For modules you can access the variable just fine, but for classes you need to use the mangled name:

  $ cat >foo.py
  __FOO = 1
  class Foo:
      __FOO = 2
  $ cat >bar.py
  import foo
  print(foo.__FOO)
  print(foo.Foo()._Foo__FOO)
  $ python3 bar.py 
  1
  2

Nope, you don't need to start the name with underscores.

    class Foo:
        bar = 'public'
        __bar = bar # stashed