ch15: draft examples

15-more-types/petbox/petbox.py

@@ -0,0 +1,47 @@
+from typing import TypeVar, Generic, Any
+
+
+class Pet:
+    """Domestic animal kept for companionship."""
+
+
+class Dog(Pet):
+    """Canis familiaris"""
+
+
+class Cat(Pet):
+    """Felis catus"""
+
+
+class Siamese(Cat):
+    """Cat breed from Thailand"""
+
+
+T = TypeVar('T')
+
+
+class Box(Generic[T]):
+    def put(self, item: T) -> None:
+        self.contents = item
+
+    def get(self) -> T:
+        return self.contents
+
+
+T_contra = TypeVar('T_contra', contravariant=True)
+
+
+class InBox(Generic[T_contra]):
+    def put(self, item: T) -> None:
+        self.contents = item
+
+
+T_co = TypeVar('T_co', covariant=True)
+
+
+class OutBox(Generic[T_co]):
+    def __init__(self, contents: Any):
+        self.contents = contents
+
+    def get(self) -> Any:
+        return self.contents

15-more-types/petbox/petbox_demo.py

@@ -0,0 +1,42 @@
+from typing import TYPE_CHECKING
+
+from petbox import *
+
+
+cat_box: Box[Cat] = Box()
+
+si = Siamese()
+
+cat_box.put(si)
+
+animal = cat_box.get()
+
+#if TYPE_CHECKING:
+#    reveal_type(animal)  # Revealed: petbox.Cat*
+
+
+################### Covariance
+
+out_box: OutBox[Cat] = OutBox(Cat())
+
+out_box_si: OutBox[Siamese] = OutBox(Siamese())
+
+## Incompatible types in assignment
+##   expression has type "OutBox[Cat]"
+#      variable has type "OutBox[Siamese]"
+# out_box_si = out_box
+
+out_box = out_box_si
+
+################### Contravariance
+
+in_box: InBox[Cat] = InBox()
+
+in_box_si: InBox[Siamese] = InBox()
+
+in_box_si = in_box
+
+## Incompatible types in assignment
+##   expression has type "InBox[Siamese]"
+##     variable has type "InBox[Cat]"
+# in_box = in_box_si