python · dolfinus · Mar 30, 2025 · Mar 31, 2025 · Mar 31, 2025 · Mar 31, 2025
diff --git a/Lib/_py_abc.py b/Lib/_py_abc.py
@@ -49,6 +49,7 @@ def __new__(mcls, name, bases, namespace, /, **kwargs):
         cls._abc_cache = WeakSet()
         cls._abc_negative_cache = WeakSet()
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
+        cls._abc_issubclasscheck_recursive = False
         return cls
 
     def register(cls, subclass):
@@ -66,7 +67,8 @@ def register(cls, subclass):
             # This would create a cycle, which is bad for the algorithm below
             raise RuntimeError("Refusing to create an inheritance cycle")
         cls._abc_registry.add(subclass)
-        ABCMeta._abc_invalidation_counter += 1  # Invalidate negative cache
+        # Invalidate negative cache
+        ABCMeta._abc_invalidation_counter += 1
         return subclass
 
     def _dump_registry(cls, file=None):
@@ -137,11 +139,29 @@ def __subclasscheck__(cls, subclass):
             if issubclass(subclass, rcls):
                 cls._abc_cache.add(subclass)
                 return True
+
         # Check if it's a subclass of a subclass (recursive)
         for scls in cls.__subclasses__():
-            if issubclass(subclass, scls):
-                cls._abc_cache.add(subclass)
+            # If inside recursive issubclass check, avoid adding classes to any cache because this
+            # may drastically increase memory usage.
+            # Unfortunately, issubclass/__subclasscheck__ don't accept third argument with context,
+            # so using global context within ABCMeta.
+            # This is done only on first method call, others will use cached result.
+            scls_is_abc = hasattr(scls, "_abc_issubclasscheck_recursive")
+            if scls_is_abc:
+                scls._abc_issubclasscheck_recursive = True
+
+            try:
+                result = issubclass(subclass, scls)
+            finally:
+                if scls_is_abc:
+                    scls._abc_issubclasscheck_recursive = False
+
+            if result:
+                if not cls._abc_issubclasscheck_recursive:
+                    cls._abc_cache.add(subclass)
                 return True
-        # No dice; update negative cache
-        cls._abc_negative_cache.add(subclass)
+
+        if not cls._abc_issubclasscheck_recursive:
+            cls._abc_negative_cache.add(subclass)
         return False
diff --git a/Lib/test/test_abc.py b/Lib/test/test_abc.py
@@ -270,29 +270,100 @@ def x(self):
             class C(metaclass=meta):
                 pass
 
