I want to access a class member from derived classes, but not outside of the class hierarchy.
I know private access, (adding 2 underscores(_) to the variable/function name).
How can declare a class member as protected in a python class ?
I use Python 2.5 and Python 3.1

You can't. Python doesn't have private, public, or protected classes like Java or other languages. That's why the underscores are used. You can simulate protected classes with a single underscore and private with double, but it doesn't actually prevent direct calling like Java.

Thank you, I understand something. But I don't mean private or protected classes,
I ask, how to declare a class member(variable or function/method) as protected ?
If I didn't understand wrong, it's possible to implement it with single underscore. True ?

Yes, you can use underscores to simulate protected members. However, there is nothing within the Python language that enforces it, i.e. someone can still use the member "in a bad way" by calling the member directly w/ the underscore.

The underscore essentially hides the member from inadvertent use, but if the "true" name is known, it can still be used. There is no way to prevent the member from being misused like in Java or C++; it's more like the honor system.