gh-70835: Clarify error message for CSV file opened with wrong newline (GH-113786)

Based on patch by SilentGhost.

2 files changed, 14 insertions, 8 deletions

diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 97b9bba..282cff4 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -297,13 +297,18 @@ class Test_Csv(unittest.TestCase):
                           [b'abc'], None)
 
     def test_read_eol(self):
-        self._read_test(['a,b'], [['a','b']])
-        self._read_test(['a,b\n'], [['a','b']])
-        self._read_test(['a,b\r\n'], [['a','b']])
-        self._read_test(['a,b\r'], [['a','b']])
-        self.assertRaises(csv.Error, self._read_test, ['a,b\rc,d'], [])
-        self.assertRaises(csv.Error, self._read_test, ['a,b\nc,d'], [])
-        self.assertRaises(csv.Error, self._read_test, ['a,b\r\nc,d'], [])
+        self._read_test(['a,b', 'c,d'], [['a','b'], ['c','d']])
+        self._read_test(['a,b\n', 'c,d\n'], [['a','b'], ['c','d']])
+        self._read_test(['a,b\r\n', 'c,d\r\n'], [['a','b'], ['c','d']])
+        self._read_test(['a,b\r', 'c,d\r'], [['a','b'], ['c','d']])
+
+        errmsg = "with newline=''"
+        with self.assertRaisesRegex(csv.Error, errmsg):
+            next(csv.reader(['a,b\rc,d']))
+        with self.assertRaisesRegex(csv.Error, errmsg):
+            next(csv.reader(['a,b\nc,d']))
+        with self.assertRaisesRegex(csv.Error, errmsg):
+            next(csv.reader(['a,b\r\nc,d']))
 
     def test_read_eof(self):
         self._read_test(['a,"'], [['a', '']])
diff --git a/Modules/_csv.c b/Modules/_csv.c
index ae6b645..d45a15a 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -837,7 +837,8 @@ parse_process_char(ReaderObj *self, _csvstate *module_state, Py_UCS4 c)
             self->state = START_RECORD;
         else {
             PyErr_Format(module_state->error_obj,
-                         "new-line character seen in unquoted field - do you need to open the file in universal-newline mode?");
+                         "new-line character seen in unquoted field - "
+                         "do you need to open the file with newline=''?");
             return -1;
         }
         break;