Allow to create the unnamed section from scratch.

May fix #123049.

Author: pslacerda

Lib/configparser.py

@@ -694,6 +694,7 @@ def add_section(self, section):
 
         if section in self._sections:
             raise DuplicateSectionError(section)
+
         self._sections[section] = self._dict()
         self._proxies[section] = SectionProxy(self, section)
 
@@ -1243,7 +1244,11 @@ def add_section(self, section):
         """Create a new section in the configuration.  Extends
         RawConfigParser.add_section by validating if the section name is
         a string."""
-        self._validate_value_types(section=section)
+        if section is UNNAMED_SECTION:
+            if not self._allow_unnamed_section:
+                raise ValueError("Unnamed section not enabled")
+        else:
+            self._validate_value_types(section=section)
         super().add_section(section)
 
     def _read_defaults(self, defaults):

Lib/test/test_configparser.py

@@ -2161,6 +2161,11 @@ def test_no_section(self):
         self.assertEqual('1', cfg2[configparser.UNNAMED_SECTION]['a'])
         self.assertEqual('2', cfg2[configparser.UNNAMED_SECTION]['b'])
 
+    def test_add_section(self):
+        cfg = configparser.ConfigParser(allow_unnamed_section=True)
+        cfg.add_section(configparser.UNNAMED_SECTION)
+        cfg.set(configparser.UNNAMED_SECTION, 'a', '1')
+        self.assertEqual('1', cfg[configparser.UNNAMED_SECTION]['a'])
 
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):