Python slots Documentation: Complete 2026 Guide

Unlock Python __slots__ documentation essentials in 2026. This powerful attribute optimizes memory usage in classes, speeding up attribute access by 3-5x compared to standard dict-based instances.

Ideal for data-heavy apps like ML models or games, __slots__ prevents dynamic attributes, enforcing structure. Our guide covers syntax, benefits, pitfalls, and best practices from official docs.

Step 1: Basic Syntax and Declaration

Declare __slots__ as a class attribute listing allowed names. No dict created for instances.

class Point: __slots__ = ['x', 'y']
p = Point(); p.x = 1 # Fast access

Step 2: Memory Savings Explained

Instances use fixed slots array, slashing overhead. Test with sys.getsizeof().

1. Without slots: ~56 bytes overhead
2. With slots: ~32 bytes
3. Scale to millions: GBs saved

Step 3: Inheritance and Limitations

Subclasses must declare own __slots__. Can't add dynamic attrs post-slots.

1. class Child(Point): __slots__ = ['z']
2. No __dict__ or __weakref__ by default
3. Use tuple for slots

Step 4: Advanced Usage in 2026 Libraries

NumPy, dataclasses integrate __slots__. Pickle support requires care.

1. @dataclass(slots=True)
2. Custom __getattr__ hacks avoided
3. Profiling tools confirm speed

Step 5: Common Errors and Fixes

Avoid modifying slots post-init. Debug with inspect module.

1. AttributeError on missing slots
2. Extend via subclassing
3. Tools: memory_profiler

Best Practices from Python Docs

Use for value objects in perf-critical codebases.

Immutable slots with properties
Combine with typing
2026: Async class optimizations

Python __slots__ Documentation: Complete 2026 Guide