import unittest

__private_stuff = 1 # if it would have single underscore, it would not be a problem.

class ComplexTestCase(unittest.TestCase):
    def test_internal_symbol(self):
        self.assertEqual(__private_stuff, 1)

unittest.main(__name__)

E
======================================================================
ERROR: test_internal_symbol (__main__.ComplexTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:filesy.py", line 7, in test_internal_symbol
    self.assertEqual(__private_stuff, 1)
NameError: global name '_ComplexTestCase__private_stuff' is not defined

----------------------------------------------------------------------

import unittest

__private_stuff = 1 # if it would have single underscore, it would bot be a problem.

class ComplexTestCase(unittest.TestCase):
    def test_internal_symbol(self):
        private_stuff = globals()['__private_stuff']
        self.assertEqual(private_stuff, 1)

unittest.main(__name__)

import unittest
import other_module

__private_stuff = 1 # if it would have single underscore, it would bot be a problem.

class ComplexTestCase(unittest.TestCase):
    def test_internal_symbol(self):
        private_stuff = getattr(other_module, '__private_stuff')
        self.assertEqual(private_stuff, 1)

unittest.main(__name__)