What is the `__slots__` attribute?

Question

Accepted Answer

**__slots__** — restrict instance attributes to a fixed set, reducing memory.

```python
class PointWithSlots:
    __slots__ = ("x", "y")

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

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

# Memory comparison
import sys

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

r = Regular(1, 2)
s = PointWithSlots(1, 2)
print(sys.getsizeof(r.__dict__))  # ~104 bytes
# s has no __dict__ attribute
```

**Benefits:**
- Lower memory per instance (~40% savings).
- Faster attribute access.
- Prevents accidental attribute creation.

**Trade-offs:**
- Cannot add attributes after creation.
- Incompatible with multiple inheritance (sometimes).

Python: What is the `slots` attribute?