Review inheritance and composition

Purpose

Hold a class hierarchy to the is-a test — inheritance only for a true specialization, composition and delegation when one object merely uses another — and flag the specific trap of subclassing a built-in container or string type. (P07, P08)

When to use

• A design uses or proposes subclassing to reuse another class's functionality.
• Code subclasses a built-in dict, list, or str and overrides its dunder methods.

Do not apply when the subclass is a genuine behavioural specialization (is-a) of its parent, or when no built-in type is being subclassed (or the collections.User* base classes are already used).

Procedure

1. Apply the is-a versus uses-a test. For each subclass, ask whether the new object is genuinely a specialized kind of its parent (is-a) or merely uses the parent as a component (uses-a / has-a). (C008)
2. Recommend composition for uses-a. Where the relationship is uses-a, recommend composition and delegation instead of inheritance — it gives looser coupling and hides the component's implementation detail. Leave true is-a specializations as inheritance.
3. Flag subclassing of built-ins. Flag any class that subclasses built-in dict, list, or str and overrides dunder methods. Their methods are implemented in C and bypass the overrides — for example, dict.update does not route through a redefined __setitem__ — so the subclass only appears to work. (C009)
4. Give the safe alternative. Recommend collections.UserDict, UserList, or UserString (whose Python-level methods do route through the overrides), or composition wrapping the built-in.

Inputs

• The class hierarchy under review, the relationship each subclass has to its parent, and whether any built-in type is being subclassed with overridden behaviour.

Output

A review/compare finding per issue: the subclass, whether it passes the is-a test, and the recommendation (composition+delegation for uses-a; User*/composition for a subclassed built-in), traced to P07 or P08. A finding that implies replacing a hierarchy with composition is handed to a design discussion rather than patched inline.

References

• pythonic-review-checklist — the inheritance/composition and built-in-subclass rows.

Provenance

Derived from principles P07 (claim C008, evidence E008) and P08 (claim C009, evidence E009), grounded in Python Distilled's object-oriented design sections (anchors python-distilled-pea-1baf485f-h0207, python-distilled-pea-1baf485f-h0210). Distillation-only source: paraphrased, no verbatim quotation.