bpo-37258: Not a bug, but added a unit test and updated documentation. (GH-14229) (GH-14231)

(cherry picked from commit 0150001)

Author: miss-islington

Doc/library/logging.rst

@@ -50,8 +50,8 @@ listed below.
 Logger Objects
 --------------
 
-Loggers have the following attributes and methods.  Note that Loggers are never
-instantiated directly, but always through the module-level function
+Loggers have the following attributes and methods.  Note that Loggers should
+*NEVER* be instantiated directly, but always through the module-level function
 ``logging.getLogger(name)``.  Multiple calls to :func:`getLogger` with the same
 name will always return a reference to the same Logger object.
 
@@ -1191,7 +1191,9 @@ functions.
    The class should define :meth:`__init__` such that only a name argument is
    required, and the :meth:`__init__` should call :meth:`Logger.__init__`. This
    function is typically called before any loggers are instantiated by applications
-   which need to use custom logger behavior.
+   which need to use custom logger behavior. After this call, as at any other
+   time, do not instantiate loggers directly using the subclass: continue to use
+   the :func:`logging.getLogger` API to get your loggers.
 
 
 .. function:: setLogRecordFactory(factory)

Lib/test/test_logging.py

@@ -3877,6 +3877,37 @@ class MyLogger(logging.Logger):
         logging.setLoggerClass(logging.Logger)
         self.assertEqual(logging.getLoggerClass(), logging.Logger)
 
+    def test_subclass_logger_cache(self):
+        # bpo-37258
+        message = []
+
+        class MyLogger(logging.getLoggerClass()):
+            def __init__(self, name='MyLogger', level=logging.NOTSET):
+                super().__init__(name, level)
+                message.append('initialized')
+
+        logging.setLoggerClass(MyLogger)
+        logger = logging.getLogger('just_some_logger')
+        self.assertEqual(message, ['initialized'])
+        stream = io.StringIO()
+        h = logging.StreamHandler(stream)
+        logger.addHandler(h)
+        try:
+            logger.setLevel(logging.DEBUG)
+            logger.debug("hello")
+            self.assertEqual(stream.getvalue().strip(), "hello")
+
+            stream.truncate(0)
+            stream.seek(0)
+
+            logger.setLevel(logging.INFO)
+            logger.debug("hello")
+            self.assertEqual(stream.getvalue(), "")
+        finally:
+            logger.removeHandler(h)
+            h.close()
+            logging.setLoggerClass(logging.Logger)
+
     @support.requires_type_collecting
     def test_logging_at_shutdown(self):
         # Issue #20037