Slots Class: How to Reduce Memory Usage in Python

Use __slots__ to prevent dynamic attribute creation and reduce per-instance memory overhead, while keeping methods intact.

Easy Python 3.9+ Aug 9, 2026 OOP & classes 12 views 0 copies

memory slots class performance oop

Python code

16 lines

Python 3.9+

class SlotsDemo:
    __slots__ = ("name", "age", "email")

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

    def describe(self):
        return f"{self.name}, {self.age}, {self.email}"

if __name__ == "__main__":
    instance = SlotsDemo("Alice", 30, "alice@example.com")
    print(instance.describe())
    print(f"Instance dict exists: {'__dict__' in dir(instance)}")
    print(f"Total size with slots: {instance.__sizeof__()} bytes")

Output

stdout

Alice, 30, alice@example.com
Instance dict exists: False
Total size with slots: 48 bytes

How it works

By defining __slots__, Python allocates a fixed-size array for the attributes listed, instead of a per-instance __dict__. This eliminates the overhead of a dictionary, which stores attribute names as strings and values, and reduces memory usage significantly, especially when creating many instances. The downside is that you can't assign attributes that are not in __slots__, preventing accidental typos and enforcing a strict attribute set. This approach is ideal for classes that act as data containers or value objects.

Common mistakes

Forgetting that __slots__ replaces __dict__, so dynamic attribute assignment fails.
Not including '__weakref__' in __slots__ if you need weak references to instances.
Defining __slots__ with a string instead of a tuple or list, which is allowed but less explicit.

Variations

Use a list instead of a tuple for __slots__, e.g., __slots__ = ['name', 'age'].
Add '__weakref__' to __slots__ to support weak references while keeping the memory savings.

Real-world use cases

Storing millions of rows from a database as lightweight record objects in a data pipeline.
Implementing value objects in a domain model where immutability and low overhead matter.
Representing game entities or simulation particles where thousands of instances are alive.

Slots Class: How to Reduce Memory Usage in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

More from OOP & classes

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