Fix uninformative error message (#830)

* Test against non-specific message

* Make error on invalid dynamic metadata more informative

Author: abravalheri

src/packaging/metadata.py

@@ -589,10 +589,12 @@ def _process_dynamic(self, value: list[str]) -> list[str]:
         for dynamic_field in map(str.lower, value):
             if dynamic_field in {"name", "version", "metadata-version"}:
                 raise self._invalid_metadata(
-                    f"{value!r} is not allowed as a dynamic field"
+                    f"{dynamic_field!r} is not allowed as a dynamic field"
                 )
             elif dynamic_field not in _EMAIL_TO_RAW_MAPPING:
-                raise self._invalid_metadata(f"{value!r} is not a valid dynamic field")
+                raise self._invalid_metadata(
+                    f"{dynamic_field!r} is not a valid dynamic field"
+                )
         return list(map(str.lower, value))
 
     def _process_provides_extra(

tests/test_metadata.py

@@ -608,8 +608,12 @@ def test_invalid_dynamic_value(self):
     def test_disallowed_dynamic(self, field_name):
         meta = metadata.Metadata.from_raw({"dynamic": [field_name]}, validate=False)
 
-        with pytest.raises(metadata.InvalidMetadata):
+        message = f"{field_name!r} is not allowed"
+        with pytest.raises(metadata.InvalidMetadata, match=message) as execinfo:
             meta.dynamic  # noqa: B018
+            # The name of the specific offending field should be used,
+            # not a list with all fields:
+            assert "[" not in str(execinfo.value)
 
     @pytest.mark.parametrize(
         "field_name",