gh-105144: abc: Suppress errors raised by unrelated other subclasses

Lib/_py_abc.py

@@ -134,14 +134,20 @@ def __subclasscheck__(cls, subclass):
             return True
         # Check if it's a subclass of a registered class (recursive)
         for rcls in cls._abc_registry:
-            if issubclass(subclass, rcls):
-                cls._abc_cache.add(subclass)
-                return True
+            try:
+                if issubclass(subclass, rcls):
+                    cls._abc_cache.add(subclass)
+                    return True
+            except Exception:
+                pass
         # Check if it's a subclass of a subclass (recursive)
         for scls in cls.__subclasses__():
-            if issubclass(subclass, scls):
-                cls._abc_cache.add(subclass)
-                return True
+            try:
+                if issubclass(subclass, scls):
+                    cls._abc_cache.add(subclass)
+                    return True
+            except Exception:
+                pass
         # No dice; update negative cache
         cls._abc_negative_cache.add(subclass)
         return False

Lib/test/test_abc.py

@@ -435,17 +435,22 @@ class C:
                 None,
                 lambda x: [],
                 lambda: 42,
-                lambda: [42],
             ]
 
             for i, func in enumerate(bogus_subclasses):
                 class S(metaclass=abc_ABCMeta):
                     __subclasses__ = func
 
-                with self.subTest(i=i):
+                with self.subTest(i=i, func=func):
                     with self.assertRaises(TypeError):
                         issubclass(int, S)
 
+            # If __subclasses__ contains non-classes, we suppress the error instead.
+            class S(metaclass=abc_ABCMeta):
+                __subclasses__ = lambda: [42]
+
+            self.assertIs(issubclass(int, S), False)
+
             # Also check that issubclass() propagates exceptions raised by
             # __subclasses__.
             exc_msg = "exception from __subclasses__"
@@ -459,6 +464,35 @@ class S(metaclass=abc_ABCMeta):
             with self.assertRaisesRegex(Exception, exc_msg):
                 issubclass(int, S)
 
+        def test_subclass_with_broken_subclasscheck(self):
+            class A(metaclass=abc_ABCMeta):
+                pass
+
+            class Unrelated: pass
+
+            self.assertIs(issubclass(Unrelated, A), False)
+
+            class BrokenMeta(abc_ABCMeta):
+                is_broken = True
+                def __subclasscheck__(cls, subclass):
+                    if not BrokenMeta.is_broken:
+                        return super().__subclasscheck__(subclass)
+                    raise Exception("broken")
+
+            class Broken(A, metaclass=BrokenMeta):
+                pass
+
+            self.assertIs(issubclass(Unrelated, A), False)
+
+            class RegisteredBroken(metaclass=BrokenMeta):
+                pass
+
+            BrokenMeta.is_broken = False
+            A.register(RegisteredBroken)
+            BrokenMeta.is_broken = True
+
+            self.assertIs(issubclass(Unrelated, A), False)
+
         def test_subclasshook(self):
             class A(metaclass=abc.ABCMeta):
                 @classmethod

Misc/NEWS.d/next/Library/2023-05-31-09-55-16.gh-issue-105144.fTqfGv.rst

@@ -0,0 +1,3 @@
+Subclass checks on :class:`abc.ABC` subclasses no longer propagate errors
+raised by subclass checks on unrelated base classes of the ABC. Patch by
+Jelle Zijlstra.

Modules/_abc.c

@@ -786,7 +786,8 @@ _abc__abc_subclasscheck_impl(PyObject *module, PyObject *self,
             goto end;
         }
         if (r < 0) {
-            goto end;
+            // Ignore subclasses that throw on issubclass().
+            PyErr_Clear();
         }
     }
 
@@ -856,8 +857,7 @@ subclasscheck_check_registry(_abc_data *impl, PyObject *subclass,
         int r = PyObject_IsSubclass(subclass, rkey);
         Py_DECREF(rkey);
         if (r < 0) {
-            ret = -1;
-            break;
+            PyErr_Clear();
         }
         if (r > 0) {
             if (_add_to_weak_set(&impl->_abc_cache, subclass) < 0) {