CSCA08 Winter 2018 Week 9:Object-Oriented Programming. Marzieh Ahmadzadeh,, Brian Harrington University of Toronto Scarborough

Transcription

1 CSCA08 Winter 2018 Week 9:Object-Oriented Programming Marzieh Ahmadzadeh,, Brian Harrington University of Toronto Scarborough

2 Data Types Up to now you have used Python s built-in data types: primitive data types: int, float and Boolean Non-primitive (object-reference) data types: str, list, tuple, set and dictionary How do you represent data for which no built-in data type is available? 2

3 Objects What is an object? An entity that has some states/attributes and behaviours. Usually represented by a noun and verb respectively. Each object has only one data type. S = { 1, 2, 3} -> data type: Set Each data type can have a number of instances (i.e. objects) S1 = { 1, 2, 3}, S2= { 2, 3, 4} 3

4 Representation : Data Types A data type is represented by a class. A state/attribute is called an instance variable/field/attribute and is represented by a variable of a proper type. A behaviour is shown by a method Instance variables methods OnlineBook _name: str _author: list _no_of_pages: int run_code(str) scroll() take_quiz() Name of the class 4

5 Representation : objects An object is an instance of a class. A monitor is an object and an instance of class ElectronicDevice. my_monior = ElectronicDevice() your_monitor = ElectronicDevice() To access an object s attributes and methods, dot operator is used. Think of an object as a remote control and state and behaviours as the buttons on the remote and pressing the buttons as dot operator. my_set.add(something) my_list.append(something) my_dict.keys() 5

6 Object Oriented Features (1) Using dot operator, your code gets aware of the object you are working with by passing a copy of the object. John.take_course( CSCA08 ) Jane.take_course( MATA08 ) Python Specific: Python requires to have one special input parameter for each and every method called self in order to take the address of the current object. self is used in method definition only (i.e. not for the method call) Behind the scenes: my_obj.method() is really just an alias of Class.method(my_obj) 6

7 Object Oriented Features (2) Each object has its own unique state each instance variable lives inside an object in the memory. Each car has its own name, colour and engine number. Each object may have its unique behaviour. Wait for it until we talk about object relationships! Pen is a data type: Both pencil and ball pen are pen and both can write ( not a unique behaviour) A ball pen cannot be erased with an eraser while a pencil can. (a unique behaviour) 7

8 Constructors A constructor is a special method that is called automatically to initialize the instance variables of an object. While other languages allow more than one constructor, Python allows only one. It is called init Use default values to simulate having more than one constructor in Python. 8

9 str Python specific: a method that is called automatically when you print the object. You can implement it as you wish and still it is called when you print the object. 9

10 Conventions Naming: Use of Camel Case for class name Use of lower case latter and underscore for methods and instance variables. External Documentation: No example is required. Add class description below class definition 10

11 Memory Model hercar = Car() hiscar = Car() 2 reference variable 2 objects hiscar = hercar 2 reference variable 1 object [reachable] 1 object [unreachabe, subject to GC] hercar = None 1 active reference variable 1 null reference variable 1 object [reachable] her = Student() him = Student() 2 reference variable 2 objects him = None 1 active reference variable 1 null reference variable 1 object [reachable] 1 object [unreachabe, subject to GC] 11