Python OOP

Python Classes and Objects

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a "blueprint" for creating objects.

Create a Class

To create a class, use the keyword class:

class MyClass:
    x = 5

Create Object

Now we can use the class named MyClass to create objects:

class MyClass:
    x = 5

p1 = MyClass()
print(p1.x)

The __init__() Function

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() function.

All classes have a function called __init__(), which is always executed when the class is being initiated.

Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:

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

p1 = Person("John", 36)

print(p1.name)
print(p1.age)

Note: The __init__() function is called automatically every time the class is being used to create a new object.

The __str__() Function

The __str__() function controls what should be returned when the class object is represented as a string.

If the __str__() function is not set, the string representation of the object will be returned:

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

p1 = Person("John", 36)

print(p1)

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

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

p1 = Person("John", 36)

print(p1)

Object Methods

Objects can also contain methods. Methods in objects are functions that belong to the object.

Let us create a method in the Person class:

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

    def myfunc(self):
        print("Hello my name is " + self.name)

p1 = Person("John", 36)
p1.myfunc()

Note: The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.

The self Parameter

The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.

It does not have to be named self, you can call it whatever you like, but it has to be the first parameter of any function in the class:

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

    def myfunc(abc):
        print("Hello my name is " + abc.name)

p1 = Person("John", 36)
p1.myfunc()

Modify Object Properties

You can modify properties on objects like this:

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

    def myfunc(self):
        print("Hello my name is " + self.name)

p1 = Person("John", 36)

p1.age = 40

print(p1.age)

Delete Object Properties

You can delete properties on objects by using the del keyword:

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

    def myfunc(self):
        print("Hello my name is " + self.name)

p1 = Person("John", 36)

del p1.age

print(p1.age)

Delete Objects

You can delete objects by using the del keyword:

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

    def myfunc(self):
        print("Hello my name is " + self.name)

p1 = Person("John", 36)

del p1

print(p1)

The pass Statement

class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.

class Person:
    pass

OOP Principles

1. Encapsulation

Encapsulation is the bundling of data and methods that work on that data within one unit (class).

class BankAccount:
    def __init__(self, account_number, initial_balance=0):
        self.account_number = account_number
        self.__balance = initial_balance  # Private attribute
    
    def deposit(self, amount):
        if amount > 0:
            self.__balance += amount
            return True
        return False
    
    def withdraw(self, amount):
        if 0 < amount <= self.__balance:
            self.__balance -= amount
            return True
        return False
    
    def get_balance(self):
        return self.__balance

# Usage
account = BankAccount("12345", 1000)
account.deposit(500)
print(account.get_balance())  # 1500

2. Abstraction

Abstraction hides complex implementation details and shows only essential features.

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def make_sound(self):
        pass
    
    @abstractmethod
    def move(self):
        pass

class Dog(Animal):
    def make_sound(self):
        return "Woof!"
    
    def move(self):
        return "Running"

class Bird(Animal):
    def make_sound(self):
        return "Tweet!"
    
    def move(self):
        return "Flying"

# Usage
dog = Dog()
print(dog.make_sound())  # Woof!
print(dog.move())        # Running

Class Variables vs Instance Variables

class Student:
    # Class variable (shared by all instances)
    school_name = "Python High School"
    student_count = 0
    
    def __init__(self, name, grade):
        # Instance variables (unique to each instance)
        self.name = name
        self.grade = grade
        Student.student_count += 1
    
    def get_info(self):
        return f"{self.name} - Grade {self.grade}"
    
    @classmethod
    def get_student_count(cls):
        return cls.student_count

# Usage
student1 = Student("Alice", 10)
student2 = Student("Bob", 11)

print(Student.school_name)        # Python High School
print(Student.get_student_count()) # 2
print(student1.get_info())        # Alice - Grade 10

Property Decorators

Python provides property decorators to create getter, setter, and deleter methods:

class Temperature:
    def __init__(self, celsius=0):
        self._celsius = celsius
    
    @property
    def celsius(self):
        return self._celsius
    
    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError("Temperature cannot be below absolute zero")
        self._celsius = value
    
    @property
    def fahrenheit(self):
        return (self._celsius * 9/5) + 32
    
    @fahrenheit.setter
    def fahrenheit(self, value):
        self._celsius = (value - 32) * 5/9

# Usage
temp = Temperature(25)
print(temp.celsius)     # 25
print(temp.fahrenheit)  # 77.0

temp.fahrenheit = 86
print(temp.celsius)     # 30.0