bpo-37868: Improve is_dataclass for instances. (GH-15325)

1 files changed, 26 insertions, 0 deletions

diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index ea42904b003..037bf4c2214 100755
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -1300,6 +1300,32 @@ class TestCase(unittest.TestCase):
         self.assertTrue(is_dataclass(d.d))
         self.assertFalse(is_dataclass(d.e))
 
+    def test_is_dataclass_when_getattr_always_returns(self):
+        # See bpo-37868.
+        class A:
+            def __getattr__(self, key):
+                return 0
+        self.assertFalse(is_dataclass(A))
+        a = A()
+
+        # Also test for an instance attribute.
+        class B:
+            pass
+        b = B()
+        b.__dataclass_fields__ = []
+
+        for obj in a, b:
+            with self.subTest(obj=obj):
+                self.assertFalse(is_dataclass(obj))
+
+                # Indirect tests for _is_dataclass_instance().
+                with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'):
+                    asdict(obj)
+                with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'):
+                    astuple(obj)
+                with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'):
+                    replace(obj, x=0)
+
     def test_helper_fields_with_class_instance(self):
         # Check that we can call fields() on either a class or instance,
         #  and get back the same thing.