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-rw-r--r--Lib/dataclasses.py2
-rwxr-xr-xLib/test/test_dataclasses.py14
-rw-r--r--Misc/NEWS.d/next/Library/2019-02-10-20-57-12.bpo-35960.bh-6Ja.rst2
3 files changed, 15 insertions, 3 deletions
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 71d9896..325b822 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -241,7 +241,7 @@ class Field:
self.hash = hash
self.compare = compare
self.metadata = (_EMPTY_METADATA
- if metadata is None or len(metadata) == 0 else
+ if metadata is None else
types.MappingProxyType(metadata))
self._field_type = None
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']
diff --git a/Misc/NEWS.d/next/Library/2019-02-10-20-57-12.bpo-35960.bh-6Ja.rst b/Misc/NEWS.d/next/Library/2019-02-10-20-57-12.bpo-35960.bh-6Ja.rst
new file mode 100644
index 0000000..6713584
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-02-10-20-57-12.bpo-35960.bh-6Ja.rst
@@ -0,0 +1,2 @@
+Fix :func:`dataclasses.field` throwing away empty mapping objects passed as
+metadata.