__* name mangling documentation

[Emile van Sebille]

"Eric Brunel" <eric.brunel at pragmadev.com> wrote in message
news:a7fcb1$1m9h$1 at norfair.nerim.net...
> Michel Pelletier wrote:
> > http://www.python.org/doc/current/ref/id-classes.html
> >
> > sort of left me hanging on why __ name mangling exists and why it
would be
> > used.

I think the question may want to be 'why does it still exist?'  Possibly
the answer is backwards compatibility.

>
> Simple: private attributes and methods:
>


>>> class C:
...     def __init__(self, val):
...             self.__priv = val
...
>>> c = C(2)
>>> c._C__priv    # they're not hidden...(if you know the rule)
2


<snip>
> I can't figure out what could be "tricky" about private attributes or
> methods: they can only be accessed from inside the class's instances,
and
> that's all there is about them... Can you provide an example of what
you
> mean by "tricky"?
>

More specifically, they're known as class-private variables and are
(normally) accessible only to the class's instance when accessed from
within the defining class :
>>> class D(C):
...     def test(self):
...             print 'my private val is ',self.__priv
...
>>> d = D(3)
>>> d.test()
my private val is
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in test
AttributeError: D instance has no attribute '_D__priv'
>>>

 http://aspn.activestate.com/ASPN/Mail/Message/886237 discusses other
problems with mangling.

--

Emile van Sebille
emile at fenx.com

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