Preserve original mapping keys when deriving CSV columns

benedict/serializers/csv.py

@@ -50,13 +50,16 @@ def encode(self, d: list[dict[str, Any]], **kwargs: Any) -> str:
         kwargs.setdefault("lineterminator", "\n")
         columns = kwargs.pop("columns", None)
         columns_row = kwargs.pop("columns_row", True)
+        # 2026-10-11: Supplied headers keep their existing writer behavior.
+        headers = columns
         if not columns and len(ls) and type_util.is_dict(ls[0]):
-            keys = [str(key) for key in ls[0].keys()]
-            columns = sorted(keys)
+            # 2026-10-11: Keep original keys for lookup while sorting their labels.
+            columns = sorted(ls[0].keys(), key=str)
+            headers = [str(key) for key in columns]
         f = StringIO()
         w = csv.writer(f, **kwargs)
         if columns_row and columns:
-            w.writerow(columns)
+            w.writerow(headers)
         for item in ls:
             if type_util.is_dict(item):
                 row = [item.get(key, "") for key in columns]

tests/dicts/io/test_io_dict_csv.py

@@ -181,6 +181,27 @@ def test_to_csv(self) -> None:
 """
         self.assertEqual(s, r)
 
+    # 2026-10-11: Column labels must not replace the keys used for row lookup.
+    def test_to_csv_with_non_string_keys(self) -> None:
+        for row, expected in (
+            ({2: "second", 1: "first"}, "1,2\nfirst,second\n"),
+            ({2: "second", "a": "first"}, "2,a\nsecond,first\n"),
+            ({2: "second", "10": "first"}, "10,2\nfirst,second\n"),
+        ):
+            with self.subTest(row=row):
+                d = IODict({"values": [row]})
+                self.assertEqual(d.to_csv(), expected)
+                self.assertEqual(
+                    d.to_csv(columns_row=False), expected.split("\n", 1)[1]
+                )
+
+    # 2026-10-11: Inferred labels stringify None while explicit columns keep it.
+    def test_to_csv_with_none_key(self) -> None:
+        d = IODict({"values": [{None: "first", "a": "second"}]})
+        self.assertEqual(d.to_csv(), "None,a\nfirst,second\n")
+        self.assertEqual(d.to_csv(columns_row=False), "first,second\n")
+        self.assertEqual(d.to_csv(columns=[None, "a"]), ",a\nfirst,second\n")
+
     def test_to_csv_with_custom_columns(self) -> None:
         d = IODict(
             {