FakeLogger: Mis-formatted log messages will raise Exception

When using the FakeLogger, have mis-formatted logging messages raise an
exception.

Normally when using the logging module, mis-formatted logging messages
will not raise an exception. Instead the exception will be printed but
not raised.

Change this behavior so that mis-formatted log messages can be caught
during unit-testing.

Closes-Bug: #1503049
Change-Id: I8d3e94d131289300ae020eb1d63306489e986335

NEWS

@@ -13,6 +13,9 @@ NEXT
   backport has significant bugfixes over older but still supported CPython
   stdlib versions. (Robert Collins)
 
+* ``fixtures.FakeLogger`` now detects incorrectly formatted log messages.
+  (John Villalovos, #1503049)
+
 1.3.1
 ~~~~~
 

fixtures/_fixtures/logger.py

@@ -14,6 +14,8 @@
 # limitations under that license.
 
 from logging import StreamHandler, getLogger, INFO, Formatter
+import six
+import sys
 
 from testtools.compat import _u
 
@@ -61,6 +63,12 @@ def _setUp(self):
             self.addCleanup(logger.removeHandler, self.handler)
 
 
+class StreamHandlerRaiseException(StreamHandler):
+    """Handler class that will raise an exception on formatting errors."""
+    def handleError(self, record):
+        six.reraise(*sys.exc_info())
+
+
 class FakeLogger(Fixture):
     """Replace a logger and capture its output."""
 
@@ -98,7 +106,7 @@ def _setUp(self):
         name = _u("pythonlogging:'%s'") % self._name
         output = self.useFixture(StringStream(name)).stream
         self._output = output
-        handler = StreamHandler(output)
+        handler = StreamHandlerRaiseException(output)
         if self._format:
             formatter = (self._formatter or Formatter)
             handler.setFormatter(formatter(self._format, self._datefmt))

fixtures/tests/_fixtures/test_logger.py

@@ -17,6 +17,7 @@
 import sys
 import time
 
+import testtools
 from testtools import TestCase
 from testtools.compat import StringIO
 
@@ -140,6 +141,11 @@ def test_logging_output_included_in_details(self):
         except:
             pass
 
+    def test_exceptionraised(self):
+        with FakeLogger():
+            with testtools.ExpectedException(TypeError):
+                logging.info("Some message", "wrongarg")
+
 
 class LogHandlerTest(TestCase, TestWithFixtures):