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From: Joseph Fox-Rabinovitz <jfoxrabinovitz@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: Unable to access module attribute with underscores in class method, Python 3
Date: Thu, 7 Jan 2016 12:33:25 -0500
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> On Thu, Jan 7, 2016 at 11:14 AM, Joseph Fox-Rabinovitz <jfoxrabinovitz@gm=
ail.com> wrote:
>
> Hi,
>
> I have a module attribute whose name starts with a pair of underscores. I=
 am apparently unable to access it directly in a class method (within the s=
ame module, but that is not relevant as far as I can tell). The following b=
it of code illustrates the situation:
>
> __a =3D 3
> class B:
>     def __init__(self):
>         global __a
>         self.a =3D __a
> b =3D B()
>
> This results in a NameError because of name-mangling, despite the global =
declaration:
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 4, in __init__
> NameError: name '_B__a' is not defined
>
> Not using global does not make a difference. I posted a similar question =
on Stack Overflow, where the only reasonable answer given was to wrap __a i=
n a container whose name is not mangled. For example, doing `self.a =3D glo=
bals()['__a']` or manually creating a dictionary with a non-mangled name an=
d accessing that.
>
> I feel that there should be some way of accessing __a within the class di=
rectly in Python 3. Clearly my expectation that global would fix the issue =
is incorrect. I would appreciate either a solution or an explanation of wha=
t is going on that would convince me that accessing a module attribute in s=
uch a way should be forbidden.
>
>     -Joseph Fox-Rabinovitz
>
> P.S. For reference, the Stack Overflow question is here: http://stackover=
flow.com/questions/34621484/how-to-access-private-variable-of-python-module=
-from-class

One more detail that makes me think that name mangling may be getting
greedy to the point of bugginess:

__a =3D 3
class B:
    def __init__(self):
        m =3D sys.modules[__name__]
        self.a =3D m.__a
b =3D B()

Raises the same exception as all the other way I tried to access __a:
'module' object has no attribute '_B__a'!

   -Joseph