bpo-39385: Add an assertNoLogs context manager to unittest.TestCase (GH-18067)

Co-authored-by: Rémi Lapeyre <remi.lapeyre@henki.fr>

Doc/library/unittest.rst

@@ -950,6 +950,9 @@ Test cases
    | :meth:`assertLogs(logger, level)                        | The ``with`` block logs on *logger*  | 3.4        |
    | <TestCase.assertLogs>`                                  | with minimum *level*                 |            |
    +---------------------------------------------------------+--------------------------------------+------------+
+   | :meth:`assertNoLogs(logger, level)                      | The ``with`` block does not log on   | 3.10       |
+   | <TestCase.assertNoLogs>`                                |  *logger* with minimum *level*       |            |
+   +---------------------------------------------------------+--------------------------------------+------------+
 
    .. method:: assertRaises(exception, callable, *args, **kwds)
                assertRaises(exception, *, msg=None)
@@ -1121,6 +1124,24 @@ Test cases
 
       .. versionadded:: 3.4
 
+   .. method:: assertNoLogs(logger=None, level=None)
+
+      A context manager to test that no messages are logged on
+      the *logger* or one of its children, with at least the given
+      *level*.
+
+      If given, *logger* should be a :class:`logging.Logger` object or a
+      :class:`str` giving the name of a logger.  The default is the root
+      logger, which will catch all messages.
+
+      If given, *level* should be either a numeric logging level or
+      its string equivalent (for example either ``"ERROR"`` or
+      :attr:`logging.ERROR`).  The default is :attr:`logging.INFO`.
+
+      Unlike :meth:`assertLogs`, nothing will be returned by the context
+      manager.
+
+      .. versionadded:: 3.10
 
    There are also other methods used to perform more specific checks, such as:
 

Lib/unittest/_log.py

@@ -26,11 +26,11 @@ class _CapturingHandler(logging.Handler):
 
 
 class _AssertLogsContext(_BaseTestCaseContext):
-    """A context manager used to implement TestCase.assertLogs()."""
+    """A context manager for assertLogs() and assertNoLogs() """
 
     LOGGING_FORMAT = "%(levelname)s:%(name)s:%(message)s"
 
-    def __init__(self, test_case, logger_name, level):
+    def __init__(self, test_case, logger_name, level, no_logs):
         _BaseTestCaseContext.__init__(self, test_case)
         self.logger_name = logger_name
         if level:
@@ -38,6 +38,7 @@ class _AssertLogsContext(_BaseTestCaseContext):
         else:
             self.level = logging.INFO
         self.msg = None
+        self.no_logs = no_logs
 
     def __enter__(self):
         if isinstance(self.logger_name, logging.Logger):
@@ -54,15 +55,30 @@ class _AssertLogsContext(_BaseTestCaseContext):
         logger.handlers = [handler]
         logger.setLevel(self.level)
         logger.propagate = False
+        if self.no_logs:
+            return
         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 self.no_logs:
+            # assertNoLogs
+            if len(self.watcher.records) > 0:
+                self._raiseFailure(
+                    "Unexpected logs found: {!r}".format(
+                        self.watcher.output
+                    )
+                )
+
+        else:
+            # assertLogs
             if len(self.watcher.records) == 0:
                 self._raiseFailure(
                     "no logs of level {} or higher triggered on {}"

Lib/unittest/case.py

@@ -295,7 +295,6 @@ class _AssertWarnsContext(_AssertRaisesBaseContext):
             self._raiseFailure("{} not triggered".format(exc_name))
 
 
-
 class _OrderedChainMap(collections.ChainMap):
     def __iter__(self):
         seen = set()
@@ -788,7 +787,16 @@ class TestCase(object):
         """
         # Lazy import to avoid importing logging if it is not needed.
         from ._log import _AssertLogsContext
-        return _AssertLogsContext(self, logger, level)
+        return _AssertLogsContext(self, logger, level, no_logs=False)
+
+    def assertNoLogs(self, logger=None, level=None):
+        """ Fail unless no log messages of level *level* or higher are emitted
+        on *logger_name* or its children.
+
+        This method must be used as a context manager.
+        """
+        from ._log import _AssertLogsContext
+        return _AssertLogsContext(self, logger, level, no_logs=True)
 
     def _getAssertEqualityFunc(self, first, second):
         """Get a detailed comparison function for the types of the two args.

