bpo-41086: Add exception for uninstantiated interpolation (configparser)

Lib/configparser.py

@@ -612,6 +612,8 @@ def __init__(self, defaults=None, dict_type=_default_dict,
             self._interpolation = self._DEFAULT_INTERPOLATION
         if self._interpolation is None:
             self._interpolation = Interpolation()
+        if not isinstance(self._interpolation, Interpolation):
+            raise TypeError("interpolation is not an instance of Interpolation")
         if converters is not _UNSET:
             self._converters.update(converters)
         if defaults:

Lib/test/test_configparser.py

@@ -1048,6 +1048,14 @@ def test_set_malformatted_interpolation(self):
         self.assertEqual(cf.get("sect", "option2"), "foo%%bar")
 
 
+class ConfigParserTestCaseInvalidInterpolationType(unittest.TestCase):
+    def test_error_on_wrong_type_for_interpolation(self):
+        for value in [configparser.ExtendedInterpolation,  42,  "a string"]:
+            with self.subTest(value=value):
+                with self.assertRaises(TypeError):
+                    configparser.ConfigParser(interpolation=value)
+
+
 class ConfigParserTestCaseNonStandardDelimiters(ConfigParserTestCase):
     delimiters = (':=', '$')
     comment_prefixes = ('//', '"')

Misc/NEWS.d/next/Library/2020-06-23-01-50-24.bpo-41086.YnOvpS.rst

@@ -0,0 +1 @@
+Make the :class:`configparser.ConfigParser` constructor raise :exc:`TypeError` if the ``interpolation`` parameter is not of type :class:`configparser.Interpolation`