added cafeteria example

15-more-types/cafeteria.py

@@ -0,0 +1,82 @@
+from typing import TypeVar, Generic
+
+
+class Beverage:
+    """Any beverage"""
+
+
+class Juice(Beverage):
+    """Any fruit juice"""
+
+
+class OrangeJuice(Juice):
+    """Delicious juice from Brazilian oranges"""
+
+
+BeverageT = TypeVar('BeverageT', covariant=True)
+
+
+class BeverageDispenser(Generic[BeverageT]):
+    def __init__(self, beverage: BeverageT) -> None:
+        self.beverage = beverage
+
+    def dispense(self) -> BeverageT:
+        return self.beverage
+
+
+class Garbage:
+    """Any garbage."""
+
+
+class Biodegradable(Garbage):
+    """Biodegradable garbage."""
+
+
+class Compostable(Biodegradable):
+    """Compostable garbage."""
+
+
+GarbageT = TypeVar('GarbageT', contravariant=True)
+
+
+class TrashCan(Generic[GarbageT]):
+    def put(self, trash) -> None:
+        """Store trash until dumped..."""
+
+
+class Cafeteria:
+    def __init__(
+        self,
+        dispenser: BeverageDispenser[Juice],
+        trash_can: TrashCan[Biodegradable]
+    ):
+        """Initialize..."""
+
+
+beverage_dispenser = BeverageDispenser(Beverage())
+juice_dispenser = BeverageDispenser(Juice())
+orange_juice_dispenser = BeverageDispenser(OrangeJuice())
+
+trash_can: TrashCan[Garbage] = TrashCan()
+bio_can: TrashCan[Biodegradable] = TrashCan()
+compost_can: TrashCan[Compostable] = TrashCan()
+
+arnold_hall = Cafeteria(juice_dispenser, bio_can)
+
+######################## covariance on 1st argument
+arnold_hall = Cafeteria(orange_juice_dispenser, trash_can)
+
+## Argument 1 to "Cafeteria" has
+## incompatible type "BeverageDispenser[Beverage]"
+##          expected "BeverageDispenser[Juice]"
+# arnold_hall = Cafeteria(beverage_dispenser, trash_can)
+
+
+######################## contravariance on 2nd argument
+
+## Argument 2 to "Cafeteria" has
+## incompatible type "TrashCan[Compostable]"
+##          expected "TrashCan[Biodegradable]"
+# arnold_hall = Cafeteria(juice_dispenser, compost_can)
+
+arnold_hall = Cafeteria(juice_dispenser, trash_can)