# Memory Optimization with __slots__ in Python

**Domain Paper: Python Performance ADRs**
**Date:** 2026-01-03
**Source:** python-numbers-everyone-should-know benchmarks

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## Executive Summary

Python's `__slots__` mechanism provides **52-70% memory reduction** when creating many instances of the same class.

**Key Finding**: For a class with 5 attributes, `__slots__` reduces instance memory from 694 bytes to 212 bytes (69% reduction).

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## Benchmark Results: Memory Footprint

### Single Instance Memory (5 Attributes)

| Type | Memory (bytes) | vs Regular Class |
|------|----------------|------------------|
| Regular class | 694 | baseline |
| `__slots__` class | 212 | -69% |
| dataclass | 694 | same as regular |
| `@dataclass(slots=True)` | 212 | -69% |
| namedtuple | 228 | -67% |

### At Scale (1,000 Instances)

| Type | Total Memory |
|------|--------------|
| List of 1,000 regular class | 165.2 KB |
| List of 1,000 `__slots__` class | 79.1 KB |

**Memory Savings**: 52% reduction at scale

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## Attribute Access Speed (Virtually Identical)

| Operation | Regular | `__slots__` |
|-----------|---------|-------------|
| Read attr | 14.1 ns | 14.1 ns |
| Write attr | 15.7 ns | 16.4 ns |

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## Trade-offs

### What __slots__ Prevents

1. **No dynamic attribute assignment**
2. **No `__dict__` access** (`vars()` doesn't work)
3. **Inheritance complications**
4. **No weak references by default**

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## Practical Rules for Coding Agents

### Rule 1: Instance Count Threshold
```
IF creating > 100 instances of the same class
AND attributes are fixed at design time
THEN consider __slots__ or @dataclass(slots=True)
```

### Rule 2: Prefer Slots Dataclass (Python 3.10+)
```python
@dataclass(slots=True)
class User:
    id: int
    name: str
    email: str
```

### Rule 3: Don't Optimize Prematurely
For < 100 instances, use regular classes for flexibility.

### Rule 4: Document the Trade-off
```python
# Using __slots__ for memory efficiency (1000+ instances expected)
__slots__ = ['x', 'y', 'z']
```

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## Summary

| Aspect | Regular Class | `__slots__` Class |
|--------|---------------|-------------------|
| Memory (5 attrs) | 694 bytes | 212 bytes |
| Read speed | 14.1 ns | 14.1 ns |
| Dynamic attributes | Yes | No |
| Best for | Flexibility | Many instances |

**Bottom Line**: Use `__slots__` (or `@dataclass(slots=True)`) when creating many instances of fixed-attribute classes. For small numbers, stick with regular classes.

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*Benchmark source: python-numbers-everyone-should-know*