Object-Oriented Programming

Object-oriented programming (OOP) is a style of programming that allows you to think of code in terms of "objects." Here's an example of a Car class:

#{{}}
class Car(object):
    num_wheels = 4

    def __init__(self, color):
        self.wheels = Car.num_wheels
        self.color = color

    def drive(self):
        if self.wheels <= Car.num_wheels:
            return self.color + ' car cannot drive!'
        return self.color + ' car goes vroom!'

    def pop_tire(self):
        if self.wheels > 0:
            self.wheels -= 1

Here's some terminology:

• class: a blueprint for how to build a certain type of object. The Car class (shown above) describes the behavior and data that all Car objects have.

• instance: a particular occurrence of a class. In Python, we create instances of a class like this:

  >>> my_car = Car('red')
  

  my_car is an instance of the Car class.

• attribute or field: a variable that belongs to the class. Think of an attribute as a quality of the object: cars have wheels and color, so we have given our Car class self.wheels and self.color attributes. We can access attributes using dot notation:

  >>> my_car.color
  'red'
  >>> my_car.wheels
  4
  

• method: Methods are just like normal functions, except that they are tied to an instance or a class. Think of a method as a "verb" of the class: cars can drive and also pop their tires, so we have given our Car class the methods drive and pop_tire. We call methods using dot notation:

  >>> my_car = Car('red')
  >>> my_car.drive()
  'red car goes vroom!'
  

• constructor: As with data abstraction, constructors describe how to build an instance of the class. Most classes have a constructor. In Python, the constructor of the class defined as __init__. For example, here is the Car class's constructor:

  def __init__(self, color):
      self.wheels = Car.num_wheels
      self.color = color
  

  The constructor takes in one argument, color. As you can see, the constructor also creates the self.wheels and self.color attributes.

• self: in Python, self is the first parameter for many methods (in this class, we will only use methods whose first parameter is self). When a method is called, self is bound to an instance of the class. For example:

  >>> my_car = Car('red')
  >>> car.drive()
  

  Notice that the drive method takes in self as an argument, but it looks like we didn't pass one in! This is because the dot notation implicitly passes in car as self for us.