What is the __class_getitem__ method?

Question

Accepted Answer

**__class_getitem__** allows **generic type syntax** with custom classes.

```python
from typing import TypeVar, Generic

K = TypeVar("K")
V = TypeVar("V")

class Registry(Generic[K, V]):
    def __init__(self):
        self._data: dict[K, V] = {}

def set(self, key: K, value: V):
        self._data[key] = value

def get(self, key: K) -> V | None:
        return self._data.get(key)

# Usage with type hints
reg: Registry[str, int] = Registry()
reg.set("count", 42)

# Without Generic — manual __class_getitem__
class Matrix:
    def __init__(self, rows, cols):
        self.rows = rows
        self.cols = cols

def __class_getitem__(cls, params):
        rows, cols = params
        return type(f"Matrix{rows}x{cols}", (cls,), {
            "rows": rows,
            "rows": cols
        })

Matrix3x3 = Matrix[3, 3]
```

**Use cases:**
- Generic collections.
- Type-safe containers.
- API design.

Python: What is the __class_getitem__ method?