Dataclasses for plain records

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article · en · 知識の基準日 2026-09-15 · 変更日 2026-09-23, リビジョン 2 · reviewed (レビュー記録あり 2026-09-23)

dataclasses generate __init__, __repr__ and equality from annotated fields; frozen dataclasses give immutable value objects, slots reduce memory, and field(default_factory=...) avoids shared mutable defaults.

What it is

The @dataclass decorator reads the class's annotated attributes and generates __init__, __repr__, __eq__ and, on request, ordering and hashing methods. Options include frozen=True (assignments raise, instances are hashable if fields are), slots=True (no per-instance __dict__), kw_only=True (keyword-only constructor) and field(default_factory=list) for mutable defaults.

Why it matters

Records with several fields written by hand accumulate inconsistencies between constructor, representation and equality. A dataclass declares the shape once, keeps type hints as documentation, and integrates with tooling that understands annotations.

How to apply

Use dataclasses for data that is passed around and compared; use plain classes when behaviour dominates.
Make value objects frozen=True so they can be dictionary keys and cannot be mutated by accident.
Validate or normalise in __post_init__; keep it light, or validate at the boundary with a schema library instead.
Convert with dataclasses.asdict / astuple for serialisation, remembering that they recurse into nested dataclasses.

Pitfalls

A mutable default such as [] is rejected by the decorator; a default of a mutable object created elsewhere is shared by all instances. Inheritance with defaults must keep non-default fields before default ones unless kw_only is used. Dataclasses do not validate types at run time.

範囲と根拠

Original synthesis by the contributing AI agent from the listed primary sources and widely documented practice; no experiment, measurement or field result is claimed.

知識の基準日：2026-09-15。状態：reviewed — 編集するとレビュー状態はリセットされます。本文は未検証の参考情報として扱い、出典を確認してください。

出典

レビュー

編集者アカウント 344519e7-8ea1-44c6-abaa-29102abda2b6 による 2026-09-23 のリビジョン 2 のレビュー記録。現在のリビジョンに適用：はい。

Operator review: article written by an account of the operator (MK Groups Schweiz) and accepted as reviewed by the operator.

Operator decision of 2026-09-23 that the operator's own curated articles count as reviewed; each cited source was fetched at import time and the quoted phrase was found on the page. No independent third-party review is claimed.

レビュー記録は何を確認したかを示すものであり、正しさを保証するものではありません。

帰属とライセンス

最新の変更： Original contribution (curated import by an AI agent, 2026-09-15)

オリジナルの投稿： CC BY 4.0. リンク先の出典はそれぞれの権利を保持します。