Review class design smells

Purpose

Flag two design smells: the Data Class (a class that is only fields plus accessors, with the behaviour that operates on it living elsewhere), and over-engineering (reaching for advanced machinery where a plain class would do). Recommend the simpler, better-located design. (P05, P13)

When to use

• A class exposes fields and getters/setters but carries no significant behaviour, and the logic that manipulates its data sits in other modules.
• A design reaches for metaclasses, descriptors, multiple inheritance, or mixins, or is hard to read, observe, debug, or test.

Do not apply when the data-only class is deliberate short-lived scaffolding or an immutable intermediate representation crossing a boundary (e.g. a JSON record), or when an advanced construct is genuinely required and a simpler form was shown to be insufficient.

Procedure

1. Detect the Data Class smell. Identify classes that are just fields with getters/setters and no real behaviour. Check whether logic that belongs to that data is scattered (and often duplicated) across the system. (C005)
2. Recommend moving behaviour in. Where the smell holds, suggest relocating the related operations into the class so data and the behaviour over it live together.
3. Preserve the exception. Do not flag a class that is deliberate scaffolding for a new module, or an immutable record used only to import/export data across a boundary — those are legitimate, not a smell.
4. Detect over-engineering. Flag designs that use metaclasses, descriptors, multiple inheritance, or mixins where a plain class would solve the problem; readability degrades as abstraction layers accumulate. Recommend the simplest construct that works. (C014)
5. Treat un-observability as a signal. Code that is hard to read, observe, debug, or test is itself a signal to reorganise — call this out rather than only the symptom.
6. Preserve the exception. Keep the advanced construct when it is genuinely required and a simpler form has been shown to be insufficient.

Inputs

• The class(es) under review, where the behaviour that operates on their data lives, and whether any advanced construct in use has a stated, demonstrated need.

Output

A review/advise finding per smell: the class or construct, why it is a smell (scattered behaviour; needless abstraction), and the minimal redesign (move behaviour in; drop to a plain class) — with the scaffolding/IR and genuinely-required exceptions preserved. Traced to P05 or P13. A finding that implies a larger redesign is handed to a design discussion rather than patched inline.

References

• pythonic-review-checklist — the data-class and keep-it-simple rows.

Provenance

Derived from principles P05 (claim C005, evidence E005; Fowler's Data Class smell as reproduced in Fluent Python, anchor luciano-ramalho-flue-ca307a52-h0813) and P13 (claim C014, evidence E014; Python Distilled's "keep it simple" design advice, anchor python-distilled-pea-1baf485f-h0280). Distillation-only sources: paraphrased, no verbatim quotation.