From 2001900b0c02a397d8cf1d776a7cc7fcb2a463e3 Mon Sep 17 00:00:00 2001
From: Licht Takeuchi <licht-t@outlook.jp>
Date: Tue, 12 Dec 2017 18:57:06 +0900
Subject: [PATCH] bpo-32255: Always quote a single empty field when write into
 a CSV file. (#4769)

This allows to distinguish an empty row from a row consisting of a single empty field.
---
 Lib/test/test_csv.py                          | 21 ++++++++++++++++++-
 Misc/ACKS                                     |  1 +
 .../2017-12-12-07-29-06.bpo-32255.2bfNmM.rst  |  3 +++
 Modules/_csv.c                                |  2 +-
 4 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2017-12-12-07-29-06.bpo-32255.2bfNmM.rst

diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 03ab1840dd0..fe248019a0f 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -207,10 +207,29 @@ class Test_Csv(unittest.TestCase):
         with TemporaryFile("w+", newline='') as fileobj:
             writer = csv.writer(fileobj)
             self.assertRaises(TypeError, writer.writerows, None)
-            writer.writerows([['a','b'],['c','d']])
+            writer.writerows([['a', 'b'], ['c', 'd']])
             fileobj.seek(0)
             self.assertEqual(fileobj.read(), "a,b\r\nc,d\r\n")
 
+    def test_writerows_with_none(self):
+        with TemporaryFile("w+", newline='') as fileobj:
+            writer = csv.writer(fileobj)
+            writer.writerows([['a', None], [None, 'd']])
+            fileobj.seek(0)
+            self.assertEqual(fileobj.read(), "a,\r\n,d\r\n")
+
+        with TemporaryFile("w+", newline='') as fileobj:
+            writer = csv.writer(fileobj)
+            writer.writerows([[None], ['a']])
+            fileobj.seek(0)
+            self.assertEqual(fileobj.read(), '""\r\na\r\n')
+
+        with TemporaryFile("w+", newline='') as fileobj:
+            writer = csv.writer(fileobj)
+            writer.writerows([['a'], [None]])
+            fileobj.seek(0)
+            self.assertEqual(fileobj.read(), 'a\r\n""\r\n')
+
     @support.cpython_only
     def test_writerows_legacy_strings(self):
         import _testcapi
diff --git a/Misc/ACKS b/Misc/ACKS
index 54d8d62b633..8303ce80050 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1554,6 +1554,7 @@ Joel Taddei
 Arfrever Frehtes Taifersar Arahesis
 Hideaki Takahashi
 Takase Arihiro
+Licht Takeuchi
 Indra Talip
 Neil Tallim
 Geoff Talvola
diff --git a/Misc/NEWS.d/next/Library/2017-12-12-07-29-06.bpo-32255.2bfNmM.rst b/Misc/NEWS.d/next/Library/2017-12-12-07-29-06.bpo-32255.2bfNmM.rst
new file mode 100644
index 00000000000..dafee67bdd0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-12-12-07-29-06.bpo-32255.2bfNmM.rst
@@ -0,0 +1,3 @@
+A single empty field is now always quoted when written into a CSV file.
+This allows to distinguish an empty row from a row consisting of a single empty field.
+Patch by Licht Takeuchi.
diff --git a/Modules/_csv.c b/Modules/_csv.c
index 5314ef6edc4..e616151bae6 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -1238,7 +1238,7 @@ csv_writerow(WriterObj *self, PyObject *seq)
     if (PyErr_Occurred())
         return NULL;
 
-    if (self->num_fields > 0 && self->rec_size == 0) {
+    if (self->num_fields > 0 && self->rec_len == 0) {
         if (dialect->quoting == QUOTE_NONE) {
             PyErr_Format(_csvstate_global->error_obj,
                 "single empty field record must be quoted");