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

[Vinay Sajip]

https://github.com/python/cpython/commit/9eb4b2c8a3387ea901dad793e8d5586880a5968e
commit: 9eb4b2c8a3387ea901dad793e8d5586880a5968e
branch: 3.7
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: Vinay Sajip <vinay_sajip at yahoo.co.uk>
date: 2019-06-19T14:46:19+01:00
summary:

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

(cherry picked from commit 015000165373f8db263ef5bc682f02d74e5782ac)

files:
M Doc/library/logging.rst
M Lib/test/test_logging.py

diff --git a/Doc/library/logging.rst b/Doc/library/logging.rst
index 0e65f316735e..73ce9ad1deec 100644
--- a/Doc/library/logging.rst
+++ b/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)
diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index 97c13a4c5214..13393cd8b372 100644
--- a/Lib/test/test_logging.py
+++ b/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