Core Python Programming (2nd Edition)

interfacing with components of the RepairShop), or aggregation, encapsulating components that are then
accessed only via defined interfaces, and again, hidden from the client programmer. Continuing our
example, the client programmer may be able to make a SmogCheck request on behalf of the Customer, but
has no ability to interact with the SmogZone part of the RepairShop, which is accessed only via internal
controls of the RepairShop when the smogCheckCar() method is called. Both forms of composition are
supported in Python.
Derivation/Inheritance/Hierarchy
Derivation describes the creation of subclasses, new classes that retain all desired data and behavior of
the existing class type but permit modification or other customization, all without having to modify the
original class definition. Inheritance describes the means by which attributes of a subclass are
"bequeathed from" an ancestor class. From our earlier example, a Mechanic may have more car skill
attributes than a Customer, but individually, each "is a" Person, so it is valid to invoke the talk() method,
which is common to all instances of Person, for either of them. Hierarchy describes multiple
"generations" of derivation which can be depicted graphically as a "family tree," with successive
subclasses having relationships with ancestor classes.
Generalization/Specialization
Generalization describes all the traits a subclass has with its parent and ancestor classes, so subclasses
are considered to have an "is-a" relationship with ancestor classes because a derived object (instance) is
an "example" of an ancestor class. For example, a Mechanic "is a" Person, a Car "is a" Vehicle, etc. In the
family tree diagram we alluded to above, we can draw lines from subclasses to ancestors indicating "isa"
relationships. Specialization is the term that describes all the customization of a subclass, i.e., what
attributes make it differ from its ancestor classes.
Polymorphism
The concept of polymorphism describes how objects can be manipulated and accessed using attributes
and behaviors they have in common without regard to their specific class. Polymorphism indicates the
presence of dynamic (aka late, runtime) binding, allowing for overriding and runtime type determination
and verification.
Introspection/Reflection
Introspection is what gives you, the programmer, the ability to perform an activity such as "manual type
checking." Also called reflection, this property describes how information about a particular object can
be accessed by itself during runtime. Would it not be great to have the ability to take an object passed
to you and be able to find out what it is capable of? This is a powerful feature that you will encounter
frequently in this chapter. The dir() and type() built-in functions would have a very difficult time
working if Python did not support some sort of introspection capability. Keep an eye out for these calls
as well as for special attributes like __dict__, __name__, and __doc__. You may even be familiar with
some of them already!