bpo-40275: Avoid importing logging in test.support (GH-19601)

Import logging lazily in assertLogs() in unittest.
Move TestHandler from test.support to logging_helper.

7 files changed, 105 insertions, 109 deletions

diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 1e6b111..c2aaecc 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -1410,11 +1410,6 @@ 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__`
diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 15cf45d..f48decc 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -15,7 +15,6 @@ import hashlib
 import importlib
 import importlib.util
 import locale
-import logging.handlers
 import os
 import platform
 import re
@@ -99,8 +98,6 @@ __all__ = [
     "open_urlresource",
     # processes
     'temp_umask', "reap_children",
-    # logging
-    "TestHandler",
     # threads
     "threading_setup", "threading_cleanup", "reap_threads", "start_threads",
     # miscellaneous
@@ -2368,37 +2365,6 @@ 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):
 
diff --git a/Lib/test/support/logging_helper.py b/Lib/test/support/logging_helper.py
new file mode 100644
index 0000000..12fcca4
--- /dev/null
+++ b/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
diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index 241ed2c..e1d0eb8 100644
--- a/Lib/test/test_logging.py
+++ b/Lib/test/test_logging.py
@@ -44,6 +44,7 @@ import tempfile
 from test.support.script_helper import assert_python_ok, assert_python_failure
 from test import support
 from test.support import socket_helper
+from test.support.logging_helper import TestHandler
 import textwrap
 import threading
 import time
@@ -3524,7 +3525,7 @@ class QueueHandlerTest(BaseTest):
     @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:
@@ -3540,7 +3541,7 @@ class QueueHandlerTest(BaseTest):
 
         # 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)
diff --git a/Lib/unittest/_log.py b/Lib/unittest/_log.py
new file mode 100644
index 0000000..94e7e75
--- /dev/null
+++ b/Lib/unittest/_log.py
@@ -0,0 +1,69 @@
+import logging
+import collections
+
+from .case import _BaseTestCaseContext
+
+
+_LoggingWatcher = collections.namedtuple("_LoggingWatcher",
+                                         ["records", "output"])
+
+class _CapturingHandler(logging.Handler):
+    """
+    A logging handler capturing all (raw and formatted) logging output.
+    """
+
+    def __init__(self):
+        logging.Handler.__init__(self)
+        self.watcher = _LoggingWatcher([], [])
+
+    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):
+    """A context manager used to implement TestCase.assertLogs()."""
+
+    LOGGING_FORMAT = "%(levelname)s:%(name)s:%(message)s"
+
+    def __init__(self, test_case, logger_name, level):
+        _BaseTestCaseContext.__init__(self, test_case)
+        self.logger_name = logger_name
+        if level:
+            self.level = logging._nameToLevel.get(level, level)
+        else:
+            self.level = logging.INFO
+        self.msg = None
+
+    def __enter__(self):
+        if isinstance(self.logger_name, logging.Logger):
+            logger = self.logger = self.logger_name
+        else:
+            logger = self.logger = logging.getLogger(self.logger_name)
+        formatter = logging.Formatter(self.LOGGING_FORMAT)
+        handler = _CapturingHandler()
+        handler.setFormatter(formatter)
+        self.watcher = handler.watcher
+        self.old_handlers = logger.handlers[:]
+        self.old_level = logger.level
+        self.old_propagate = logger.propagate
+        logger.handlers = [handler]
+        logger.setLevel(self.level)
+        logger.propagate = False
+        return handler.watcher
+
+    def __exit__(self, exc_type, exc_value, tb):
+        self.logger.handlers = self.old_handlers
+        self.logger.propagate = self.old_propagate
+        self.logger.setLevel(self.old_level)
+        if exc_type is not None:
+            # let unexpected exceptions pass through
+            return False
+        if len(self.watcher.records) == 0:
+            self._raiseFailure(
+                "no logs of level {} or higher triggered on {}"
+                .format(logging.getLevelName(self.level), self.logger.name))
diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index d0ee561..f8bc865 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -3,7 +3,6 @@
 import sys
 import functools
 import difflib
-import logging
 import pprint
 import re
 import warnings
@@ -297,73 +296,6 @@ class _AssertWarnsContext(_AssertRaisesBaseContext):
 
 
 
-_LoggingWatcher = collections.namedtuple("_LoggingWatcher",
-                                         ["records", "output"])
-
-
-class _CapturingHandler(logging.Handler):
-    """
-    A logging handler capturing all (raw and formatted) logging output.
-    """
-
-    def __init__(self):
-        logging.Handler.__init__(self)
-        self.watcher = _LoggingWatcher([], [])
-
-    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):
-    """A context manager used to implement TestCase.assertLogs()."""
-
-    LOGGING_FORMAT = "%(levelname)s:%(name)s:%(message)s"
-
-    def __init__(self, test_case, logger_name, level):
-        _BaseTestCaseContext.__init__(self, test_case)
-        self.logger_name = logger_name
-        if level:
-            self.level = logging._nameToLevel.get(level, level)
-        else:
-            self.level = logging.INFO
-        self.msg = None
-
-    def __enter__(self):
-        if isinstance(self.logger_name, logging.Logger):
-            logger = self.logger = self.logger_name
-        else:
-            logger = self.logger = logging.getLogger(self.logger_name)
-        formatter = logging.Formatter(self.LOGGING_FORMAT)
-        handler = _CapturingHandler()
-        handler.setFormatter(formatter)
-        self.watcher = handler.watcher
-        self.old_handlers = logger.handlers[:]
-        self.old_level = logger.level
-        self.old_propagate = logger.propagate
-        logger.handlers = [handler]
-        logger.setLevel(self.level)
-        logger.propagate = False
-        return handler.watcher
-
-    def __exit__(self, exc_type, exc_value, tb):
-        self.logger.handlers = self.old_handlers
-        self.logger.propagate = self.old_propagate
-        self.logger.setLevel(self.old_level)
-        if exc_type is not None:
-            # let unexpected exceptions pass through
-            return False
-        if len(self.watcher.records) == 0:
-            self._raiseFailure(
-                "no logs of level {} or higher triggered on {}"
-                .format(logging.getLevelName(self.level), self.logger.name))
-
-
 class _OrderedChainMap(collections.ChainMap):
     def __iter__(self):
         seen = set()
@@ -854,6 +786,8 @@ class TestCase(object):
             self.assertEqual(cm.output, ['INFO:foo:first message',
                                          'ERROR:foo.bar:second message'])
         """
+        # Lazy import to avoid importing logging if it is not needed.
+        from ._log import _AssertLogsContext
         return _AssertLogsContext(self, logger, level)
 
     def _getAssertEqualityFunc(self, first, second):
diff --git a/Misc/NEWS.d/next/Library/2020-04-20-19-06-55.bpo-40275.9UcN2g.rst b/Misc/NEWS.d/next/Library/2020-04-20-19-06-55.bpo-40275.9UcN2g.rst
new file mode 100644
index 0000000..09e0a97
--- /dev/null
+++ b/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.