Review slots and memory

Purpose

Treat __slots__ as a targeted memory optimization, recommended only for classes instantiated in very large numbers with a fixed attribute set, and always reported with its trade-offs — not as a default. (P12)

When to use

• A class is instantiated in very large numbers with a fixed, known set of attributes, and memory use is a real concern.

Do not recommend __slots__ when instance counts are small, the attribute set is dynamic, or the class relies on inheritance or on the instance __dict__.

Procedure

1. Confirm the qualifying conditions. Recommend __slots__ only when both hold: the class is instantiated in large numbers, and its attribute set is fixed and known. By default an instance stores attributes in a per-instance __dict__; __slots__ replaces that dict with a compact array, which can cut memory substantially at scale. (C013)
2. State the trade-offs alongside the benefit. Always report that __slots__ interacts badly with inheritance (especially multiple inheritance) and removes the instance __dict__, so dynamic attribute assignment and code that relies on __dict__ stop working. Present it as a targeted optimization, not a reflex.
3. Do not recommend it outside scope. For small instance counts, dynamic attributes, or classes built on inheritance / __dict__, advise against __slots__.
4. Keep memory in scope, not runtime speed. This is a memory optimization; it is not a substitute for runtime/algorithmic performance work, which is out of this reviewer's scope.

Inputs

• The class under review, its approximate instance count at runtime, whether its attribute set is fixed, and whether it participates in inheritance or relies on __dict__.

Output

An advise/review finding: whether __slots__ is warranted here, the memory benefit, and the inheritance/__dict__ trade-offs stated alongside — never a bare "add __slots__". Traced to P12.

References

• pythonic-review-checklist — the __slots__ row.

Provenance

Derived from principle P12 (claim C013, evidence E013; medium confidence), grounded in Python Distilled's discussion of __slots__ and its trade-offs (anchor python-distilled-pea-1baf485f-h0260). Distillation-only source: paraphrased, no verbatim quotation.