David's Blog

Chapter 5 Object-oriented programming.

By David Li on Fri, 14 September 2024

Object-oriented programming (OOP) is a programming paradigm that is based on the concept of “objects”, which can contain data (referred to as “attributes” or “properties”) and methods (referred to as “behaviors” or “functions”) that operate on that data.

The main idea behind OOP is to model real-world entities as objects in the program. For example, in a car racing game, we might create an object for each car that contains attributes such as its speed, position, and direction, as well as methods for accelerating, braking, and turning. We might also create objects for the race track, the obstacles on the track, and the other drivers in the race.

By modeling real-world entities as objects in the program, we can create a more intuitive and natural representation of the problem domain. We can also encapsulate data and behavior within objects, which helps to reduce complexity and make the code more modular and reusable.

In OOP, objects are usually defined using classes, which are templates or blueprints for creating objects of a certain type. For example, we might define a Car class that contains the attributes and methods for a car object. We can then create multiple car objects from this class, each with its own set of attribute values.

Here’s an example of what a Car class might look like in Python:

class Car:
 def __init__(self, speed, position):
 self.speed = speed
 self.position = position
 
 def accelerate(self, amount):
 self.speed += amount
 
 def brake(self, amount):
 self.speed -= amount
 
 def turn(self, direction):
 # logic for turning the car
 pass

In this example, the Car class defines a constructor method (__init__) that initializes the speed and position attributes of a car object. It also defines three methods for accelerating, braking, and turning the car.

To create a car object from this class, we would use code like this:

my_car = Car(speed=0, position=0)

This creates a new car object with an initial speed of 0 and an initial position of 0. We can then call the methods of the car object to make it accelerate, brake, and turn.

Overall, object-oriented programming provides a powerful and flexible way to model complex systems and solve problems in a more intuitive and natural way. By representing real-world entities as objects in the program, we can create more modular, reusable, and maintainable code.

In Python, an object is an instance of a class, which is a template or blueprint that defines the data (attributes) and behavior (methods) of a certain type of object. An object can be thought of as a self-contained entity that contains both data and the operations that can be performed on that data.

Objects in Python are used to represent real-world entities by mapping their attributes and behaviors to the attributes and methods of a class. For example, a car object might have attributes such as its make, model, and color, as well as methods for accelerating, braking, and turning. Similarly, a user object in a social media application might have attributes such as their name, age, and email address, as well as methods for posting messages, commenting on posts, and following other users.

Here’s an example of how we might create a Car object in Python:

class Car:
 def __init__(self, make, model, color):
 self.make = make
 self.model = model
 self.color = color
 self.speed = 0
 
 def accelerate(self, amount):
 self.speed += amount
 
 def brake(self, amount):
 self.speed -= amount
 
 def turn(self, direction):
 # logic for turning the car
 pass

my_car = Car(make="Toyota", model="Corolla", color="red")

In this example, the Car class defines a constructor method (__init__) that initializes the make, model, and color attributes of a car object. It also defines methods for accelerating, braking, and turning the car. We then create a Car object called my_car with the make “Toyota”, model “Corolla”, and color “red”.

Once we have created a Car object, we can access its attributes and call its methods using dot notation. For example, we can print the make of my_car by writing print(my_car.make).

Overall, objects in Python provide a powerful way to represent real-world entities with their attributes and behaviors. By encapsulating data and methods within an object, we can create more modular, reusable, and maintainable code.

In Python, a class is a blueprint or template for creating objects. A class defines a set of attributes (data) and methods (behavior) that all objects created from that class will have.

To define a class in Python, we use the class keyword followed by the name of the class. Here’s an example of a simple class definition:

class Person:
 def __init__(self, name, age):
 self.name = name
 self.age = age

 def say_hello(self):
 print("Hello, my name is", self.name)

In this example, we define a Person class with two attributes (name and age) and one method (say_hello). The __init__ method is a special method that gets called when an object is created from the class. It initializes the name and age attributes of the object.

To create an object from a class, we call the class like a function, passing in any required arguments. This creates a new instance of the class, which we can then manipulate and interact with using its attributes and methods. Here’s an example of creating an object from the Person class and calling its say_hello method:

person = Person(name="Alice", age=25)
person.say_hello() # prints "Hello, my name is Alice"

In this example, we create a new Person object called person with the name “Alice” and age 25. We then call its say_hello method, which prints out a message containing the object’s name attribute.

Overall, classes in Python provide a powerful way to define the blueprint for creating objects with shared attributes and behavior. By encapsulating data and methods within a class, we can create more modular, reusable, and maintainable code.

To create a class in Python, we use the class keyword followed by the name of the class. Here’s an example of a simple class definition:

class Person:
 def __init__(self, name, age):
 self.name = name
 self.age = age

 def say_hello(self):
 print("Hello, my name is", self.name)

In this example, we define a Person class with two attributes (name and age) and one method (say_hello). The __init__ method is a special method that gets called when an object is created from the class. It initializes the name and age attributes of the object.

The self parameter in the __init__ and other method definitions refers to the object itself, and allows us to access and modify the object’s attributes and call its methods.

To create an object from a class, we call the class like a function, passing in any required arguments. This creates a new instance of the class, which we can then manipulate and interact with using its attributes and methods. Here’s an example of creating an object from the Person class and calling its say_hello method:

person = Person(name="Alice", age=25)
person.say_hello() # prints "Hello, my name is Alice"

In this example, we create a new Person object called person with the name “Alice” and age 25. We then call its say_hello method, which prints out a message containing the object’s name attribute.

Overall, classes in Python provide a powerful way to define the blueprint for creating objects with shared attributes and behavior. By encapsulating data and methods within a class, we can create more modular, reusable, and maintainable code.

