puzzled about class attribute resolution and mangling

[Brian van den Broek]

James Stroud said unto the world upon 2005-12-09 20:39:
> Brian van den Broek wrote:
> 
>>Hi all,
>>
>>I've the following code snippet that puzzles me:
>>
>>class Base(object):
>>    __v, u = "Base v", "Base u"
>>    def __init__(self):
>>        print self.__v, self.u
>>
>>class Derived(Base):
>>    __v, u = "Derived v", "Derived u"
>>    def __init__(self):
>>        print self.__v, self.u
>>        super(Derived, self).__init__()
>>
>>d = Derived()
>>
>>When run (Python 2.4.2, IDLE 1.1.2), it produces:
>>
>> >>>
>>Derived v Derived u
>>Base v Derived u
>> >>>
>>
>>What I expected was that all four emitted strings would contain "Derived".

<snip me -- Brian -- speculating on locus of my confusion>

>>Thanks and best,
>>
>>Brian vdB
> 
> 
> This is name mangling at work. Mangling turns self.__v in the Derrived class's 
> __init__ method to self._Derrived__v and self.__v in the Base class's __init__ 
> method to self._Base__v. These are different names bound to different values and 
> are reflected as such. self.u is the same name in both cases and the value was 
> bound in the Derrived class, and not re-bound in the Base class.
> 
> James

Thanks for the reply, James. Rereading the relevant section of the
Tutorial:

>> Any identifier of the form __spam (at least two leading
>> underscores, at most one trailing underscore) is textually replaced
>> with _classname__spam, where classname is the current class name
>> with leading underscore(s) stripped.
<http://docs.python.org/tut/node11.html#SECTION0011600000000000000000>

I'm not sure how I came to think that names were mangled only with
respect to calling code from outside class definitions.

As I'd been confused, I naturally started thinking of ways to clarify
the docs. But, I've come to think that the error was wholly mine.

Thanks for the help,

Brian vdB