Introduction to OOP in Python
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This document provides an introduction to object oriented programming in Python. It discusses why OOP is useful, defines some key concepts like classes, objects, methods, and variables. It provides an example of modeling a Taxi using a Taxi class with attributes like driver name and methods like pickUpPassenger. It shows how to define a class, create objects from the class, and call methods on those objects. It also introduces the concept of class variables that are shared among all objects versus instance variables that are unique to each object.
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Introduction to OOP in Python
- 1. Introduction to Object Oriented Programming in Python Aleksander Fabijan
- 2. Why OOP (from last time)? • Often we describe real-life objects in code (which is easier with OOP), • Code get’s more manageable, • With OPP, it is easy to reuse previously written code, • … • Every bigger real-life project will be written in OOP.
- 3. Object oriented programming • Object in Python is a representation of a person, a place, a bank account, a car, or any item that the program should handle. • Object Oriented Programming Recipe: • (1) define classes (these are descriptions) • (2) make object instances out of classes. Class Car Car instances
- 5. OOP with a Taxi Example • To learn OOP, we will use an example of a Taxi.
- 6. Example Object - Taxi DATA • DriverName • OnDuty • NumPassenger • Cities BEHAVIOR • PickUpPassenger • DropOffPassenger • SetDriverName • GetDriverName Every object has two main components: Data (the attributes about it) Behavior (the methods)
- 7. • DriverName: string • OnDuty: Boolean • NumPassenger: int • Cities:list Taxi • PickUpPassenger():int • DropOffPassenger(): int • SetDriverName(string) • GetDriverName:string
- 8. Creating a simple class in Python • In a class, we describe (1) how the object will look like, and (2) what behavior it will have. • Classes are the blueprints for a new data type in your program! • A class should have at minimum*: • A name (e.g. Class Taxi) • A constructor method (that describes how to create a new object, __init__)
- 10. The __init__ method • __init__ is a special method in Python classes, • The __init__ method is the constructor method for a class • __init__ is called when ever an object of the class is constructed. Programmer makes a new object __init__ method is called Behaviour within the __init__ method executes myTaxi = Taxi(“Aleks”) … Self.taxiDriver = “Aleks” …
- 11. Example Object - Taxi DATA • DriverName • OnDuty • NumPassenger • Cities
- 12. Creating an object from the class • From one class, you make objects (instances).
- 13. Object Creation Flow • To create a new object, use the class name
- 14. Object Creation Flow • To create a new object, use the class name • When you create a new object, the __init__ method from the class is called with the parameters that were passed.
- 15. Object Creation Flow • The __init__method Is called
- 16. Example Object - Taxi DATA • DriverName • OnDuty • NumPassenger • Cities BEHAVIOR • PickUpPassenger • DropOffPassenger • SetDriverName • GetDriverName
- 17. Behavior of a class • classes/objects can have methods just like functions except that they have an extra self variable at the beginning. • An object method takes as the first parameter the object (self) and can accept any number of other parameters.
- 18. Example Object Method This method changes the name of the taxi driver for the passed object (self).
- 19. Another example of an object method
- 20. Exercise: Write a class that describes a Bus. • A bus is created with a number of seats, a color, and is driven by a bus driver that has a name. New passengers can get in the bus, and existing bus passengers can leave their seat and get of the bus. Number of buss passengers can’t be smaller than 0. Instructions • Start by drawing a class diagram • Create a class for the bus in python • Create two objects of your class
- 21. Object vs. Class Variables • Most variables are object specific (for example, the variable number of passengers in a taxi is different for every taxi that we create). • Some variables are common to all objects (for example, if we want to count the number of taxis that we have in our company) T1 T2 T7 Class Taxi -> number_of_taxi_vehicles = 7
- 22. Accessing class variables • To access a class variable within a method, we use the @classmethod decorator, and pass the class to the method.
- 23. Example use of class variable
- 24. Exercise2: Update your class Bus. • As an owner of the bus company, I wish to keep track of the number of busses that people buy (create). Instructions • Create a class variable • Increment a class variable on __init__ • Create a @classmethod to print its value
- 25. Exercise 3: Hippo ZOO • A local ZOO keeper wants to model his collection of Hippos in the Zoo. • Every hippo has a name and size (in kg). In the morning, a zoo keeper feeds the hypo so his weight increases. During the day the Hippo exercises, therefore his weight goes down. • A zoo keeper also needs a way to keep track how many Hippos he has in the ZOO. • Model this problem (1) with a Class diagram & (2) in Python OOP code.
- 26. Example Solution
- 27. Takeaways • Today, we learned how to create simple classes and objects. • A class is a blueprint for objects, and it contains attributes and behavior. • We added to our classes a number of: • instance methods & instance variables (for example, color, size, etc.). • class methods & class variables (for exmaple, a number of taxis). • We learned that new objects are created from a class through the __init__ method.
- 28. For Next time In our next lecture, we will learn how to reuse the code that we have written today while describing other objects. We will learn about: • Inheritance. (TransportVehicle – Taxi) • Encapsulation (e.g. private functions) • Polymorphism (defining the same function for different types of objects)