Undo log level set by caplog.set_level at the end of the test

Otherwise this leaks the log level information to other tests

Ref: pytest-dev#3013

_pytest/logging.py

@@ -129,6 +129,17 @@ class LogCaptureFixture(object):
     def __init__(self, item):
         """Creates a new funcarg."""
         self._item = item
+        self._initial_log_levels = {}  # type: Dict[str, int] # dict of log name -> log level
+
+    def _finalize(self):
+        """Finalizes the fixture.
+
+        This restores the log levels changed by :meth:`set_level`.
+        """
+        # restore log levels
+        for logger_name, level in self._initial_log_levels.items():
+            logger = logging.getLogger(logger_name)
+            logger.setLevel(level)
 
     @property
     def handler(self):
@@ -160,27 +171,30 @@ def clear(self):
         self.handler.records = []
 
     def set_level(self, level, logger=None):
-        """Sets the level for capturing of logs.
+        """Sets the level for capturing of logs. The level will be restored to its previous value at the end of
+        the test.
 
         :param int level: the logger to level.
         :param str logger: the logger to update the level. If not given, the root logger level is updated.
+
+        .. versionchanged:: 3.4
+            The levels of the loggers changed by this function will be restored to their initial values at the
+            end of the test.
         """
-        logger = logging.getLogger(logger)
+        logger_name = logger
+        logger = logging.getLogger(logger_name)
+        self._initial_log_levels.setdefault(logger_name, logger.level)
         logger.setLevel(level)
 
     @contextmanager
     def at_level(self, level, logger=None):
-        """Context manager that sets the level for capturing of logs.
+        """Context manager that sets the level for capturing of logs. After the end of the 'with' statement the
+        level is restored to its original value.
 
-        By default, the level is set on the handler used to capture
-        logs. Specify a logger name to instead set the level of any
-        logger.
+        :param int level: the logger to level.
+        :param str logger: the logger to update the level. If not given, the root logger level is updated.
         """
-        if logger is None:
-            logger = self.handler
-        else:
-            logger = logging.getLogger(logger)
-
+        logger = logging.getLogger(logger)
         orig_level = logger.level
         logger.setLevel(level)
         try:
@@ -199,7 +213,9 @@ def caplog(request):
     * caplog.records()       -> list of logging.LogRecord instances
     * caplog.record_tuples() -> list of (logger_name, level, message) tuples
     """
-    return LogCaptureFixture(request.node)
+    result = LogCaptureFixture(request.node)
+    yield result
+    result._finalize()
 
 
 def get_actual_log_level(config, *setting_names):

testing/logging/test_fixture.py

@@ -26,6 +26,30 @@ def test_change_level(caplog):
     assert 'CRITICAL' in caplog.text
 
 
+def test_change_level_undo(testdir):
+    """Ensure that 'set_level' is undone after the end of the test"""
+    testdir.makepyfile('''
+        import logging
+
+        def test1(caplog):
+            caplog.set_level(logging.INFO)
+            # using + operator here so fnmatch_lines doesn't match the code in the traceback
+            logging.info('log from ' + 'test1')
+            assert 0
+
+        def test2(caplog):
+            # using + operator here so fnmatch_lines doesn't match the code in the traceback
+            logging.info('log from ' + 'test2')
+            assert 0
+    ''')
+    result = testdir.runpytest_subprocess()
+    result.stdout.fnmatch_lines([
+        '*log from test1*',
+        '*2 failed in *',
+    ])
+    assert 'log from test2' not in result.stdout.str()
+
+
 def test_with_statement(caplog):
     with caplog.at_level(logging.INFO):
         logger.debug('handler DEBUG level')
@@ -42,13 +66,15 @@ def test_with_statement(caplog):
 
 
 def test_log_access(caplog):
+    caplog.set_level(logging.INFO)
     logger.info('boo %s', 'arg')
     assert caplog.records[0].levelname == 'INFO'
     assert caplog.records[0].msg == 'boo %s'
     assert 'boo arg' in caplog.text
 
 
 def test_record_tuples(caplog):
+    caplog.set_level(logging.INFO)
     logger.info('boo %s', 'arg')
 
     assert caplog.record_tuples == [
@@ -57,13 +83,15 @@ def test_record_tuples(caplog):
 
 
 def test_unicode(caplog):
+    caplog.set_level(logging.INFO)
     logger.info(u'bū')
     assert caplog.records[0].levelname == 'INFO'
     assert caplog.records[0].msg == u'bū'
     assert u'bū' in caplog.text
 
 
 def test_clear(caplog):
+    caplog.set_level(logging.INFO)
     logger.info(u'bū')
     assert len(caplog.records)
     caplog.clear()