Unable to access module attribute with underscores in class method, Python 3

[Joseph Fox-Rabinovitz]

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
same module, but that is not relevant as far as I can tell). The following
bit of code illustrates the situation:

__a = 3
class B:
    def __init__(self):
        global __a
        self.a = __a
b = 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 in a
container whose name is not mangled. For example, doing `self.a =
globals()['__a']` or manually creating a dictionary with a non-mangled name
and accessing that.

I feel that there should be some way of accessing __a within the class
directly in Python 3. Clearly my expectation that global would fix the
issue is incorrect. I would appreciate either a solution or an explanation
of what is going on that would convince me that accessing a module
attribute in such a way should be forbidden.

    -Joseph Fox-Rabinovitz

P.S. For reference, the Stack Overflow question is here:
http://stackoverflow.com/questions/34621484/how-to-access-private-variable-of-python-module-from-class