__slots__ in Classes — Saving Memory

A common scenario is building classes for data models.
By default, each instance stores attributes in a dictionary, which takes memory.
With __slots__, you can restrict attributes and reduce memory usage.

Without slots

import sys

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

p = Person("Alice", 30)
print(sys.getsizeof(p.__dict__))

# size of underlying dict

With slots

class PersonSlots:
    __slots__ = ("name", "age")

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

p = PersonSlots("Bob", 25)
print(hasattr(p, "__dict__"))

# False

Why use slots?

• Saves memory for large numbers of objects.
• Prevents creation of new attributes not listed.
• Slight speed improvement in attribute access.

Conclusion

Use __slots__ in performance-sensitive cases with many instances.
For general use, normal classes are more flexible.