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Classes and Objects Review

curriculum/challenges/english/blocks/review-classes-and-objects/67f39d848979082814c07d9c.md

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--description--

Python Classes and Objects

  • Class Definition: A class is a blueprint for creating objects. It defines the behavior an object will have through its attributes and methods. Here is a basic example of a class definition in Python:
python
class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        print(f'{self.name.upper()} says woof woof!')
  • Creating Objects: Objects are instances of a class. They are created by calling the class with the necessary arguments.
python
dog1 = Dog('Jack', 3)
dog2 = Dog('Thatcher', 5)

dog1.bark()  # JACK says woof woof!
dog2.bark()  # THATCHER says woof woof!
  • Calling Methods on Objects: You can call methods on objects to perform actions or retrieve information.
python
object_name1.method_name()
object_name2.method_name()
  • Difference Between Class and Object: A class is a reusable template, while an object is a specific instance of that class with actual data.

Attributes

  • Instance Attributes: Defined in __init__() using self, and unique to each object.
  • Class Attributes: Defined directly inside the class and shared by all instances.
python
class Dog:
    species = 'French Bulldog'  # Class attribute

    def __init__(self, name):
        self.name = name  # Instance attribute

print(Dog.species) # French Bulldog

jack = Dog('Jack')
print(jack.name)     # Jack
print(jack.species)  # French Bulldog

Methods

  • Methods: Functions defined inside a class that operate on the object's attributes.
python
class Car:
    def __init__(self, color, model):
        self.color = color
        self.model = model

    def describe(self):
        return f'This car is a {self.color} {self.model}'

my_car_1 = Car('red', 'Tesla Model S')
print(my_car_1.describe())  # This car is a red Tesla Model S
  • Accessing Methods: Call methods on objects using the dot notation. Here is an example of calling the describe method on two different car objects:
python
class Car:
    def __init__(self, color, model):
        self.color = color  
        self.model = model  

    def describe(self):
        return f'This car is a {self.color} {self.model}'

my_car_1 = Car('red', 'Tesla Model S')
my_car_2 = Car('green', 'Lamborghini Revuelto')

print(my_car_1.describe()) # Calling method using the dot notation

print(my_car_2.describe()) # Calling method using the dot notation

Dunder (Magic) Methods

  • Definition: Special methods that start and end with a double underscore (e.g., __init__, __len__, __str__, __eq__). Python uses them internally for built-in operations.
python
class Book:
    def __init__(self, title, pages):
        self.title = title
        self.pages = pages

    def __len__(self):
        return self.pages

    def __str__(self):
        return f"'{self.title}' has {self.pages} pages"

    def __eq__(self, other):
        return self.pages == other.pages

book1 = Book('Built Wealth Like a Boss', 420)
print(len(book1))        # 420
print(str(book1))        # 'Built Wealth Like a Boss' has 420 pages

  • Calling dunder methods indirectly: You don't need to call dunder methods directly. Instead, Python automatically calls them when certain actions happen. These operations include:

    • arithmetic operations like addition, subtraction, multiplication, division, and others. In addition, __add__() is called, __sub__() for subtraction, __mul__() for multiplication, and __truediv__() for division.

    • string operations like concatenation, repetition, formatting, and conversion to text. __add__() is called for concatenation, __mul__() for repetition, __format__() for formatting, __str__() and __repr__() for text conversion, and so on.

    • comparison operations like equality, less-than, greater-than, and others. __eq__() is called for equality checks, __lt__() for less-than, __gt__() for greater-than, and so on.

    • iteration operations like making an object iterable and advancing through items. __iter__() is called to return an iterator and  __next__() to fetch the next item.

Real World Example: Shopping Cart

  • Cart Class with Dunder Methods: Allows adding, removing, iterating, and checking contents with built-in behavior.
python
class Cart:
    def __init__(self):
        self.items = []

    def add(self, item):
        self.items.append(item)

    def remove(self, item):
        if item in self.items:
            self.items.remove(item)
        else:
            print(f'{item} is not in cart')

    def list_items(self):
        return self.items

    def __len__(self):
        return len(self.items)

    def __getitem__(self, index):
        return self.items[index]

    def __contains__(self, item):
        return item in self.items

    def __iter__(self):
        return iter(self.items)

cart = Cart()
cart.add('Laptop')
print(len(cart))        # 1
print('Laptop' in cart) # True

--assignment--

Review the Classes and Objects topics and concepts.