Unlock Massive Memory Savings in Python with __slots__: A Deep Dive

Introduction

Many Python programs suffer from high memory usage when creating large numbers of objects. This article introduces the __slots__ magic attribute that can dramatically reduce memory consumption and speed up attribute access.

Memory Overhead of Regular Classes

By default each instance stores attributes in a __dict__, which adds significant overhead.

class RegularUser:
    def __init__(self, name, age, email):
        self.name = name
        self.age = age
        self.email = email

users = [RegularUser(f"user{i}", i % 100, f"user{i}@example.com") for i in range(1000000)]

Using __slots__

Defining __slots__ removes the per‑instance __dict__ and fixes the set of attributes.

class SlotsUser:
    __slots__ = ['name', 'age', 'email']

    def __init__(self, name, age, email):
        self.name = name
        self.age = age
        self.email = email

Memory measurement shows a reduction from ~328 bytes per object to ~96 bytes, a saving of about 70 %.

Performance Test

Attribute access on a slots class is noticeably faster.

def test_access_speed():
    regular = RegularUser("test", 25, "[email protected]")
    slots = SlotsUser("test", 25, "[email protected]")
    # timing loops omitted for brevity

When to Use __slots__

Creating millions of objects (e.g., game entities, data records).

Memory‑sensitive applications.

Performance‑critical attribute access.

Limitations

Cannot add attributes dynamically.

Inheritance requires careful handling of __slots__.

Incompatible with features that rely on __dict__ (e.g., some decorators, weak references).

Best Practices

Declare all attributes in __slots__.

Use explicit inheritance to extend slots.

Provide clear error messages for illegal attribute assignments.

Example: Game Development

class GameEntity:
    __slots__ = ['x', 'y', 'width', 'height', 'health', 'speed', 'texture']
    def __init__(self, x, y, width, height):
        self.x = x
        self.y = y
        self.width = width
        self.height = height
        self.health = 100
        self.speed = 1.0
        self.texture = None

Creating 10 000 entities saves roughly 2 MB of memory.

When Not to Use __slots__

When you need to add attributes at runtime.

When using libraries that expect a __dict__.

When the number of objects is small.

During early development when requirements may change.