Object oriented programming

Transcription

1 Object oriented programming In object oriented programming programs are built of classes that describe objects or concepts which we normally can associate to in the real world. It could be persons, buildings, storms or anything. Person A Class Person name (name) dob (day of birth) country (country of residence) Classes have attribute(s) that describe properties or the state of an object move(country) (change country) Classes have method(s) that describe behavior or change the state of an object

2 Classes and objects The class definition Person describes the attributes and the methods of a Person object. The class definition is not a Person object (an instance of a Person). To create a Person object (an instance of the Person class) you must call the Person class. Class definition - Person Person objects Sven Svensson Lloyd

3 Class definition A class definition is used to create a class in Python class Person name (name) dob (day of birth) country (country of residence) Constructor The attributes of the class move(country) (change country)

4 Constructor - def init () The first method in a class is the constructor. The constructor creates an instance of an object and assigns values to the attributes. Note that the first argument is self. This is used by convention to indicate the reference to the current instance of the class and to indicate the attributes arguments constructor attributes

5 Create an object To create a Person object you call the Person class as: my_person = Person( Sven Svensson, , Sweden ) No value given for self. It is the object itself Sven Svensson, , Sweden

6 Create an object my_person = Person( Sven Svensson, , Sweden ) The attribute name will be Sven Svensson Person name (name) dob (day of birth) country (country of residence) Sven Svensson, , Sweden move(country) (change country)

7 Create an object my_person = Person( Sven Svensson, , Sweden ) The attribute dob will be Person name (name) dob (day of birth) country (country of residence) Sven Svensson, , Sweden move(country) (change country)

8 Create an object my_person = Person( Sven Svensson, , Sweden ) The attribute country will be Sweden Person name (name) dob (day of birth) country (country of residence) Sven Svensson, , Sweden move(country) (change country)

9 Add methods to the class Here the method move() is added to change country of residence for a Person object. Note that a method is created with the keyword def as functions. Person name (name) dob (day of birth) country (country of residence) def move(self, new_country): self.country = new_country move(country) (change country) The method move

10 Add methods to the class Note that the method also has self as first argument self as first argument def move(self, new_country): self.country = new_country

11 Add methods to the class The method changes the value of the attribute self.country The method move changes the value of the attribute self.country def move(self, new_country): self.country = new_country

12 Calling the move() method my_person = Person( Sven Svensson, , Sweden ) my_person.move( Denmark ) After this method call the attribute self.country will have the value Denmark Sven Svensson, , Denmark def move(self, new_country): self.country = new_country

13 Printing a class object To make nice printouts of objects you can add the function str (self) This function is called by the print statement Note that you need return str (self) def str (self): return 'Name: {}, Day-of-birth: {}\ncurrently living in {}'.format( self.name, self.dob, self.country) def move(self, new_country): self.country = new_country

14 Printing a class object print my_person Name: Sven Svensson, Day-of-birth: Currently living in Denmark str (self) def str (self): return 'Name: {}, Day-of-birth: {}\ncurrently living in {}'.format( self.name, self.dob, self.country) def move(self, new_country): self.country = new_country

15 Inheritance You can create subclasses that inherits attributes and methods from a superclass. Superclass Person Person name (name) dob (day of birth) country (country of residence) Student(Person) university department Subclass Student move(country) str (change country)

16 Inheritance The class Student will have the attributes from the class Person, and also the additional attributes of the Student class. Superclass Person Person name (name) dob (day of birth) country (country of residence) Student(Person) university department Subclass Student move(country) str (change country)

17 Inheritance - constructor You put a Person object into the Student class definition class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department

18 Inheritance - constructor All the attributes of the Person class given class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department

19 Inheritance - constructor And the attributes of the Student class class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department

20 Inheritance - constructor class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department First the constructor in the Person class is called with the attributes that are inherited from the Person class

21 Inheritance - constructor class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department Then the values of the attributes that are unique to the Student class are given to their resp. attribute.

22 Create a Student object My_Student = Student( Bo Falk', ' ','Sweden','Lund University','Physical Geography') class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department

23 Printing a Student object Since the Student class has more attributes than the Person class we should add a new str (self) method. class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department def str (self): return 'Name: {}, Day-of-birth: {}\ncurrently studying {} at {}\nliving in {}'.format( self.name, self.dob, self.department, self.university, self.country) Also prints university and department

24 Printing a Student object Print my_student Name: Bo Falk, Day-of-birth: Currently studying Physical Geography at Lund University Living in Sweden class Student(Person): def init (self, name, day_of_birth, country, university, department): Person. init (self, name, day_of_birth, country) self.university = university self.department = department def str (self): return 'Name: {}, Day-of-birth: {}\ncurrently studying {} at {}\nliving in {}'.format( self.name, self.dob, self.department, self.university, self.country)

25 Inheritance which method is called When you call a method, Python will search in the current class. If the method is there it will be called. If not, Python will go up one level to the superclass and search for the method there. Person name (name) dob (day of birth) country (country of residence) Student(Person) university department move(country) str (change country) str

26 Inheritance which method is called my_student.move('norway') First Python search for a move() method in the Student class Person name (name) dob (day of birth) country (country of residence) Student(Person) university department move(country) str (change country) str

27 Inheritance which method is called There is no move() method so Python call the move() method in the superclass Person my_student.move('norway') Person name (name) dob (day of birth) country (country of residence) Student(Person) university department move(country) str (change country) str

28 Inheritance which method is called print my_student First Python search for the str method in the Student class and calls that method Person name (name) dob (day of birth) country (country of residence) Student(Person) university department move(country) str (change country) str