In Python, the self keyword is used to refer to the current object within a class method. When a method is called on an object, the object itself is automatically passed as the first argument to the method, which is typically named self by convention.

Here’s an example of a class method that uses self to access and modify the object’s attributes:

class Person:
 def __init__(self, name, age):
 self.name = name
 self.age = age

 def say_hello(self):
 print("Hello, my name is", self.name)

 def celebrate_birthday(self):
 self.age += 1
 print("Happy birthday! You are now", self.age, "years old.")

In this example, we define a Person class with an __init__ method that initializes the object’s name and age attributes, a say_hello method that prints a message containing the object’s name attribute, and a celebrate_birthday method that increments the object’s age attribute and prints a birthday message.

To call a method on a Person object and access its attributes, we create an object of the Person class and call its methods using the . operator, passing self implicitly:

person = Person(name="Alice", age=25)
person.say_hello() # prints "Hello, my name is Alice"
person.celebrate_birthday() # prints "Happy birthday! You are now 26 years old."

In this example, we create a Person object called person with the name “Alice” and age 25. We then call its say_hello method and celebrate_birthday method, which access and modify the object’s name and age attributes using the self keyword.

In Python, the __str__ method is a special method that allows us to define a string representation of an object. When an object is printed or converted to a string using the str function, Python calls the object’s __str__ method to obtain a string representation of the object.

Here’s an example of a class with a __str__ method:

class Person:
 def __init__(self, name, age):
 self.name = name
 self.age = age

 def say_hello(self):
 print("Hello, my name is", self.name)

 def celebrate_birthday(self):
 self.age += 1
 print("Happy birthday! You are now", self.age, "years old.")

 def __str__(self):
 return f"{self.name} ({self.age})"

In this example, we define a Person class with an __init__ method, a say_hello method, a celebrate_birthday method, and a __str__ method. The __str__ method returns a string representation of the Person object in the format “name (age)“.

To print a Person object using its __str__ method, we can simply use the print function or convert the object to a string using the str function:

person = Person(name="Alice", age=25)
print(person) # prints "Alice (25)"
print(str(person)) # converts the object to a string and prints "Alice (25)"

In this example, we create a Person object called person with the name “Alice” and age 25. We then print the person object using the print function and the str function, which call the person object’s __str__ method to obtain a string representation of the object.

In Python, we create objects from classes by instantiating them. When we instantiate an object, we create a new instance of the class with its own set of attributes and methods.

To instantiate an object from a class in Python, we call the class’s constructor method, which is called __init__. The __init__ method is a special method that is called when a new instance of the class is created, and it is used to set the initial state of the object by assigning values to its attributes.

Here’s an example of a class with an __init__ method:

class Person:
 def __init__(self, name, age):
 self.name = name
 self.age = age

 def say_hello(self):
 print("Hello, my name is", self.name)

 def celebrate_birthday(self):
 self.age += 1
 print("Happy birthday! You are now", self.age, "years old.")

In this example, we define a Person class with an __init__ method, a say_hello method, and a celebrate_birthday method. The __init__ method takes two arguments, name and age, and assigns them to the object’s name and age attributes, respectively.

To instantiate a Person object, we simply call the Person constructor with the desired values for name and age:

person = Person(name="Alice", age=25)

In this example, we create a new Person object called person with the name “Alice” and age 25. The Person constructor assigns these values to the person object’s name and age attributes. We can now call the say_hello and celebrate_birthday methods on the person object:

person.say_hello() # prints "Hello, my name is Alice"
person.celebrate_birthday() # prints "Happy birthday! You are now 26 years old."

In this example, we call the say_hello method on the person object, which prints “Hello, my name is Alice”. We then call the celebrate_birthday method on the person object, which increments the person object’s age attribute by 1 and prints “Happy birthday! You are now 26 years old.”

In Python, we access the attributes and methods of an object using dot notation and parentheses, respectively.

To access an object’s attribute using dot notation, we simply write the object’s name followed by a dot (.) and the name of the attribute. For example, if we have a Person object with a name attribute, we can access the name attribute using dot notation as follows:

person = Person(name="Alice", age=25)
print(person.name) # prints "Alice"

In this example, we create a Person object called person with a name attribute set to “Alice”. We then use dot notation to access the person object’s name attribute and print its value, which is “Alice”.

To call an object’s method using parentheses, we write the object’s name followed by a dot (.) and the name of the method, followed by an open parenthesis (() and any arguments to the method (if applicable), followed by a close parenthesis ()). For example, if we have a Person object with a say_hello method, we can call the say_hello method using parentheses as follows:

person = Person(name="Alice", age=25)
person.say_hello() # prints "Hello, my name is Alice"

In this example, we create a Person object called person with a say_hello method. We then call the person object’s say_hello method using parentheses, which prints “Hello, my name is Alice”.

Here’s an example of a Person class that models a real-world entity:

class Person:
 def __init__(self, name, age):
 self.name = name
 self.age = age

 def say_hello(self):
 print(f"Hello, my name is {self.name} and I am {self.age} years old.")

In this example, we define a Person class with a constructor method (__init__) that takes name and age parameters and initializes corresponding instance variables (self.name and self.age). The say_hello method prints a greeting that includes the person’s name and age.

To create an instance of the Person class and call its say_hello method, we could do the following:

person = Person(name="Alice", age=25)
person.say_hello() # prints "Hello, my name is Alice and I am 25 years old."

In this example, we create a Person object called person with a name attribute set to “Alice” and an age attribute set to 25. We then call the person object’s say_hello method, which prints “Hello, my name is Alice and I am 25 years old.”

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