python-crash-course-book/chap9_classes.md

Chapter 9. Classes

Objectives:

• Making an object from a class is called instantiation, and you work with instances of a class.
• In this chapter you’ll write classes and create instances of those classes.
• You’ll specify the kind of information that can be stored in instances, and you’ll define actions that can be taken with these instances.
• You’ll also write classes that extend the functionality of existing classes, so similar classes can share code efficiently.
• You’ll store your classes in modules and import classes written by other programmers into your own program files.

Creating and using a class

class Dog:
    """A Simple attempt to model a dog"""

    def __init__(self, name, age):
        """Initialize name and age attributes."""
        self.name = name
        self.age = age
    
    def sit(self):
        """Simulate a dog sitting in response to a command."""
        print(f"{self.name} is now sitting.")

    def roll_over(self):
        """Simulate rolling over in response to a command"""
        print(f"{self.name} rolled over!")

init() method

• A special method python run automatically whenever we create a new instance.
• self in parameters is required in method definition, always come first
  • So method call will automatically pass the self argument, it's a reference to itself.

Making an Instance from a Class

my_dog = Dog('Willie', 6)

(f"My dog's name is {my_dog.name}.")
print(f"My dog is {my_dog.age} years old.")

Accessing Attributes

my_dog.name

Calling Methods

class Dog:
    --snip--

my_dog = Dog('Willie', 6)
my_dog.sit()
my_dog.roll_over()

Creating Multiple Instances

class Dog:
    --snip--

my_dog = Dog('Willie', 6)
your_dog = Dog('Lucy', 3)

print(f"My dog's name is {my_dog.name}.")
print(f"My dog is {my_dog.age} years old.")
my_dog.sit()

print(f"\nYour dog's name is {your_dog.name}.")
print(f"Your dog is {your_dog.age} years old.")
your_dog.sit();

Working with Classes and Instances

After we create class, we can edit object

• need write getter/setter functions to access attributes
• may need default value for attribute

class Car:
    """A simple attempt to represent a car."""

    def __init__(self, make, model, year):
        """Initialize attributes to describe a car."""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 0

    def get_descriptive_name(self):
        """Return a neatly formatted descriptive name."""
        long_name = f"{self.year} {self.make} {self.model}"
        return long_name.title()

    def read_odometer(self):
           """Print a statement showing the car's mileage."""
           print(f"This car has {self.odometer_reading} miles on it.")
    
    def update_odometer(self, mileage):
        """Set the odometer reading to the given value."""
        self.odometer_reading = mileage

my_new_car = Car('audi', 'a4', 2019)
print(my_new_car.get_descriptive_name())
my_new_car.update_odometer(23)
my_new_car.read_odometer()

• get_descriptive_name() is a getter function, so we can get private attributes
• odometer_reading has default value as 0

Inheritance

Use inheritance when the class is a specialized version of another class.

• When one class inherits from another, it takes on the attributes and methods of the first class.
• Original class is called parent class, New class is called child class
• Child class can define new attributes/methods

init() for a Child Class

Creating a __init__() of a child class:

1. Call __init__() from parent class, to initialize parent's attributes