gh-156955: Keep the line terminator test additions in their own tests

Restore test_write_lineterminator to its original body and move the
reviewer's suggestion into test_write_lineterminator_in_field, so the
existing test keeps its charter -- the terminator is emitted between
records -- and the new coverage stands on its own.

Also strip a trailing space from the NEWS entry, which was failing the
trim-trailing-whitespace pre-commit hook and turning the lint job red.

Author: brittanyrey

Lib/test/test_csv.py

@@ -260,6 +260,19 @@ def test_write_escape(self):
                          escapechar='\\', quoting=csv.QUOTE_MINIMAL)
 
     def test_write_lineterminator(self):
+        for lineterminator in '\r\n', '\n', '\r', '!@#', '\0':
+            with self.subTest(lineterminator=lineterminator):
+                with StringIO() as sio:
+                    writer = csv.writer(sio, lineterminator=lineterminator)
+                    writer.writerow(['a', 'b'])
+                    writer.writerow([1, 2])
+                    writer.writerow(['\r', '\n'])
+                    self.assertEqual(sio.getvalue(),
+                                     f'a,b{lineterminator}'
+                                     f'1,2{lineterminator}'
+                                     f'"\r","\n"{lineterminator}')
+
+    def test_write_lineterminator_in_field(self):
         for lineterminator in ('\r\n', '\n', '\r', '!@#', '\0', '\x85',
                                '\u2028', '\U0001f600'):
             with self.subTest(lineterminator=lineterminator):
@@ -276,10 +289,6 @@ def test_write_lineterminator(self):
                                      f'"a{lineterminator[-1]}b",c{lineterminator}')
 
     def test_write_lineterminator_quoting(self):
-        # Every character of the line terminator forces quoting, not just the
-        # last one, and no other character does.  Each terminator is paired
-        # with characters that bracket it in code point order, to pin that
-        # boundary.
         for lineterminator, plain in ('!@#', ' ?A'), ('\u2028', '\u2027\u2029'):
             with self.subTest(lineterminator=lineterminator):
                 for c in lineterminator:
@@ -289,9 +298,6 @@ def test_write_lineterminator_quoting(self):
                                  lineterminator=lineterminator)
 
     def test_write_empty_lineterminator(self):
-        # An empty line terminator separates nothing and, having no characters
-        # of its own, forces no quoting -- not even of '\0', the lowest code
-        # point.  '\r' and '\n' are quoted whatever the terminator is.
         with StringIO() as sio:
             writer = csv.writer(sio, lineterminator='')
             writer.writerow(['a', 'b'])

Misc/NEWS.d/next/Library/2026-09-02-14-30-00.gh-issue-156955.Kv3Qa1.rst

@@ -1,2 +1,2 @@
-Speed up :func:`csv.writer` by up to 2.9x when fields contain no special 
+Speed up :func:`csv.writer` by up to 2.9x when fields contain no special
 characters.