多重継承と MRO

導入

多重継承の構文

class A:
    def hello(self): return 'A'
class B:
    def world(self): return 'B'
class C(A, B): pass
c = C()
print(c.hello(), c.world())

メソッド解決順序（MRO）

class A:
    def greet(self): return 'A'
class B:
    def greet(self): return 'B'
class C(A, B): pass
print(C().greet())  # A wins
print(C.__mro__)

C3線形化

class A: pass
class B(A): pass
class C(A): pass
class D(B, C): pass
print([c.__name__ for c in D.__mro__])

ダイヤモンド継承

class A:
    def __init__(self): print('A')
class B(A):
    def __init__(self): super().__init__(); print('B')
class C(A):
    def __init__(self): super().__init__(); print('C')
class D(B, C):
    def __init__(self): super().__init__(); print('D')
D()

ミックスイン

class JSONMixin:
    def to_json(self):
        import json
        return json.dumps(vars(self))
class User(JSONMixin):
    def __init__(self, name, age): self.name,self.age=name,age
print(User('Alice', 30).to_json())

ミックスインの設計ルール

class LoggingMixin:
    def log(self, msg): print(f'[{self.__class__.__name__}] {msg}')
class Service(LoggingMixin):
    def run(self): self.log('running')
Service().run()

多重継承でのsuper()

class Base:
    def action(self): return 'Base'
class A(Base):
    def action(self): return 'A->' + super().action()
class B(Base):
    def action(self): return 'B->' + super().action()
class C(A, B):
    def action(self): return 'C->' + super().action()
print(C().action())

動的なクラス作成のためのtype()

def hello(self): return 'hello'
MyClass = type('MyClass', (object,), {'hello': hello})
print(MyClass().hello())

MROの確認

class A: pass
class B(A): pass
class C(B): pass
import inspect
print([c.__name__ for c in inspect.getmro(C)])

多重継承を避ける場合

# Prefer:
class User:
    def __init__(self, name, logger):
        self.name = name
        self.logger = logger  # composition
print('composition preferred')

クイックチェック

復習

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