Lib/unittest/test/test_case.py

@@ -1681,6 +1681,81 @@ test case
                 with self.assertLogs('foo'):
                     log_quux.error("1")
 
+    def testAssertLogsUnexpectedException(self):
+        # Check unexpected exception will go through.
+        with self.assertRaises(ZeroDivisionError):
+            with self.assertLogs():
+                raise ZeroDivisionError("Unexpected")
+
+    def testAssertNoLogsDefault(self):
+        with self.assertRaises(self.failureException) as cm:
+            with self.assertNoLogs():
+                log_foo.info("1")
+                log_foobar.debug("2")
+        self.assertEqual(
+            str(cm.exception),
+            "Unexpected logs found: ['INFO:foo:1']",
+        )
+
+    def testAssertNoLogsFailureFoundLogs(self):
+        with self.assertRaises(self.failureException) as cm:
+            with self.assertNoLogs():
+                log_quux.error("1")
+                log_foo.error("foo")
+
+        self.assertEqual(
+            str(cm.exception),
+            "Unexpected logs found: ['ERROR:quux:1', 'ERROR:foo:foo']",
+        )
+
+    def testAssertNoLogsPerLogger(self):
+        with self.assertNoStderr():
+            with self.assertLogs(log_quux):
+                with self.assertNoLogs(logger=log_foo):
+                    log_quux.error("1")
+
+    def testAssertNoLogsFailurePerLogger(self):
+        # Failure due to unexpected logs for the given logger or its
+        # children.
+        with self.assertRaises(self.failureException) as cm:
+            with self.assertLogs(log_quux):
+                with self.assertNoLogs(logger=log_foo):
+                    log_quux.error("1")
+                    log_foobar.info("2")
+        self.assertEqual(
+            str(cm.exception),
+            "Unexpected logs found: ['INFO:foo.bar:2']",
+        )
+
+    def testAssertNoLogsPerLevel(self):
+        # Check per-level filtering
+        with self.assertNoStderr():
+            with self.assertNoLogs(level="ERROR"):
+                log_foo.info("foo")
+                log_quux.debug("1")
+
+    def testAssertNoLogsFailurePerLevel(self):
+        # Failure due to unexpected logs at the specified level.
+        with self.assertRaises(self.failureException) as cm:
+            with self.assertNoLogs(level="DEBUG"):
+                log_foo.debug("foo")
+                log_quux.debug("1")
+        self.assertEqual(
+            str(cm.exception),
+            "Unexpected logs found: ['DEBUG:foo:foo', 'DEBUG:quux:1']",
+        )
+
+    def testAssertNoLogsUnexpectedException(self):
+        # Check unexpected exception will go through.
+        with self.assertRaises(ZeroDivisionError):
+            with self.assertNoLogs():
+                raise ZeroDivisionError("Unexpected")
+
+    def testAssertNoLogsYieldsNone(self):
+        with self.assertNoLogs() as value:
+            pass
+        self.assertIsNone(value)
+
     def testDeprecatedMethodNames(self):
         """
         Test that the deprecated methods raise a DeprecationWarning. See #9424.

Misc/NEWS.d/next/Library/2020-04-23-18-21-19.bpo-39385.MIAyS7.rst

@@ -0,0 +1,3 @@
+A new test assertion context-manager, :func:`unittest.assertNoLogs` will
+ensure a given block of code emits no log messages using the logging module.
+Contributed by Kit Yan Choi.