Py-OOP-Cheatsheet

# ===========================
# Python OOP Cheat Sheet
# ===========================

# ---------------------------
# 1. Class & Object
# ---------------------------

# Define a class
class Person:
    species = "Homo sapiens"  # Class variable (shared by all instances)

    # Constructor (initializer)
    def __init__(self, name, age):
        self.name = name        # Instance variable
        self.age = age

    # Instance method
    def greet(self):
        print(f"Hello, my name is {self.name} and I'm {self.age} years old.")

    # Class method (decorated with @classmethod)
    @classmethod
    def species_info(cls):
        print(f"Species: {cls.species}")

    # Static method (independent of class or instance)
    @staticmethod
    def is_adult(age):
        return age >= 18


# Create object
p1 = Person("Han", 22)
p1.greet()
Person.species_info()
print(Person.is_adult(20))

# ---------------------------
# 2. Encapsulation
# ---------------------------

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance  # Private attribute (double underscore)

    def deposit(self, amount):
        self.__balance += amount

    def withdraw(self, amount):
        if amount <= self.__balance:
            self.__balance -= amount
        else:
            print("Insufficient funds.")

    def get_balance(self):
        return self.__balance

acct = BankAccount(1000)
acct.deposit(500)
print(acct.get_balance())  # 1500

# ---------------------------
# 3. Inheritance
# ---------------------------

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        print("Some generic sound")

# Subclass inherits from parent class
class Dog(Animal):
    def __init__(self, name, breed):
        super().__init__(name)  # Call parent constructor
        self.breed = breed

    # Method overriding
    def speak(self):
        print(f"{self.name} says Woof!")

dog = Dog("Buddy", "Golden Retriever")
dog.speak()

# ---------------------------
# 4. Polymorphism
# ---------------------------

class Cat(Animal):
    def speak(self):
        print(f"{self.name} says Meow!")

animals = [Dog("Fido", "Labrador"), Cat("Whiskers")]
for a in animals:
    a.speak()  # Calls different methods depending on object type

# ---------------------------
# 5. Abstract Class
# ---------------------------

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    def area(self):
        return 3.14 * self.radius ** 2

circle = Circle(5)
print(circle.area())

# ---------------------------
# 6. Property Decorator
# ---------------------------

class Student:
    def __init__(self, name):
        self._name = name

    @property
    def name(self):
        print("Getting name...")
        return self._name

    @name.setter
    def name(self, value):
        print("Setting name...")
        self._name = value

s = Student("Han")
print(s.name)
s.name = "Han Li"

# ---------------------------
# 7. Magic / Dunder Methods
# ---------------------------

class Vector:
    def __init__(self, x, y):
        self.x, self.y = x, y

    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

v1 = Vector(1, 2)
v2 = Vector(3, 4)
print(v1 + v2)  # Vector(4, 6)
print(v1 == Vector(1, 2))  # True

# ---------------------------
# 8. Composition
# ---------------------------

class Engine:
    def start(self):
        print("Engine starting...")

class Car:
    def __init__(self):
        self.engine = Engine()  # Contains an Engine object (composition)

    def start(self):
        self.engine.start()
        print("Car is running.")

car = Car()
car.start()

# ---------------------------
# 9. Class Relationship Checks
# ---------------------------

print(isinstance(dog, Dog))        # True
print(isinstance(dog, Animal))     # True
print(issubclass(Dog, Animal))     # True

# ---------------------------
# 10. Data Class (@dataclass)
# ---------------------------

from dataclasses import dataclass

@dataclass
class Point:
    x: int
    y: int

p = Point(3, 4)
print(p)