__(self,other):
return int.__sub__(other,self)
>>> a=Nint(5)
>>> b=Nint(3)
>>> 1+b
-2
>>> b+1
4
