The implementation adds a few special read-only attributes to several object types, where they are relevant:
x.__dict__is a dictionary of some sort used to store an object's (writable) attributes;
x.__methods__lists the methods of many built-in object types, e.g.,
['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort'];
x.__members__lists data attributes;
x.__class__is the class to which a class instance belongs;
x.__bases__is the tuple of base classes of a class object.