+        def test_isinstance_direct_inheritance(self):
+            class A(metaclass=abc_ABCMeta):
+                pass
+            class B(A):
+                pass
+            class C(A):
+                pass
+            a = A()
+            b = B()
+            c = C()
+            # trigger caching
+            for _ in range(2):
+                self.assertIsInstance(a, A)
+                self.assertIsInstance(a, (A,))
+                self.assertNotIsInstance(a, B)
+                self.assertNotIsInstance(a, (B,))
+                self.assertNotIsInstance(a, C)
+                self.assertNotIsInstance(a, (C,))
+
+                self.assertIsInstance(b, B)
+                self.assertIsInstance(b, (B,))
+                self.assertIsInstance(b, A)
+                self.assertIsInstance(b, (A,))
+                self.assertNotIsInstance(b, C)
+                self.assertNotIsInstance(b, (C,))
+
+                self.assertIsInstance(c, C)
+                self.assertIsInstance(c, (C,))
+                self.assertIsInstance(c, A)
+                self.assertIsInstance(c, (A,))
+                self.assertNotIsInstance(c, B)
+                self.assertNotIsInstance(c, (B,))
+
+                self.assertIsSubclass(B, A)
+                self.assertIsSubclass(B, (A,))
                self.assertIsSubclass(C, A)
+                self.assertIsSubclass(C, (A,))
+                self.assertNotIsSubclass(B, C)
+                self.assertNotIsSubclass(B, (C,))
+                self.assertNotIsSubclass(C, B)
+                self.assertNotIsSubclass(C, (B,))
+                self.assertNotIsSubclass(A, B)
+                self.assertNotIsSubclass(A, (B,))
+                self.assertNotIsSubclass(A, C)
+                self.assertNotIsSubclass(A, (C,))
+
         def test_registration_basics(self):
             class A(metaclass=abc_ABCMeta):
                 pass
             class B(object):
                 pass
+            a = A()
             b = B()
-            self.assertNotIsSubclass(B, A)
-            self.assertNotIsSubclass(B, (A,))
-            self.assertNotIsInstance(b, A)
-            self.assertNotIsInstance(b, (A,))
+
+            # trigger caching
+            for _ in range(2):
+                self.assertNotIsSubclass(B, A)
+                self.assertNotIsSubclass(B, (A,))
+                self.assertNotIsInstance(b, A)
+                self.assertNotIsInstance(b, (A,))
+
+                self.assertNotIsSubclass(A, B)
+                self.assertNotIsSubclass(A, (B,))
+                self.assertNotIsInstance(a, B)
+                self.assertNotIsInstance(a, (B,))
+
             B1 = A.register(B)
-            self.assertIsSubclass(B, A)
-            self.assertIsSubclass(B, (A,))
-            self.assertIsInstance(b, A)
-            self.assertIsInstance(b, (A,))
-            self.assertIs(B1, B)
+            # trigger caching
+            for _ in range(2):
+                self.assertIsSubclass(B, A)
+                self.assertIsSubclass(B, (A,))
+                self.assertIsInstance(b, A)
+                self.assertIsInstance(b, (A,))
+                self.assertIs(B1, B)
+
+                self.assertNotIsSubclass(A, B)
+                self.assertNotIsSubclass(A, (B,))
+                self.assertNotIsInstance(a, B)
+                self.assertNotIsInstance(a, (B,))
+
             class C(B):
                 pass
             c = C()
-            self.assertIsSubclass(C, A)
-            self.assertIsSubclass(C, (A,))
-            self.assertIsInstance(c, A)
-            self.assertIsInstance(c, (A,))
+            # trigger caching
+            for _ in range(2):
+                self.assertIsSubclass(C, A)
+                self.assertIsSubclass(C, (A,))
+                self.assertIsInstance(c, A)
+                self.assertIsInstance(c, (A,))
+
+                self.assertNotIsSubclass(A, C)
+                self.assertNotIsSubclass(A, (C,))
+                self.assertNotIsInstance(a, C)
+                self.assertNotIsInstance(a, (C,))
 
