[Twisted-Python] What's the purpose of __?

[Phil Christensen]

On Feb 15, 2011, at 5:36 AM, Attila Nagy wrote:
> Hi,
> 
> I'm trying to do an LMTP implementation based on smtp.py and came to the 
> issue of class private variables with double underscores.
> Examples:
> http://twistedmatrix.com/trac/browser/trunk/twisted/mail/smtp.py#L746
> http://twistedmatrix.com/trac/browser/trunk/twisted/mail/smtp.py#L815
> and a lot of occurrences in this file.
> 
> This makes me a problem, because I override some functions in a class, 
> named LMTP, so these cannot work together.
> 
> What's the rationale of using these, instead of static names?

I've got no idea of the rationale for using double-underscore notation, but why does that prevent you from subclassing it? Just add the class name to it for access:

class A:
    def __init__(self):
        self.__test = 'test'

a = A()

print a._A__test



-phil