A dictionary or other mapping object used to store an object’s (writable) attributes.
The Python Standard Library » 4. Built-in Types » 4.13. Special Attributes
The implementation adds a few special read-only attributes to several object types, where they are relevant. Some of these are not reported by the dir() built-in function.
This attribute is a tuple of classes that are considered when looking for base classes during method resolution.
This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in __mro__.
Think Python 2e
by Allen B. Downey