Object-Oriented Programming Fundamentals

Object Orientation

• Object-oriented algorithm simulates activities of several explicit actors, each with specific steps to perform.
• Each actor is free to use any appropriate technique to accomplish its task.
• Combined effort of all these simulated actors results in the goal of the algorithm.

Object Orientation Principles

• Encapsulation: everything for an object is defined/enclosed in a class.
• Inheritance: new objects can be based on existing objects.
• Polymorphism: operations can act on more than one type.

Classes

• A class is a template/blueprint for a new data type.
• The class keyword defines a new class with its heading ending with a colon and body indented.
• A class contains object methods and object variables.
• Object methods define the actions or operations which can be applied to an object.
• Object variables define the data/properties which need to be stored for each object.

Class Example

• A class can be defined with a constructor, object methods, and object variables.
• Example: class Person: ...

Object Methods

• Methods are operations which can be applied to an object.
• Methods for the class are defined in its body and conform to the same rules as functions.
• Object methods have self as the first parameter.
• There are standard methods using the double underscore naming convention, such as __init__(self) and __str__(self).
• User-defined methods can also be defined.

self

• The self keyword refers to the current object instance.
• self needs to be used explicitly when specifying object methods and variables.

init(self)

• The __init__(self) method is called the constructor and is called when a new object is created.
• The constructor is used to initialize the object variables.
• The constructor can use default parameters.

Object Variables

• Object variables, also called instance variables, are used to define the data which needs to be stored for each object.
• Object variables are prefaced with self.
• The constructor __init__(self) is used to initialize the object variables.

str(self)

• The __str__(self) method converts an object to a string and is called when an object is printed out.
• The __str__(self) method typically returns the concatenation of the object variables.

Class Reuse

• Once a class is defined, it can be reused over and over again in different programs.
• A class should be saved in a filename, with only one copy.
• Class variables are defined at the top of the class block without the self keyword.

Class Variables and Methods

• Class methods are defined without self as a parameter.
• Class variables and methods are referenced with the class name.

None

• The None keyword typically defines that a variable has no value.
• A variable can be checked if it is None.

Inheritance

• Inheritance is the process by which a new class is created from another class.
• The original class is called the base/parent/super class.
• The new class is called a derived/child/sub class.
• A derived class automatically inherits all the object variables and methods of the base class.

Inheritance Example

• A derived class can inherit characteristics and behaviors from its base class.
• Example: Children often inherit certain characteristics from their parents.

Multiple Inheritance

• Python allows multiple inheritance, meaning that a base class can inherit from more than one super class.
• Super classes are separated by commas.

Method Overriding

• Subclasses can override methods inherited from their superclasses.