Python Classes and Principles of Computer Science

1. Lecture 04 Classes

2. Python has a number of classes built-in lists, dictionaries, sets, int, float, boolean, strings We can define our own classes creates a new type of object in Python Classes consist of: state variables (sometimes called instance variables) methods (functions that are linked to a particular instance of the class) 2 COMPSCI 105 - Principles of Computer Science class name_of_the_class : definition of the class goes here

3. Pass is a statement that does nothing It is often used as a placeholder when developing code 3 COMPSCI 105 - Principles of Computer Science class Point: pass >>> p = Point() >>> p <__main__.Point object at 0x02702570> >>> p.x = 2 >>> p.y = 4 >>> p.x 2 >>> p.y 4

4. Classes are designed to help build modular code Can be defined within a module that also contains application code Multiple classes can be defined in the same file In this course, we will typically store each class in their own module To use the class in another module, you will need to import the module 4 COMPSCI 105 - Principles of Computer Science class Point: pass Saved in a file called Geometry.py from Geometry import Point p = Point() Use the class like this

5. 5 COMPSCI 105 - Principles of Computer Science from Geometry import Point p = Point() p.x = 5 p.y = 7 p x: 5 y: 7

6. We want to define the Point class so we can write code that sets the initial values of some variables First, we need to define a special method of the Point class called a constructor The constructor is called whenever you create an object of the Point class. 6 COMPSCI 105 - Principles of Computer Science from Geometry import Point p = Point(5, 7)

7. Each class should contain a constructor method Name of the method is __init__ The method always has at least one parameter, called self Self is a reference to the object that we are creating The constructor can have other parameters 7 COMPSCI 105 - Principles of Computer Science class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y from Geometry import Point p = Point(5, 7) print(p.x) print(p.y)

8. Defining more methods A method to shift a point by a given amount in horizontal and vertical directions Note: the method is named normally, but has the additional parameter (self) as the first parameter All methods that are called on an instance of an object need the self parameter 8 COMPSCI 105 - Principles of Computer Science class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y def translate(self, dx, dy): self.x += dx self.y += dy

9. Methods are defined to accept self as the first parameter We call the method using: object_name.method(params) Method call is translated to: method(object_name, params) 9 COMPSCI 105 - Principles of Computer Science class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y def translate(self, dx, dy): self.x += dx self.y += dy from Geometry import Point p = Point(0,0) p.translate(10,5)

10. Define a class that will be used to represent a square with a given side length. Your class should include a constructor that will allow the square to be used as follows: 10 COMPSCI 105 - Principles of Computer Science from Geometry import Square side = 10 s = Square(side)

11. Add a method to the class to calculate the perimeter of the square. The following code shows how the method may be used. 11 COMPSCI 105 - Principles of Computer Science from Geometry import Square side = 10 s = Square(side) p = s.perimeter()

12. Add a method to the class to change the size of the square using a scaling factor. For example, if you scale the square by a factor of 2, then the sides of the square will be twice as long. The following code shows how the method may be used. 12 COMPSCI 105 - Principles of Computer Science from Geometry import Square side = 10 s = Square(side) big_s = s.scale(2)

13. Add a method that compares the size of the square with the size of another square. The method should be called bigger_than() and should accept a square as a parameter 13 COMPSCI 105 - Principles of Computer Science from Geometry import Square s = Square(6) t = Square(7) if s.bigger_than(t): print(The first square is bigger)

14. Classes are defined using the syntax: Classes contain methods that define the behaviour and operations on objects of that class We override the default behaviours for built-in methods/operations Additional Resources http://interactivepython.org/runestone/static/thinkcspy/Classes/fractions.html 14 COMPSCI 105 - Principles of Computer Science class name_of_the_class : definition of the class goes here