Class variables vs Instance variables

class Dog:
    tricks = []  # Class variable
 
    def add_trick(self, trick):
        self.tricks.append(trick)
 
# Tạo 2 dogs
fido = Dog()
buddy = Dog()
 
# Fido học trick
fido.add_trick("roll over")
print(fido.tricks)  # ['roll over']
 
# Buddy cũng biết trick của Fido?!
print(buddy.tricks)  # ['roll over'] ❌ What?!

class Dog:
    tricks = []  # Shared bởi TẤT CẢ instances!
 
# Tất cả dogs dùng CHUNG list
fido = Dog()
buddy = Dog()
 
print(fido.tricks is buddy.tricks)  # True - Cùng object!

class Dog:
    def __init__(self):
        self.tricks = []  # Mỗi instance có list riêng
 
fido = Dog()
buddy = Dog()
 
print(fido.tricks is buddy.tricks)  # False - Khác object

class Dog:
    def __init__(self, name):
        self.name = name    # Instance variable
        self.tricks = []    # Instance variable
 
    def add_trick(self, trick):
        self.tricks.append(trick)
 
fido = Dog("Fido")
buddy = Dog("Buddy")
 
fido.add_trick("roll over")
print(fido.tricks)   # ['roll over']
print(buddy.tricks)  # [] ✅

class Circle:
    pi = 3.14159  # Class variable - constant
 
    def __init__(self, radius):
        self.radius = radius  # Instance variable
 
    def area(self):
        return Circle.pi * self.radius ** 2

class Database:
    connection_pool = []  # Shared pool
    max_connections = 10  # Config
 
    def __init__(self, db_name):
        self.db_name = db_name  # Instance-specific

class MyClass:
    class_var = "class"
 
    def __init__(self):
        self.instance_var = "instance"
 
obj = MyClass()
 
# Instance variable
print(obj.instance_var)  # "instance"
 
# Class variable qua instance
print(obj.class_var)     # "class"
 
# Class variable qua class
print(MyClass.class_var) # "class"

class Counter:
    count = 0  # Class variable
 
c1 = Counter()
c2 = Counter()
 
# Đọc qua instance
print(c1.count)  # 0 (từ class)
 
# Gán qua instance → Tạo instance variable!
c1.count = 10
print(c1.count)          # 10 (instance var)
print(c2.count)          # 0 (class var)
print(Counter.count)     # 0 (class var)

class MyClass:
    class_var = "shared"  # Class variable
 
    def __init__(self):
        self.instance_var = "unique"  # Instance variable