From d4fbf8f6feabc5b57211600c5e1a9dd4241ec7a3 Mon Sep 17 00:00:00 2001
From: tonghuaroot <tonghuaroot@gmail.com>
Date: Fri, 24 Jul 2026 20:13:19 +0800
Subject: [PATCH] gh-154596: Fix configparser write() losing unnamed-section
 items to DEFAULT

---
 Lib/configparser.py                                   |  4 ++--
 Lib/test/test_configparser.py                         | 11 +++++++++++
 .../2026-07-24-20-00-00.gh-issue-154596.CfgWrt.rst    |  3 +++
 3 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-24-20-00-00.gh-issue-154596.CfgWrt.rst

diff --git a/Lib/configparser.py b/Lib/configparser.py
index 3c452afe8ade485..9a85d2325c0337b 100644
--- a/Lib/configparser.py
+++ b/Lib/configparser.py
@@ -971,11 +971,11 @@ def write(self, fp, space_around_delimiters=True):
             d = " {} ".format(self._delimiters[0])
         else:
             d = self._delimiters[0]
+        if UNNAMED_SECTION in self._sections and self._sections[UNNAMED_SECTION]:
+            self._write_section(fp, UNNAMED_SECTION, self._sections[UNNAMED_SECTION].items(), d, unnamed=True)
         if self._defaults:
             self._write_section(fp, self.default_section,
                                     self._defaults.items(), d)
-        if UNNAMED_SECTION in self._sections and self._sections[UNNAMED_SECTION]:
-            self._write_section(fp, UNNAMED_SECTION, self._sections[UNNAMED_SECTION].items(), d, unnamed=True)
 
         for section in self._sections:
             if section is UNNAMED_SECTION:
diff --git a/Lib/test/test_configparser.py b/Lib/test/test_configparser.py
index 4783943f71a1092..974d75cbc402b66 100644
--- a/Lib/test/test_configparser.py
+++ b/Lib/test/test_configparser.py
@@ -2250,6 +2250,17 @@ def test_add_section(self):
         cfg.write(output)
         self.assertEqual(output.getvalue(), 'a = 1\n\n')
 
+    def test_write_roundtrip_with_default(self):
+        US = configparser.UNNAMED_SECTION
+        cfg = configparser.ConfigParser(allow_unnamed_section=True)
+        cfg.read_string('key1 = val1\n[DEFAULT]\ndkey = dval\n[sect1]\nkey2 = val2\n')
+        out = io.StringIO()
+        cfg.write(out)
+        cfg2 = configparser.ConfigParser(allow_unnamed_section=True)
+        cfg2.read_string(out.getvalue())
+        self.assertIn('key1', cfg2[US])
+        self.assertNotIn('key1', cfg2.defaults())
+
     def test_disabled_error(self):
         with self.assertRaises(configparser.MissingSectionHeaderError):
             configparser.ConfigParser().read_string("a = 1")
diff --git a/Misc/NEWS.d/next/Library/2026-07-24-20-00-00.gh-issue-154596.CfgWrt.rst b/Misc/NEWS.d/next/Library/2026-07-24-20-00-00.gh-issue-154596.CfgWrt.rst
new file mode 100644
index 000000000000000..79d6a6f7219f1ff
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-24-20-00-00.gh-issue-154596.CfgWrt.rst
@@ -0,0 +1,3 @@
+Fix :meth:`configparser.ConfigParser.write` outputting unnamed-section
+items after the ``[DEFAULT]`` header, causing them to be absorbed into
+DEFAULT on re-read.  Patch by tonghuaroot.