PythonのMixinクラス入門 多重継承の落とし穴と安全な設計

# 多重継承の基本例: 左側のクラスが優先される
class A:
    def greet(self):
        return "A.greet"

class B:
    def greet(self):
        return "B.greet"

class C(A, B):  # Aが左、Bが右
    pass

c = C()
print(c.greet())  # A.greetが呼ばれる
print([cls.__name__ for cls in C.mro()])  # 解決順を確認

# 悪い例: 親を直接呼ぶためBase.__init__が二重に実行される
class Base:
    def __init__(self):
        print("Base init")

class Left(Base):
    def __init__(self):
        print("Left init start")
        Base.__init__(self)  # 直接呼び出し
        print("Left init end")

class Right(Base):
    def __init__(self):
        print("Right init start")
        Base.__init__(self)  # 直接呼び出し
        print("Right init end")

class Child(Left, Right):
    def __init__(self):
        print("Child init start")
        Left.__init__(self)   # 直接呼び出し
        Right.__init__(self)  # 直接呼び出し
        print("Child init end")

Child()

# C3の順序を確認する: mro()はクラス解決の完全な順序を返す
class A: pass
class B: pass
class C(A, B): pass
print([cls.__name__ for cls in C.mro()])

# 良い例: 協調的なsuperで一度ずつ初期化される
class Base:
    def __init__(self, **kwargs):
        print("Base init")
        # object.__init__は引数を受け取らないが、**kwargsは空でもOK
        super().__init__(**kwargs)

class Left(Base):
    def __init__(self, **kwargs):
        print("Left init")
        super().__init__(**kwargs)

class Right(Base):
    def __init__(self, **kwargs):
        print("Right init")
        super().__init__(**kwargs)

class Child(Left, Right):
    def __init__(self, **kwargs):
        print("Child init")
        super().__init__(**kwargs)

x = Child()
print([cls.__name__ for cls in Child.mro()])

class A: pass
class B: pass
class C(A, B): pass
class D(B, A): pass

print("C.mro:", [cls.__name__ for cls in C.mro()])
print("D.mro:", [cls.__name__ for cls in D.mro()])

# 代表的なMixinの例: 文字列表現、タイムスタンプ、ID必須のsave
from datetime import datetime

class ReprMixin:
    """__str__を提供し、repr_attrsがあればそれを使って表示する"""
    def __str__(self):
        attrs = getattr(self, "repr_attrs", None)
        if attrs:
            kv = ", ".join(f"{k}={getattr(self, k)!r}" for k in attrs)
        else:
            kv = ", ".join(f"{k}={v!r}" for k, v in self.__dict__.items())
        return f"{self.__class__.__name__}({kv})"

class TimestampMixin:
    """生成時刻を自動付与"""
    def __init__(self, **kwargs):
        self.created_at = datetime.now()
        super().__init__(**kwargs)

class SaveIdRequiredMixin:
    """id属性を要求し、save機能を提供"""
    def save(self):
        if not hasattr(self, "id"):
            # 必須属性の明示的チェック
            raise AttributeError("SaveIdRequiredMixin requires 'id' attribute")
        print(f"Saving object with id={self.id}")

class User(TimestampMixin, ReprMixin, SaveIdRequiredMixin):
    def __init__(self, user_id: int, name: str, **kwargs):
        self.id = user_id
        self.name = name
        self.repr_attrs = ("id", "name")
        super().__init__(**kwargs)

u = User(1, "Alice")
print(u)          # ReprMixinの効果
u.save()          # SaveIdRequiredMixinの効果
print(isinstance(u.created_at, datetime))  # TimestampMixinの効果
print([cls.__name__ for cls in User.mro()])

class RequiresSessionMixin:
    """self.session(HTTPクライアント)が必要"""
    def fetch(self, url: str) -> str:
        if not hasattr(self, "session"):
            raise AttributeError("RequiresSessionMixin requires 'session'")
        # 実際にはrequestsなどを使って取得する想定
        return f"GET {url} via {self.session}"

# 同名メソッドを協調的に連鎖させる例
class LogStartMixin:
    def process(self, data, **kwargs):
        print("start")
        return super().process(data, **kwargs)

class LogEndMixin:
    def process(self, data, **kwargs):
        result = super().process(data, **kwargs)
        print("end")
        return result

class Core:
    def process(self, data, **kwargs):
        print(f"core {data}")
        return data.upper()

class Pipeline(LogStartMixin, LogEndMixin, Core):
    pass

p = Pipeline()
out = p.process("abc")
print("OUT:", out)
print([cls.__name__ for cls in Pipeline.mro()])

# コンポジションの例: ロガーを注入して使う
class Logger:
    def log(self, msg: str):
        print(f"[LOG] {msg}")

class Uploader:
    def __init__(self, logger: Logger | None = None):
        self.logger = logger or Logger()

    def upload(self, path: str):
        self.logger.log(f"upload {path}")
        return True

u = Uploader()
ok = u.upload("file.txt")
print("OK?", ok)