bpo-40275: Avoid importing logging in test.support

Doc/library/test.rst

@@ -1460,11 +1460,6 @@ The :mod:`test.support` module defines the following classes:
       Run *test* and return the result.
 
 
-.. class:: TestHandler(logging.handlers.BufferingHandler)
-
-   Class for logging support.
-
-
 .. class:: FakePath(path)
 
    Simple :term:`path-like object`.  It implements the :meth:`__fspath__`

Lib/test/support/__init__.py

@@ -16,7 +16,6 @@
 import importlib
 import importlib.util
 import locale
-import logging.handlers
 import nntplib
 import os
 import platform
@@ -109,8 +108,6 @@
     "bind_unix_socket",
     # processes
     'temp_umask', "reap_children",
-    # logging
-    "TestHandler",
     # threads
     "threading_setup", "threading_cleanup", "reap_threads", "start_threads",
     # miscellaneous
@@ -2522,37 +2519,6 @@ def optim_args_from_interpreter_flags():
     optimization settings in sys.flags."""
     return subprocess._optim_args_from_interpreter_flags()
 
-#============================================================
-# Support for assertions about logging.
-#============================================================
-
-class TestHandler(logging.handlers.BufferingHandler):
-    def __init__(self, matcher):
-        # BufferingHandler takes a "capacity" argument
-        # so as to know when to flush. As we're overriding
-        # shouldFlush anyway, we can set a capacity of zero.
-        # You can call flush() manually to clear out the
-        # buffer.
-        logging.handlers.BufferingHandler.__init__(self, 0)
-        self.matcher = matcher
-
-    def shouldFlush(self):
-        return False
-
-    def emit(self, record):
-        self.format(record)
-        self.buffer.append(record.__dict__)
-
-    def matches(self, **kwargs):
-        """
-        Look for a saved dict whose keys/values match the supplied arguments.
-        """
-        result = False
-        for d in self.buffer:
-            if self.matcher.matches(d, **kwargs):
-                result = True
-                break
-        return result
 
 class Matcher(object):
 

Lib/test/support/logging_helper.py

@@ -0,0 +1,29 @@
+import logging.handlers
+
+class TestHandler(logging.handlers.BufferingHandler):
+    def __init__(self, matcher):
+        # BufferingHandler takes a "capacity" argument
+        # so as to know when to flush. As we're overriding
+        # shouldFlush anyway, we can set a capacity of zero.
+        # You can call flush() manually to clear out the
+        # buffer.
+        logging.handlers.BufferingHandler.__init__(self, 0)
+        self.matcher = matcher
+
+    def shouldFlush(self):
+        return False
+
+    def emit(self, record):
+        self.format(record)
+        self.buffer.append(record.__dict__)
+
+    def matches(self, **kwargs):
+        """
+        Look for a saved dict whose keys/values match the supplied arguments.
+        """
+        result = False
+        for d in self.buffer:
+            if self.matcher.matches(d, **kwargs):
+                result = True
+                break
+        return result

Lib/test/test_logging.py

@@ -43,6 +43,7 @@
 import tempfile
 from test.support.script_helper import assert_python_ok, assert_python_failure
 from test import support
+from test.support.logging_helper import TestHandler
 import textwrap
 import threading
 import time
@@ -3523,7 +3524,7 @@ def test_formatting(self):
     @unittest.skipUnless(hasattr(logging.handlers, 'QueueListener'),
                          'logging.handlers.QueueListener required for this test')
     def test_queue_listener(self):
-        handler = support.TestHandler(support.Matcher())
+        handler = TestHandler(support.Matcher())
         listener = logging.handlers.QueueListener(self.queue, handler)
         listener.start()
         try:
@@ -3539,7 +3540,7 @@ def test_queue_listener(self):
 
         # Now test with respect_handler_level set
 
-        handler = support.TestHandler(support.Matcher())
+        handler = TestHandler(support.Matcher())
         handler.setLevel(logging.CRITICAL)
         listener = logging.handlers.QueueListener(self.queue, handler,
                                                   respect_handler_level=True)

Lib/unittest/case.py

@@ -3,7 +3,6 @@
 import sys
 import functools
 import difflib
-import logging
 import pprint
 import re
 import warnings
@@ -16,6 +15,8 @@
 from .util import (strclass, safe_repr, _count_diff_all_purpose,
                    _count_diff_hashable, _common_shorten_repr)
 
+logging = None  # imported lazily
+
 __unittest = True
 
 _subtest_msg_sentinel = object()
@@ -300,24 +301,33 @@ def __exit__(self, exc_type, exc_value, tb):
 _LoggingWatcher = collections.namedtuple("_LoggingWatcher",
                                          ["records", "output"])
 
+# Import logging lazily for assertLogs().
+# assertLogs() is one of the least used assertions, and most tests do
+# not need to import logging.
 
-class _CapturingHandler(logging.Handler):
-    """
-    A logging handler capturing all (raw and formatted) logging output.
-    """
+def _lazy_logging_import():
+    global logging
+    global _CapturingHandler
+    if logging:
+        return
 
-    def __init__(self):
-        logging.Handler.__init__(self)
-        self.watcher = _LoggingWatcher([], [])
+    import logging
+    class _CapturingHandler(logging.Handler):
+        """
+        A logging handler capturing all (raw and formatted) logging output.
+        """
 
-    def flush(self):
-        pass
+        def __init__(self):
+            logging.Handler.__init__(self)
+            self.watcher = _LoggingWatcher([], [])
 
-    def emit(self, record):
-        self.watcher.records.append(record)
-        msg = self.format(record)
-        self.watcher.output.append(msg)
+        def flush(self):
+            pass
 
+        def emit(self, record):
+            self.watcher.records.append(record)
+            msg = self.format(record)
+            self.watcher.output.append(msg)
 
 
 class _AssertLogsContext(_BaseTestCaseContext):
@@ -328,6 +338,7 @@ class _AssertLogsContext(_BaseTestCaseContext):
     def __init__(self, test_case, logger_name, level):
         _BaseTestCaseContext.__init__(self, test_case)
         self.logger_name = logger_name
+        _lazy_logging_import()
         if level:
             self.level = logging._nameToLevel.get(level, level)
         else:

Misc/NEWS.d/next/Library/2020-04-20-19-06-55.bpo-40275.9UcN2g.rst

@@ -0,0 +1,2 @@
+The :mod:`logging` package is now imported lazily in :mod:`unittest` only
+when the :meth:`~unittest.TestCase.assertLogs` assertion is used.