inherrited private class varriables

[holger krekel]

Uwe Mayer wrote:
> hi,
> 
> why can't i access a class private varriable from within a predecessor?
> f.e.
> 
> class parent:
>   __length = 5
>   def __init__(self): pass
>   def action(self):
>     print self.__class__.__length
> 
> class child(parent):
>   __length = 10
>   def __init__(self): pass
> 
> test = child()
> test.action()
> 
> will print 5, instead of 10. why?

short answer: 

private names are obtained by textual replacement.  
child.__length and parent.__length may look similar 
but the interpreter really sees [*]

    child._child__length  and   parent._parent__length

and refuses to override.  Think 'macro-preprocessor'.
              
long answer from http://www.python.org/doc/1.5/tut/node67.html :

    Any identifier of the form __spam (at least two leading 
    underscores, at most one trailing underscore) is now textually 
    replaced with _classname__spam, where classname is the current 
    class name with leading underscore(s) stripped.  This mangling 
    is done without regard of the syntactic position of the
    identifier, so it can be used to define class-private instance 
    and class variables, methods, as well as globals, and even to 
    store instance variables private to this class on instances 
    of other classes.

regards,

    holger



[*] if you want a proof 'import dis ; dis.dis(parent.action)'