Learning Python

Inheritance Searches Namespace Trees

The whole point of a namespace tool like the class statement is to support name inheritance. In Python, inheritance happens when an object is qualified, and involves searching an attribute definition tree (one or more namespaces). Every time you use an expression of the form object.attr where object is an instance or class object, Python searches the namespace tree at and above object, for the first attr it can find. Because lower definitions in the tree override higher ones, inheritance forms the basis of specialization.

Attribute Tree Construction

Figure 6.4 sketches the way namespace trees are constructed. In general:

• Instance attributes are generated by assignments to self attributes in methods.

• Class attributes are created by statements (assignments) in class statements.

• Superclass links are made by listing classes in parentheses in a class statement header.

The net result is a tree of attribute namespaces, which grows from an instance, to the class it was generated from, to all the superclasses listed in the class headers. Python searches upward in this tree from instances to superclasses, each time you use qualification to fetch an attribute name from an instance object.^[46]

Figure 6-4. Namespaces tree construction and inheritance

Specializing Inherited Methods

The tree-searching model of inheritance we just described turns out to be a great ...