Issue #18244: Adopt C3-based linearization in functools.singledispatch for improved ABC support

Lib/test/test_functools.py

@@ -929,22 +929,55 @@ class TestSingleDispatch(unittest.TestCase):
         self.assertEqual(g(rnd), ("Number got rounded",))
 
     def test_compose_mro(self):
+        # None of the examples in this test depend on haystack ordering.
         c = collections
         mro = functools._compose_mro
         bases = [c.Sequence, c.MutableMapping, c.Mapping, c.Set]
         for haystack in permutations(bases):
             m = mro(dict, haystack)
-            self.assertEqual(m, [dict, c.MutableMapping, c.Mapping, object])
+            self.assertEqual(m, [dict, c.MutableMapping, c.Mapping, c.Sized,
+                                 c.Iterable, c.Container, object])
         bases = [c.Container, c.Mapping, c.MutableMapping, c.OrderedDict]
         for haystack in permutations(bases):
             m = mro(c.ChainMap, haystack)
             self.assertEqual(m, [c.ChainMap, c.MutableMapping, c.Mapping,
                                  c.Sized, c.Iterable, c.Container, object])
-        # Note: The MRO order below depends on haystack ordering.
-        m = mro(c.defaultdict, [c.Sized, c.Container, str])
-        self.assertEqual(m, [c.defaultdict, dict, c.Container, c.Sized, object])
-        m = mro(c.defaultdict, [c.Container, c.Sized, str])
-        self.assertEqual(m, [c.defaultdict, dict, c.Sized, c.Container, object])
+
+        # If there's a generic function with implementations registered for
+        # both Sized and Container, passing a defaultdict to it results in an
+        # ambiguous dispatch which will cause a RuntimeError (see
+        # test_mro_conflicts).
+        bases = [c.Container, c.Sized, str]
+        for haystack in permutations(bases):
+            m = mro(c.defaultdict, [c.Sized, c.Container, str])
+            self.assertEqual(m, [c.defaultdict, dict, c.Sized, c.Container,
+                                 object])
+
+        # MutableSequence below is registered directly on D. In other words, it
+        # preceeds MutableMapping which means single dispatch will always
+        # choose MutableSequence here.
+        class D(c.defaultdict):
+            pass
+        c.MutableSequence.register(D)
+        bases = [c.MutableSequence, c.MutableMapping]
+        for haystack in permutations(bases):
+            m = mro(D, bases)
+            self.assertEqual(m, [D, c.MutableSequence, c.Sequence,
+                                 c.defaultdict, dict, c.MutableMapping,
+                                 c.Mapping, c.Sized, c.Iterable, c.Container,
+                                 object])
+
+        # Container and Callable are registered on different base classes and
+        # a generic function supporting both should always pick the Callable
+        # implementation if a C instance is passed.
+        class C(c.defaultdict):
+            def __call__(self):
+                pass
+        bases = [c.Sized, c.Callable, c.Container, c.Mapping]
+        for haystack in permutations(bases):
+            m = mro(C, haystack)
+            self.assertEqual(m, [C, c.Callable, c.defaultdict, dict, c.Mapping,
+                                 c.Sized, c.Iterable, c.Container, object])
 
     def test_register_abc(self):
         c = collections
@@ -1040,17 +1073,37 @@ class TestSingleDispatch(unittest.TestCase):
         self.assertEqual(g(f), "frozen-set")
         self.assertEqual(g(t), "tuple")
 
