Correctly use the ExceptionGroup shim only when needed

CHANGELOG.rst

@@ -6,6 +6,8 @@ Changelog
 
 * Do specifier matching correctly when the specifier contains an epoch number
   and has more components than the version (:issue:`683`)
+* Fix a bug preventing the use of the built in ``ExceptionGroup`` on versions of
+  Python that support it (:issue:`725`)
 
 23.2 - 2023-10-01
 ~~~~~~~~~~~~~~~~~

src/packaging/metadata.py

@@ -41,10 +41,10 @@ def __init_subclass__(*_args, **_kwargs):
 
 
 try:
-    ExceptionGroup = __builtins__.ExceptionGroup  # type: ignore[attr-defined]
-except AttributeError:
+    ExceptionGroup
+except NameError:  # pragma: no cover
 
-    class ExceptionGroup(Exception):  # type: ignore[no-redef]  # noqa: N818
+    class ExceptionGroup(Exception):  # noqa: N818
         """A minimal implementation of :external:exc:`ExceptionGroup` from Python 3.11.
 
         If :external:exc:`ExceptionGroup` is already defined by Python itself,
@@ -61,6 +61,9 @@ def __init__(self, message: str, exceptions: List[Exception]) -> None:
         def __repr__(self) -> str:
             return f"{self.__class__.__name__}({self.message!r}, {self.exceptions!r})"
 
+else:  # pragma: no cover
+    ExceptionGroup = ExceptionGroup
+
 
 class InvalidMetadata(ValueError):
     """A metadata field contains invalid data."""
@@ -677,7 +680,7 @@ def from_raw(cls, data: RawMetadata, *, validate: bool = True) -> "Metadata":
         ins._raw = data.copy()  # Mutations occur due to caching enriched values.
 
         if validate:
-            exceptions: List[InvalidMetadata] = []
+            exceptions: List[Exception] = []
             try:
                 metadata_version = ins.metadata_version
                 metadata_age = _VALID_METADATA_VERSIONS.index(metadata_version)
@@ -732,10 +735,10 @@ def from_email(
         If *validate* is true, the metadata will be validated. All exceptions
         related to validation will be gathered and raised as an :class:`ExceptionGroup`.
         """
-        exceptions: list[InvalidMetadata] = []
         raw, unparsed = parse_email(data)
 
         if validate:
+            exceptions: list[Exception] = []
             for unparsed_key in unparsed:
                 if unparsed_key in _EMAIL_TO_RAW_MAPPING:
                     message = f"{unparsed_key!r} has invalid data"
@@ -749,8 +752,9 @@ def from_email(
         try:
             return cls.from_raw(raw, validate=validate)
         except ExceptionGroup as exc_group:
-            exceptions.extend(exc_group.exceptions)
-            raise ExceptionGroup("invalid or unparsed metadata", exceptions) from None
+            raise ExceptionGroup(
+                "invalid or unparsed metadata", exc_group.exceptions
+            ) from None
 
     metadata_version: _Validator[_MetadataVersion] = _Validator()
     """:external:ref:`core-metadata-metadata-version`

tests/test_metadata.py

@@ -251,7 +251,7 @@ def test_attributes(self):
         individual_exception = Exception("not important")
         exc = metadata.ExceptionGroup("message", [individual_exception])
         assert exc.message == "message"
-        assert exc.exceptions == [individual_exception]
+        assert list(exc.exceptions) == [individual_exception]
 
     def test_repr(self):
         individual_exception = RuntimeError("not important")