bpo-35960: Fix dataclasses.field throwing away empty metadata. (GH-11815)

1 files changed, 12 insertions, 2 deletions

diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index ff6060c..9c83459 100755
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -1737,23 +1737,33 @@ class TestCase(unittest.TestCase):
                 i: int = field(metadata=0)
 
         # Make sure an empty dict works.
+        d = {}
         @dataclass
         class C:
-            i: int = field(metadata={})
+            i: int = field(metadata=d)
         self.assertFalse(fields(C)[0].metadata)
         self.assertEqual(len(fields(C)[0].metadata), 0)
+        # Update should work (see bpo-35960).
+        d['foo'] = 1
+        self.assertEqual(len(fields(C)[0].metadata), 1)
+        self.assertEqual(fields(C)[0].metadata['foo'], 1)
         with self.assertRaisesRegex(TypeError,
                                     'does not support item assignment'):
             fields(C)[0].metadata['test'] = 3
 
         # Make sure a non-empty dict works.
+        d = {'test': 10, 'bar': '42', 3: 'three'}
         @dataclass
         class C:
-            i: int = field(metadata={'test': 10, 'bar': '42', 3: 'three'})
+            i: int = field(metadata=d)
         self.assertEqual(len(fields(C)[0].metadata), 3)
         self.assertEqual(fields(C)[0].metadata['test'], 10)
         self.assertEqual(fields(C)[0].metadata['bar'], '42')
         self.assertEqual(fields(C)[0].metadata[3], 'three')
+        # Update should work.
+        d['foo'] = 1
+        self.assertEqual(len(fields(C)[0].metadata), 4)
+        self.assertEqual(fields(C)[0].metadata['foo'], 1)
         with self.assertRaises(KeyError):
             # Non-existent key.
             fields(C)[0].metadata['baz']