Data update

Task/Constrained-genericity/Python/constrained-genericity.py

@@ -0,0 +1,42 @@
+"""Constrained genericity. Requires Python >= 3.9."""
+
+from typing import Generic
+from typing import Protocol
+from typing import TypeVar
+from typing import runtime_checkable
+
+T = TypeVar("T", covariant=True)
+
+
+@runtime_checkable
+class Edible(Protocol[T]):
+    def eat(self) -> T: ...
+
+
+class FoodBox(Generic[T]):
+    def __init__(self, *food: Edible[T]):
+        # Runtime type checking
+        for item in food:
+            if not isinstance(item, Edible):
+                raise TypeError(f"expected food, found {item.__class__.__name__}")
+
+        self.contents = food
+
+
+class Cheese:
+    def eat(self) -> None:
+        print("eating cheese")
+
+
+class Shoe:
+    def wear(self) -> None:
+        print("wearing shoe")
+
+
+if __name__ == "__main__":
+    box = FoodBox[None](Cheese())
+    for food in box.contents:
+        food.eat()
+
+    # This fails static type checking.
+    box = FoodBox[None](Cheese(), Shoe())