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. https://docs.python.org/3/library/stdtypes.html#object.__dict__
This attribute is a tuple of classes that are considered when looking for base classes during method resolution.