【Python】クラス継承時のクラス変数とインスタンス変数

class Dog:
    species = "Canis familiaris"  # クラス変数

    def __init__(self, name, age):
        self.name = name  # インスタンス変数
        self.age = age    # インスタンス変数

# インスタンス生成
dog1 = Dog("Buddy", 2)
dog2 = Dog("Lucy", 5)

print(Dog.species)  # Canis familiaris
print(dog1.species)  # Canis familiaris
print(dog2.species)  # Canis familiaris

# クラス変数を変更
Dog.species = "Canis lupus familiaris"
print(Dog.species)  # Canis lupus familiaris
print(dog1.species)  # Canis lupus familiaris
print(dog2.species)  # Canis lupus familiaris

print(dog1.name)  # Buddy
print(dog2.name)  # Lucy

dog1.name = "Max"
print(dog1.name)  # Max
print(dog2.name)  # Lucy

class Animal:
    species = "Unknown"  # クラス変数

    def __init__(self, name):
        self.name = name  # インスタンス変数

class Dog(Animal):
    species = "Canis familiaris"

dog = Dog("Buddy")
print(dog.species)  # Canis familiaris
print(dog.name)     # Buddy

class Parent:
    shared = []

    def __init__(self, value):
        self.shared.append(value)

class Child(Parent):
    pass

p1 = Parent(1)
c1 = Child(2)

print(p1.shared)  # [1, 2]
print(c1.shared)  # [1, 2]

# クラス変数の独立性を保つ方法
class Parent:
    def __init__(self, value):
        if not hasattr(self.__class__, 'shared'):
            self.__class__.shared = []
        self.__class__.shared.append(value)

p2 = Parent(3)
c2 = Child(4)

print(p2.shared)  # [3]
print(c2.shared)  # [4]