Index: Lib/test/test_cfgparser.py
===================================================================
--- Lib/test/test_cfgparser.py	(revision 41859)
+++ Lib/test/test_cfgparser.py	(working copy)
@@ -415,11 +415,153 @@
         self.assertRaises(TypeError, cf.set, "sect", "option2", object())
 
 
+example = """\
+# Leading comment.
+Rem Another comment.
+REM Yet more comments.
+reM Strange comment.
+
+;Notice the blank lines above and below.
+
+[sect1]
+opt1=2
+opt2=This is a value with
+    a continuation.
+opt3:another option.
+opt4=5 ; There's a comment here.
+[sect2]
+opt5 = old
+opt6 = 4
+opt7=a value with
+    a continuation ; and a comment.
+; This is an empty section.
+[sect3]
+"""
+stripped = """\
+[DEFAULT]
+default = not to be written
+
+[sect1]
+opt1 = 2
+opt2 = This is a value with
+\ta continuation.
+opt3 = option
+\twith a newline
+opt4 = 5
+
+[sect2]
+opt5 = new
+opt7 = new2
+opt8 = add
+
+[sect3]
+
+
+"""
+class UpdateFileTest(unittest.TestCase):
+    """Class which tests the update_file() method of the ConfigParser
+    class against the test cases.
+    """
+    def setUp(self):
+        defaults = {"default" : "not to be written"}
+        self.config = ConfigParser.RawConfigParser(defaults)
+        self.example_fp = StringIO.StringIO(example)
+        self.example_fp.seek(0)
+        self.config.readfp(self.example_fp)
+        self.example_fp.seek(0)
+        self.config.set("sect2", "opt5", "new")
+        self.config.set("sect2", "opt8", "add")
+        self.config.set("sect1", "opt3", "option\nwith a newline")
+        self.config.set("sect2", "opt7", "new2")
+        self.config.remove_option("sect2", "opt6")
+
+    def test_update_empty(self):
+        empty = StringIO.StringIO()
+        self.config.update_file(empty)
+        self.assertEqual(empty.getvalue(), stripped)
+
+    def test_continuation_comment(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("opt7=new2 ; and a comment."
+                     in self.example_fp.getvalue())
+
+    def test_add_continuation(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("opt3:option\n\twith a newline"
+                     in self.example_fp.getvalue())
+
+    def test_removed_option(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("opt6" not in self.example_fp.getvalue())
+
+    def test_blank_sorted(self):
+        empty = StringIO.StringIO()
+        self.config.update_file(empty)
+        empty.seek(0)
+        self._test_ordering(empty)
+
+    def test_ordering_preserved(self):
+        self.config.update_file(self.example_fp)
+        self.example_fp.seek(0)
+        self._test_ordering(self.example_fp)
+
+    def _test_ordering(self, result):
+        sect = '1'
+        opt = '1'
+        for line in result.readlines():
+            if line.startswith("[sect"):
+                self.assert_(line.startswith("[sect" + sect))
+                sect = str(int(sect)+1)
+            elif line.startswith("opt"):
+                self.assert_(line.startswith("opt" + opt))
+                opt = str(int(opt)+1)
+                # We remove #6.
+                if opt == '6':
+                    opt = '7'
+
+    def test_blank_lines_preserved(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("\n;Notice the blank lines above and below.\n" in
+                     self.example_fp.getvalue())
+
+    def test_values_updated(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("opt5=new" in self.example_fp.getvalue())
+
+    def test_defaults_included(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("default" in self.example_fp.getvalue())
+
+    def test_name_excluded(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("__name__" not in self.example_fp.getvalue())
+
+    def test_preserve_comments(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("# Leading comment." in self.example_fp.getvalue())
+        self.assert_("Rem Another comment." in self.example_fp.getvalue())
+        self.assert_("REM Yet more comments."
+                     in self.example_fp.getvalue())
+        self.assert_("reM Strange comment." in self.example_fp.getvalue())
+        self.assert_(";Notice the blank lines above and below."
+                     in self.example_fp.getvalue())
+        self.assert_("opt4=5 ; There's a comment here."
+                     in self.example_fp.getvalue())
+
+    def test_add_missing(self):
+        self.config.update_file(self.example_fp)
+        self.assert_("opt8" in self.example_fp.getvalue())
+
+    def test_not_add_missing(self):
+        self.config.update_file(self.example_fp, False)
+        self.assert_("opt8" not in self.example_fp.getvalue())
+
 def test_main():
     test_support.run_unittest(
         ConfigParserTestCase,
         RawConfigParserTestCase,
-        SafeConfigParserTestCase
+        SafeConfigParserTestCase,
+        UpdateFileTest,
     )
 
 if __name__ == "__main__":