Fix inherited metadata field validation and serialization

src/packaging/metadata.py

@@ -860,7 +860,7 @@ def from_raw(cls, data: RawMetadata, *, validate: bool = True) -> Metadata:
                         # Can't use getattr() as that triggers descriptor protocol which
                         # will fail due to no value for the instance argument.
                         try:
-                            field_metadata_version = cls.__dict__[key].added
+                            field_metadata_version = Metadata.__dict__[key].added
                         except KeyError:
                             exc = InvalidMetadata(key, f"unrecognized field: {key!r}")
                             collector.error(exc)
@@ -1033,7 +1033,7 @@ def _write_metadata(self, message: RFC822Message) -> None:
         """
         Return an RFC822 message with the metadata.
         """
-        for name, validator in self.__class__.__dict__.items():
+        for name, validator in Metadata.__dict__.items():
             if isinstance(validator, _Validator) and name != "description":
                 value = getattr(self, name)
                 email_name = _RAW_TO_EMAIL_MAPPING[name]

tests/test_metadata.py

@@ -318,6 +318,29 @@ def test_repr(self) -> None:
 
 
 class TestMetadata:
+    @pytest.mark.parametrize("validate", [False, True])
+    @pytest.mark.parametrize("from_email", [False, True])
+    def test_subclass_round_trip(self, validate: bool, from_email: bool) -> None:
+        class CustomMetadata(metadata.Metadata):
+            pass
+
+        raw: metadata.RawMetadata = {
+            "metadata_version": "2.6",
+            "name": "example",
+            "version": "1.0",
+            "summary": "An example package",
+            "requires_dist": ["dependency>=2"],
+            "description": "Package description\n",
+        }
+        expected = metadata.Metadata.from_raw(raw).as_rfc822().as_bytes()
+        if from_email:
+            parsed = CustomMetadata.from_email(expected, validate=validate)
+        else:
+            parsed = CustomMetadata.from_raw(raw, validate=validate)
+
+        assert isinstance(parsed, CustomMetadata)
+        assert parsed.as_rfc822().as_bytes() == expected
+
     def _invalid_with_cause(
         self,
         meta: metadata.Metadata,