updates to ch.14

14-inheritance/diamond2.py

@@ -0,0 +1,67 @@
+"""
+unrelated.py: examples with ``super()`` in a sibling class.
+
+``U`` is unrelated (does not subclass ``Root``)
+
+Calling ``ping`` on an instance of ``U`` fails::
+
+# tag::UNRELATED_DEMO_1[]
+    >>> u = U()
+    >>> u.ping()
+    Traceback (most recent call last):
+      ...
+    AttributeError: 'super' object has no attribute 'ping'
+
+# end::UNRELATED_DEMO_1[]
+
+
+But if ``U`` is part of a cooperative arrangement of base classes,
+its ``ping`` method works::
+
+# tag::UNRELATED_DEMO_2[]
+
+    >>> leaf2 = LeafUA()
+    >>> leaf2.ping()
+    <instance of LeafUA>.ping() in LeafUA
+    <instance of LeafUA>.ping() in U
+    <instance of LeafUA>.ping() in A
+    <instance of LeafUA>.ping() in Root
+    >>> LeafUA.__mro__  # doctest:+NORMALIZE_WHITESPACE
+    (<class 'diamond2.LeafUA'>, <class 'diamond2.U'>,
+     <class 'diamond.A'>, <class 'diamond.Root'>, <class 'object'>)
+
+# end::UNRELATED_DEMO_2[]
+
+
+Here ``U.ping`` is never called because ``Root.ping`` does not call ``super``.
+
+    >>> o6 = LeafAU()
+    >>> o6.ping()
+    <instance of LeafAU>.ping() in LeafAU
+    <instance of LeafAU>.ping() in A
+    <instance of LeafAU>.ping() in Root
+    >>> LeafAU.__mro__  # doctest:+NORMALIZE_WHITESPACE
+    (<class 'diamond2.LeafAU'>, <class 'diamond.A'>, <class 'diamond.Root'>,
+     <class 'diamond2.U'>, <class 'object'>)
+
+"""
+
+# tag::DIAMOND_CLASSES[]
+from diamond import A  # <1>
+
+class U():  # <2>
+    def ping(self):
+        print(f'{self}.ping() in U')
+        super().ping()  # <3>
+
+class LeafUA(U, A):  # <4>
+    def ping(self):
+        print(f'{self}.ping() in LeafUA')
+        super().ping()
+# end::DIAMOND_CLASSES[]
+
+class LeafAU(A, U):
+    def ping(self):
+        print(f'{self}.ping() in LeafAU')
+        super().ping()
+