What is the __class_getitem__ for generic types?

Question

Accepted Answer

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

```python
from typing import TypeVar, Generic

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

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

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

def __getitem__(self, key: K) -> V:
        return self._data[key]

# Type-safe usage
users: TypedDict[str, int] = TypedDict()
users["alice"] = 25  # OK
# users["alice"] = "twenty-five"  # Type error!

# Custom class with __class_getitem__
class Matrix:
    def __init__(self, rows: int, cols: int):
        self.data = [[0] * cols for _ in range(rows)]

def __class_getitem__(cls, params):
        rows, cols = params
        class TypedMatrix(cls):
            pass
        TypedMatrix.rows = rows
        TypedMatrix.cols = cols
        return TypedMatrix

Matrix3x3 = Matrix[3, 3]
m = Matrix3x3()  # rows=3, cols=3
```

**Benefits:**
- Type-safe generics.
- Better IDE support.
- Self-documenting code.

Python: What is the __class_getitem__ for generic types?