         def test_register_as_class_deco(self):
             class A(metaclass=abc_ABCMeta):
@@ -377,39 +448,73 @@ class A(metaclass=abc_ABCMeta):
                 pass
             self.assertIsSubclass(A, A)
             self.assertIsSubclass(A, (A,))
+
             class B(metaclass=abc_ABCMeta):
                 pass
             self.assertNotIsSubclass(A, B)
             self.assertNotIsSubclass(A, (B,))
             self.assertNotIsSubclass(B, A)
             self.assertNotIsSubclass(B, (A,))
+
             class C(metaclass=abc_ABCMeta):
                 pass
             A.register(B)
             class B1(B):
                 pass
-            self.assertIsSubclass(B1, A)
-            self.assertIsSubclass(B1, (A,))
+            # trigger caching
+            for _ in range(2):
+                self.assertIsSubclass(B1, A)
+                self.assertIsSubclass(B1, (A,))
+
             class C1(C):
                 pass
             B1.register(C1)
-            self.assertNotIsSubclass(C, B)
-            self.assertNotIsSubclass(C, (B,))
-            self.assertNotIsSubclass(C, B1)
-            self.assertNotIsSubclass(C, (B1,))
-            self.assertIsSubclass(C1, A)
-            self.assertIsSubclass(C1, (A,))
-            self.assertIsSubclass(C1, B)
-            self.assertIsSubclass(C1, (B,))
-            self.assertIsSubclass(C1, B1)
-            self.assertIsSubclass(C1, (B1,))
+            # trigger caching
+            for _ in range(2):
+                self.assertNotIsSubclass(C, B)
+                self.assertNotIsSubclass(C, (B,))
+                self.assertNotIsSubclass(C, B1)
+                self.assertNotIsSubclass(C, (B1,))
+                self.assertIsSubclass(C1, A)
+                self.assertIsSubclass(C1, (A,))
+                self.assertIsSubclass(C1, B)
+                self.assertIsSubclass(C1, (B,))
+                self.assertIsSubclass(C1, B1)
+                self.assertIsSubclass(C1, (B1,))
+
             C1.register(int)
             class MyInt(int):
                 pass
-            self.assertIsSubclass(MyInt, A)
-            self.assertIsSubclass(MyInt, (A,))
-            self.assertIsInstance(42, A)
-            self.assertIsInstance(42, (A,))
+            # trigger caching
+            for _ in range(2):
+                self.assertIsSubclass(MyInt, A)
+                self.assertIsSubclass(MyInt, (A,))
+                self.assertIsInstance(42, A)
+                self.assertIsInstance(42, (A,))
+
+        def test_custom_subclasses(self):
+            class A: pass
+            class B: pass
+
+            class Parent1(metaclass=abc_ABCMeta):
+                @classmethod
+                def __subclasses__(cls):
+                    return [A]
+
+            class Parent2(metaclass=abc_ABCMeta):
+                __subclasses__ = lambda: [A]
+
+            # trigger caching
+            for _ in range(2):
+                self.assertIsInstance(A(), Parent1)
+                self.assertIsSubclass(A, Parent1)
+                self.assertNotIsInstance(B(), Parent1)
+                self.assertNotIsSubclass(B, Parent1)
+
+                self.assertIsInstance(A(), Parent2)
+                self.assertIsSubclass(A, Parent2)
+                self.assertNotIsInstance(B(), Parent2)
+                self.assertNotIsSubclass(B, Parent2)
 
         def test_issubclass_bad_arguments(self):
             class A(metaclass=abc_ABCMeta):
@@ -522,7 +627,6 @@ def foo(self):
             self.assertEqual(A.__abstractmethods__, set())
             A()
 
-
         def test_update_new_abstractmethods(self):
             class A(metaclass=abc_ABCMeta):
                 @abc.abstractmethod

diff --git a/Lib/test/test_isinstance.py b/Lib/test/test_isinstance.py
@@ -351,6 +351,28 @@ class B:
         with support.infinite_recursion(25):
             self.assertRaises(RecursionError, issubclass, X(), int)
 
+    def test_custom_subclasses_are_ignored(self):
+        class A: pass
+        class B: pass
+
+        class Parent1:
+            @classmethod
+            def __subclasses__(cls):
+                return [A, B]
+
+        class Parent2:
+            __subclasses__ = lambda: [A, B]
+
+        self.assertNotIsInstance(A(), Parent1)
+        self.assertNotIsInstance(B(), Parent1)
+        self.assertNotIsSubclass(A, Parent1)
+        self.assertNotIsSubclass(B, Parent1)
+
+        self.assertNotIsInstance(A(), Parent2)
+        self.assertNotIsInstance(B(), Parent2)
+        self.assertNotIsSubclass(A, Parent2)
+        self.assertNotIsSubclass(B, Parent2)
+
 
 def blowstack(fxn, arg, compare_to):
     # Make sure that calling isinstance with a deeply nested tuple for its

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-06-13-17-22-34.gh-issue-92810.Tb6x6C.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-06-13-17-22-34.gh-issue-92810.Tb6x6C.rst
@@ -0,0 +1,2 @@
+Reduce memory usage by :meth:`~type.__subclasscheck__`
+for :class:`abc.ABCMeta` and large class trees
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Reduce memory usage by :meth:`~type.__subclasscheck__`
		for :class:`abc.ABCMeta` and large class trees
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