From a2e7acd17d64cfef3f502473c3b56db8cce24234 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C5=81ukasz=20Langa?= <lukasz@langa.pl>
Date: Tue, 1 Jan 2013 23:45:33 +0100
Subject: [PATCH] configparser: preserve section order when using `__setitem__`
 (issue #16820)

---
 Lib/configparser.py        |  3 ++-
 Lib/test/test_cfgparser.py | 26 ++++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/Lib/configparser.py b/Lib/configparser.py
index 243efec4bd3..e5536a0024b 100644
--- a/Lib/configparser.py
+++ b/Lib/configparser.py
@@ -959,7 +959,8 @@ class RawConfigParser(MutableMapping):
 
         # XXX this is not atomic if read_dict fails at any point. Then again,
         # no update method in configparser is atomic in this implementation.
-        self.remove_section(key)
+        if key in self._sections:
+            self._sections[key].clear()
         self.read_dict({key: value})
 
     def __delitem__(self, key):
diff --git a/Lib/test/test_cfgparser.py b/Lib/test/test_cfgparser.py
index 7a9e8a8c1f3..cec9b44e19e 100644
--- a/Lib/test/test_cfgparser.py
+++ b/Lib/test/test_cfgparser.py
@@ -797,6 +797,32 @@ boolean {0[0]} NO
         self.assertEqual(set(cf.sections()), set())
         self.assertEqual(set(cf[self.default_section].keys()), {'foo'})
 
+    def test_setitem(self):
+        cf = self.fromstring("""
+            [section1]
+            name1 {0[0]} value1
+            [section2]
+            name2 {0[0]} value2
+            [section3]
+            name3 {0[0]} value3
+        """.format(self.delimiters), defaults={"nameD": "valueD"})
+        self.assertEqual(set(cf['section1'].keys()), {'name1', 'named'})
+        self.assertEqual(set(cf['section2'].keys()), {'name2', 'named'})
+        self.assertEqual(set(cf['section3'].keys()), {'name3', 'named'})
+        self.assertEqual(cf['section1']['name1'], 'value1')
+        self.assertEqual(cf['section2']['name2'], 'value2')
+        self.assertEqual(cf['section3']['name3'], 'value3')
+        self.assertEqual(cf.sections(), ['section1', 'section2', 'section3'])
+        cf['section2'] = {'name22': 'value22'}
+        self.assertEqual(set(cf['section2'].keys()), {'name22', 'named'})
+        self.assertEqual(cf['section2']['name22'], 'value22')
+        self.assertNotIn('name2', cf['section2'])
+        self.assertEqual(cf.sections(), ['section1', 'section2', 'section3'])
+        cf['section3'] = {}
+        self.assertEqual(set(cf['section3'].keys()), {'named'})
+        self.assertNotIn('name3', cf['section3'])
+        self.assertEqual(cf.sections(), ['section1', 'section2', 'section3'])
+
 
 class StrictTestCase(BasicTestCase):
     config_class = configparser.RawConfigParser