bpo-39142: Avoid converting namedtuple instances to ConvertingTuple. (GH-17773)

This uses the heuristic of assuming a named tuple is a subclass of
tuple with a _fields attribute. This change means that contents of
a named tuple wouldn't be converted - if a user wants to have
ConvertingTuple functionality from a namedtuple, they will have to
implement it themselves.

Lib/logging/config.py

@@ -448,7 +448,7 @@ class BaseConfigurator(object):
             value = ConvertingList(value)
             value.configurator = self
         elif not isinstance(value, ConvertingTuple) and\
-                 isinstance(value, tuple):
+                 isinstance(value, tuple) and not hasattr(value, '_fields'):
             value = ConvertingTuple(value)
             value.configurator = self
         elif isinstance(value, str): # str for py3k

Lib/test/test_logging.py

@@ -3379,6 +3379,37 @@ class ConfigDictTest(BaseTest):
         self.assertRaises(ValueError, bc.convert, 'cfg://!')
         self.assertRaises(KeyError, bc.convert, 'cfg://adict[2]')
 
+    def test_namedtuple(self):
+        # see bpo-39142
+        from collections import namedtuple
+
+        class MyHandler(logging.StreamHandler):
+            def __init__(self, resource, *args, **kwargs):
+                super().__init__(*args, **kwargs)
+                self.resource: namedtuple = resource
+
+            def emit(self, record):
+                record.msg += f' {self.resource.type}'
+                return super().emit(record)
+
+        Resource = namedtuple('Resource', ['type', 'labels'])
+        resource = Resource(type='my_type', labels=['a'])
+
+        config = {
+            'version': 1,
+            'handlers': {
+                'myhandler': {
+                    '()': MyHandler,
+                    'resource': resource
+                }
+            },
+            'root':  {'level': 'INFO', 'handlers': ['myhandler']},
+        }
+        with support.captured_stderr() as stderr:
+            self.apply_config(config)
+            logging.info('some log')
+        self.assertEqual(stderr.getvalue(), 'some log my_type\n')
+
 class ManagerTest(BaseTest):
     def test_manager_loggerclass(self):
         logged = []

Misc/NEWS.d/next/Library/2019-12-31-19-27-23.bpo-39142.oqU5iD.rst

@@ -0,0 +1,5 @@
+A change was made to logging.config.dictConfig to avoid converting instances
+of named tuples to ConvertingTuple. It's assumed that named tuples are too
+specialised to be treated like ordinary tuples; if a user of named tuples
+requires ConvertingTuple functionality, they will have to implement that
+themselves in their named tuple class.