fix: handle special case when import-names is empty

Signed-off-by: Henry Schreiner <[email protected]>

src/packaging/metadata.py

@@ -447,6 +447,11 @@ def parse_email(data: bytes | str) -> tuple[RawMetadata, dict[str, list[str]]]:
         # of unparsed stuff.
         if raw_name in _STRING_FIELDS and len(value) == 1:
             raw[raw_name] = value[0]
+        # If this is import_names, we need to special case the empty field
+        # case, which converts to an empty list instead of None. We can't let
+        # the empty case slip through, as it will fail validation.
+        elif raw_name == "import_names" and value == [""]:
+            raw[raw_name] = []
         # If this is one of our list of string fields, then we can just assign
         # the value, since email *only* has strings, and our get_all() call
         # above ensures that this is a list.
@@ -959,6 +964,8 @@ def _write_metadata(self, message: RFC822Message) -> None:
                             message[email_name] = f"{label}, {url}"
                     elif email_name == "keywords":
                         message[email_name] = ",".join(value)
+                    elif email_name == "import-name" and value == []:
+                        message[email_name] = ""
                     elif isinstance(value, list):
                         for item in value:
                             message[email_name] = str(item)

tests/test_metadata.py

@@ -295,6 +295,13 @@ def test_from_email(self):
         )
 
         assert meta.metadata_version == metadata_version
+        assert meta.import_names is None
+
+    def test_from_email_empty_import_name(self):
+        meta = metadata.Metadata.from_email(
+            "Metadata-Version: 2.5\nImport-Name:\n", validate=False
+        )
+        assert meta.import_names == []
 
     def test_from_email_unparsed(self):
         with pytest.raises(ExceptionGroup) as exc_info:
@@ -963,6 +970,49 @@ def test_modern_license(self):
 
         assert core_metadata.get_payload() is None
 
+    def test__import_names(self):
+        meta = metadata.Metadata.from_raw(
+            {
+                "metadata_version": "2.5",
+                "name": "full_metadata",
+                "version": "3.2.1",
+                "import_names": ["one", "two"],
+                "import_namespaces": ["three"],
+            }
+        )
+
+        core_metadata = meta.as_rfc822()
+        assert core_metadata.items() == [
+            ("metadata-version", "2.5"),
+            ("name", "full_metadata"),
+            ("version", "3.2.1"),
+            ("import-name", "one"),
+            ("import-name", "two"),
+            ("import-namespace", "three"),
+        ]
+
+        assert core_metadata.get_payload() is None
+
+    def test_empty_import_names(self):
+        meta = metadata.Metadata.from_raw(
+            {
+                "metadata_version": "2.5",
+                "name": "full_metadata",
+                "version": "3.2.1",
+                "import_names": [],
+            }
+        )
+
+        core_metadata = meta.as_rfc822()
+        assert core_metadata.items() == [
+            ("metadata-version", "2.5"),
+            ("name", "full_metadata"),
+            ("version", "3.2.1"),
+            ("import-name", ""),
+        ]
+
+        assert core_metadata.get_payload() is None
+
     @pytest.mark.parametrize(
         ("items", "data"),
         [