+    def test_c3_abc(self):
+        c = collections
+        mro = functools._c3_mro
+        class A(object):
+            pass
+        class B(A):
+            def __len__(self):
+                return 0   # implies Sized
+        @c.Container.register
+        class C(object):
+            pass
+        class D(object):
+            pass   # unrelated
+        class X(D, C, B):
+            def __call__(self):
+                pass   # implies Callable
+        expected = [X, c.Callable, D, C, c.Container, B, c.Sized, A, object]
+        for abcs in permutations([c.Sized, c.Callable, c.Container]):
+            self.assertEqual(mro(X, abcs=abcs), expected)
+        # unrelated ABCs don't appear in the resulting MRO
+        many_abcs = [c.Mapping, c.Sized, c.Callable, c.Container, c.Iterable]
+        self.assertEqual(mro(X, abcs=many_abcs), expected)
+
     def test_mro_conflicts(self):
         c = collections
-
         @functools.singledispatch
         def g(arg):
             return "base"
-
         class O(c.Sized):
             def __len__(self):
                 return 0
-
         o = O()
         self.assertEqual(g(o), "base")
         g.register(c.Iterable, lambda arg: "iterable")
@@ -1062,35 +1115,114 @@ class TestSingleDispatch(unittest.TestCase):
         self.assertEqual(g(o), "sized")   # because it's explicitly in __mro__
         c.Container.register(O)
         self.assertEqual(g(o), "sized")   # see above: Sized is in __mro__
-
+        c.Set.register(O)
+        self.assertEqual(g(o), "set")     # because c.Set is a subclass of
+                                          # c.Sized and c.Container
         class P:
             pass
-
         p = P()
         self.assertEqual(g(p), "base")
         c.Iterable.register(P)
         self.assertEqual(g(p), "iterable")
         c.Container.register(P)
-        with self.assertRaises(RuntimeError) as re:
+        with self.assertRaises(RuntimeError) as re_one:
             g(p)
-            self.assertEqual(
-                str(re),
-                ("Ambiguous dispatch: <class 'collections.abc.Container'> "
-                    "or <class 'collections.abc.Iterable'>"),
-            )
-
+        self.assertIn(
+            str(re_one.exception),
+            (("Ambiguous dispatch: <class 'collections.abc.Container'> "
+              "or <class 'collections.abc.Iterable'>"),
+             ("Ambiguous dispatch: <class 'collections.abc.Iterable'> "
+              "or <class 'collections.abc.Container'>")),
+        )
         class Q(c.Sized):
             def __len__(self):
                 return 0
-
         q = Q()
         self.assertEqual(g(q), "sized")
         c.Iterable.register(Q)
         self.assertEqual(g(q), "sized")   # because it's explicitly in __mro__
         c.Set.register(Q)
         self.assertEqual(g(q), "set")     # because c.Set is a subclass of
-                                          # c.Sized which is explicitly in
-                                          # __mro__
+                                          # c.Sized and c.Iterable
+        @functools.singledispatch
+        def h(arg):
+            return "base"
+        @h.register(c.Sized)
+        def _(arg):
+            return "sized"
+        @h.register(c.Container)
+        def _(arg):
+            return "container"
+        # Even though Sized and Container are explicit bases of MutableMapping,
+        # this ABC is implicitly registered on defaultdict which makes all of
+        # MutableMapping's bases implicit as well from defaultdict's
+        # perspective.
+        with self.assertRaises(RuntimeError) as re_two:
+            h(c.defaultdict(lambda: 0))
+        self.assertIn(
+            str(re_two.exception),
+            (("Ambiguous dispatch: <class 'collections.abc.Container'> "
+              "or <class 'collections.abc.Sized'>"),
+             ("Ambiguous dispatch: <class 'collections.abc.Sized'> "
+              "or <class 'collections.abc.Container'>")),
+        )
+        class R(c.defaultdict):
+            pass
+        c.MutableSequence.register(R)
+        @functools.singledispatch
+        def i(arg):
+            return "base"
+        @i.register(c.MutableMapping)
+        def _(arg):
+            return "mapping"
+        @i.register(c.MutableSequence)
+        def _(arg):
+            return "sequence"
+        r = R()
+        self.assertEqual(i(r), "sequence")
+        class S:
+            pass
+        class T(S, c.Sized):
+            def __len__(self):
+                return 0
+        t = T()
+        self.assertEqual(h(t), "sized")
+        c.Container.register(T)
+        self.assertEqual(h(t), "sized")   # because it's explicitly in the MRO
+        class U:
+            def __len__(self):
+                return 0
+        u = U()
+        self.assertEqual(h(u), "sized")   # implicit Sized subclass inferred
+                                          # from the existence of __len__()
+        c.Container.register(U)
+        # There is no preference for registered versus inferred ABCs.
+        with self.assertRaises(RuntimeError) as re_three:
+            h(u)
+        self.assertIn(
+            str(re_three.exception),
+            (("Ambiguous dispatch: <class 'collections.abc.Container'> "
+              "or <class 'collections.abc.Sized'>"),
+             ("Ambiguous dispatch: <class 'collections.abc.Sized'> "
+              "or <class 'collections.abc.Container'>")),
+        )
+        class V(c.Sized, S):
+            def __len__(self):
+                return 0
+        @functools.singledispatch
+        def j(arg):
+            return "base"
+        @j.register(S)
+        def _(arg):
+            return "s"
+        @j.register(c.Container)
+        def _(arg):
+            return "container"
+        v = V()
+        self.assertEqual(j(v), "s")
+        c.Container.register(V)
+        self.assertEqual(j(v), "container")   # because it ends up right after
+                                              # Sized in the MRO
 
     def test_cache_invalidation(self):
         from collections import UserDict