Most Java and C++ programmers that come to Python are shocked about the lack of private/protected modifiers on fields and methods. And it's pretty hard to convince them that it's actually a feature, it doesn't hurt maintainability of large codebase and actually makes you think more about the API of your classes.
Today I learned a small trick (it was available since Java 1.2, I just didn't notice it) that makes this "protection" in Java even more worthless:
Field f = bar.getClass().getDeclaredField("private_value");
f.get(bar) // you can now read
f.set(bar, 3) // and write the private attribute
And it's actually a feature that's used a lot: in unit testing frameworks, in serialization, etc. The only difference is that you have to go all the way through Reflection API which is terribly slow.