Name Mangling example.

name_mangling.py

class Foo(object):
    def __init__(self, **kwargs):
        self.__attr = kwargs.pop('attr', None)

    @property
    def bar(self):
        return self.__attr


def test_foo():
    foo = Foo()
    assert hasattr(foo, '_Foo__attr')
    assert foo._Foo__attr is None


def test_foo_custom_attr():
    foo = Foo(attr='bar')
    assert hasattr(foo, '_Foo__attr')
    assert foo._Foo__attr == 'bar'


def test_foo_bar():
    foo = Foo()
    assert foo.bar is None


def test_foo_bar_custom_attr():
    foo = Foo(attr='bar')
    assert foo.bar == 'bar'


if __name__ == '__main__':
    """
    Name mangling of double underscore attributes.

    tl;dr __attr attributes are class private.

    For more background see http://stackoverflow.com/questions/1301346/the-meaning-of-a-single-and-a-double-underscore-before-an-object-name-in-python 
    """
    test_foo()
    test_foo_custom_attr()
    test_foo_bar()
    test_foo_bar_custom_attr()