What are __slots__?

__slots__ declares a fixed set of attribute names for instances. CPython then stores those attributes in a compact structure instead of a per-instance __dict__ (by default), which can save memory when you create many instances.

Effects:

• Faster attribute access and lower memory for large populations of objects.
• You cannot assign arbitrary new attributes unless you also include __dict__ in slots.
• Inheritance needs care: subclasses may need their own __slots__.
• Some features (weakrefs) need explicit __weakref__ in slots.

class PointDict:
    def __init__(self, x, y):
        self.x = x
        self.y = y

class PointSlots:
    __slots__ = ("x", "y")

    def __init__(self, x, y):
        self.x = x
        self.y = y

p = PointSlots(1, 2)
print(p.x, p.y)
# p.z = 3   # AttributeError: 'PointSlots' object has no attribute 'z'
# print(p.__dict__)  # AttributeError (no instance dict)

When to use: Millions of simple objects (graph nodes, particles, rows). Skip for most app code; clarity beats micro-optimization until profiling says otherwise.