gh-155436: Fix configparser.getboolean() for value-less options

Lib/configparser.py

@@ -1222,7 +1222,7 @@ def _unify_values(self, section, vars):
     def _convert_to_boolean(self, value):
         """Return a boolean value translating from other types if necessary.
         """
-        if value.lower() not in self.BOOLEAN_STATES:
+        if value is None or value.lower() not in self.BOOLEAN_STATES:
             raise ValueError('Not a boolean: %s' % value)
         return self.BOOLEAN_STATES[value.lower()]
 

Lib/test/test_configparser.py

@@ -1338,6 +1338,14 @@ def test_other_errors(self):
 class ConfigParserTestCaseNoValue(ConfigParserTestCase):
     allow_no_value = True
 
+    def test_getboolean_with_no_value(self):
+        cf = self.fromstring("[section]\noption\n")
+
+        with self.assertRaisesRegex(ValueError, "Not a boolean: None"):
+            cf.getboolean("section", "option")
+        with self.assertRaisesRegex(ValueError, "Not a boolean: None"):
+            cf["section"].getboolean("option")
+
 
 class NoValueAndExtendedInterpolation(CfgParserTestCaseClass):
     interpolation = configparser.ExtendedInterpolation()

Misc/NEWS.d/next/Library/2026-08-09-22-50-19.gh-issue-155436.gyB8_I.rst

@@ -0,0 +1,3 @@
+:class:`configparser.ConfigParser` now raises :exc:`ValueError` instead of
+:exc:`AttributeError` when :meth:`~configparser.ConfigParser.getboolean` is
+called on an option without a value while ``allow_no_value=True``.