__slots__ in Python: What It Is and When to Use It

__slots__ is a special attribute in Python classes that limits the attributes an instance can have, saving memory by avoiding the creation of a dynamic dictionary for each object. It helps make objects smaller and faster by fixing the allowed attributes at class creation.

How It Works

Normally, Python stores an object's attributes in a dictionary called __dict__. This dictionary allows you to add or change attributes freely, like a flexible toolbox. But this flexibility uses extra memory for each object.

When you define __slots__ in a class, you tell Python exactly which attributes the objects of that class can have. This is like giving each object a fixed set of labeled boxes instead of a big toolbox. Because Python knows the exact attributes ahead of time, it doesn't need to create a dictionary for each object, saving memory and sometimes making attribute access faster.

Example

This example shows a class with and without __slots__. The class with __slots__ only allows the attributes listed and uses less memory.

python

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

class PersonWithSlots:
    __slots__ = ['name', 'age']
    def __init__(self, name, age):
        self.name = name
        self.age = age

p1 = PersonWithoutSlots('Alice', 30)
p2 = PersonWithSlots('Bob', 25)

print(hasattr(p1, '__dict__'))  # True
print(hasattr(p2, '__dict__'))  # False

# Trying to add a new attribute not in __slots__ raises an error
try:
    p2.height = 170
except AttributeError as e:
    print(e)

Output

True
False
'PersonWithSlots' object has no attribute 'height'

When to Use

Use __slots__ when you create many instances of a class and want to save memory. This is common in programs that handle large data sets or many objects, like games, simulations, or data processing.

However, __slots__ restricts flexibility: you cannot add new attributes not listed in __slots__, and some Python features like multiple inheritance can be tricky with it. So use it when memory is important and your class design is stable.

Key Points

• __slots__ saves memory by preventing the creation of __dict__ for each instance.
• It fixes the set of allowed attributes, disallowing new ones.
• It can make attribute access slightly faster.
• It is best for classes with many instances and stable attribute sets.
• It limits flexibility and can complicate inheritance.

Key Takeaways

__slots__ reduces memory use by fixing allowed attributes in a class.

It disables the dynamic attribute dictionary, so you cannot add new attributes at runtime.

Use it when you have many objects and want to optimize memory and speed.

Avoid __slots__ if you need flexible or complex class designs.

It can improve performance but requires careful planning of class attributes.