It makes it nearly impossible to reference that attribute by accident. Basically, if class Foo defines "self.__bar", the Python interpreter sees the leading "__" and mangles it. It's the lightweight equivalent of a private attribute in other languages, with the difference that Python will let you attempt to access it anyway. But if you do, and it makes your code utterly brittle and prone to explosion, no one will give you sympathy or help you fix it. They're much more likely to tell you not to do that.