An important thing to keep in mind with Python is when different bits of code are executed, along with the fact that class and def statements are executed when seen, not just stored away for later. With a def it’s a little easier to understand because the only thing being executed is whatever is between the ()s — so in your code above
def __init__( SELF, name = MyClass.__DefaultName ):
when Python sees MyClass.__DefaultName it tries to find it first in the local namespace, then the module (aka global) namespace, and finally in builtins.
What is the local namespace when executing a class statement? That’s the fun part — it is what will become the class namespace, but at the moment it is anonymous — so you can’t reference it by name (aka MyClass in your code) but you can directly reference anything that has already been defined… and what has been defined? In your class example only one thing — __DefaultName. So your __init__ should look like this:
def __init__( SELF, name=__DefaultName ):
Keep in mind that you can’t change MyClass._MyClass__DefaultName later and have those changes show up in new instances. Why? Because the __init__ def is executed only once, and whatever value __DefaultName had at that moment has been stored away and will always be used — unless you directly specify a new value when you create a new instance:
my_instance = MyClass(name="new name")