Merge: #19449: Handle non-string keys when generating 'fieldnames' error.

2 files changed, 13 insertions, 1 deletions

diff --git a/Lib/csv.py b/Lib/csv.py
index da3bc44..a56eed8 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -146,7 +146,7 @@ class DictWriter:
             wrong_fields = [k for k in rowdict if k not in self.fieldnames]
             if wrong_fields:
                 raise ValueError("dict contains fields not in fieldnames: "
-                                 + ", ".join(wrong_fields))
+                                 + ", ".join([repr(x) for x in wrong_fields]))
         return [rowdict.get(key, self.restval) for key in self.fieldnames]
 
     def writerow(self, rowdict):
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index cf0b09c..01fba21 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -579,6 +579,18 @@ class TestDictFields(unittest.TestCase):
         fileobj = StringIO()
         self.assertRaises(TypeError, csv.DictWriter, fileobj)
 
+    def test_write_fields_not_in_fieldnames(self):
+        with TemporaryFile("w+", newline='') as fileobj:
+            writer = csv.DictWriter(fileobj, fieldnames = ["f1", "f2", "f3"])
+            # Of special note is the non-string key (issue 19449)
+            with self.assertRaises(ValueError) as cx:
+                writer.writerow({"f4": 10, "f2": "spam", 1: "abc"})
+            exception = str(cx.exception)
+            self.assertIn("fieldnames", exception)
+            self.assertIn("'f4'", exception)
+            self.assertNotIn("'f2'", exception)
+            self.assertIn("1", exception)
+
     def test_read_dict_fields(self):
         with TemporaryFile("w+") as fileobj:
             fileobj.write("1,2,abc\r\n")