Classes & objects

A class is a blueprint

• A class defines a new kind of thing — a user-defined type.
• An object is one item made from that class.
• One Dog class can make many dog objects, each with its own data.

init and attributes

• __init__ is a special method that runs when you make an object.
• self refers to the object being made.
• Values stored on self are the object's attributes (its data).

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

p = Point(2, 5)
print(p.x)
print(p.y)

Methods

• A method is a function defined inside a class.
• Its first parameter is always self, the object it works on.
• Call it with a dot: object.method().

class Greeter:
    def __init__(self, name):
        self.name = name

    def hello(self):
        return "Hi, " + self.name

g = Greeter("Sam")
print(g.hello())

Enumerated types

• An enumerated type is a fixed set of named values.
• from enum import Enum lets you define one as a class.
• It is clearer than using loose numbers like 1, 2, 3.

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

print(Color.GREEN)
print(Color.GREEN.value)

In Cambridge pseudocode

• A CLASS block lists its data and a NEW constructor.

CLASS Point
    PRIVATE X : INTEGER
    PRIVATE Y : INTEGER

    PUBLIC PROCEDURE NEW(startX : INTEGER, startY : INTEGER)
        X ← startX
        Y ← startY
    ENDPROCEDURE
ENDCLASS

DECLARE p : Point
p ← NEW Point(2, 5)

Common mistakes

• Every method takes self as its first parameter.
• __init__ runs automatically when you create an object.

Now you try

• Define each class with __init__ and any methods it needs.
• Press Check answer to test your code.