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Python Classes and Objects
A Basic Introduction
Coming up: Topics 1
Topics
• Objects and Classes • Abstraction • Encapsulation • MessagesWhat are objects
• An object is a datatype that stores data, but ALSO has operations defined to act on the data. It knows stuff and can do stuff.
• Generally represent: - tangible entities (e.g., student, airline ticket, etc.) - intangible entities (e.g., data stream) • Interactions between objects define the system operation (through message passing)What are Objects
• A Circle drawn on the screen: • Has attributes (knows stuff): - radius, center, color • Has methods (can do stuff): - move - change colorDesign of Circle object
• A Circle object: - center, which remembers the center point of the circle, - radius, which stores the length of the circle's radius. - color, which stores the color • The draw method examines the center and radius to decide which pixels in a window should be colored. • The move method sets the center to another location, and redraws the circleDesign of Circle
• All objects are said to be an instance of some class. The class of an object determines which attributes the object will have.
• A class is a description of what its instances will know and do.Circle: classes and objects
Circle Class Attributes: - location, radius,color Methods: - draw, move Classes are blueprints or directions on how to create an object Objects are instantiations of the class (attributes are set) 3 circle objects are shown (each has different attribute values)
Circle Class
class Circle(object):def __init__(self, center, radius): self.center = center self.radius = radius def draw(self, canvas): rad = self.radius x1 = self.center[0]-rad y1 = self.center[1]-rad x2 = self.center[0]+rad y2 = self.center[1]+rad canvas.create_oval(x1, y1, x2, y2, fill='green') def move(self, x, y): self.center = [x, y]
Beginning of the class definition The constructor. This is called when someone creates a new Circle, these assignments create attributes. A method that uses attributes to draw the circle A method that sets the center to a new location and then redraws it
objects/CircleModule.pyConstructors
• The objcet's constructor method is named __init__ • The primary duty of the constructor is to set the state of the object's attributes (instance variables)• Constructors may have default parameters • Calling an object's constructor (via the class name) is a
signal to the interpreter to create (instantiate) a new object of the data type of the class - myCircle = Circle([10,30], 20) # Never pass "self", it's automatic
Creating a Circle
myCircle = Circle([10,30], 20) • This statement creates a new Circle object and stores a reference to it in the variable myCircle. • The parameters to the constructor are used to initialize some of the instance variables (center and radius) inside myCircle.Creating a Circle
myCircle = Circle([10,30], 20) • Once the object has been created, it can be manipulated by calling on its methods: myCircle.draw(canvas) myCircle.move(x,y)Objects and Classes
• myCircle = Circle([10,30], 20) • myOtherCircle = Circle([4,60], 10) • myCircle and myOtherCircle are objects or instances of theClass Circle
• The circle class defines what a circle knows (attributes) andwhat it does (methods)... but to have a circle, you need to construct an object from that class definition
• Similar to a "list". Python defines what a list is, and can do (slicing, indexing, length(...), etc... but until you create one, you don't really have one