Fixed #18773 -- Added logging for template variable resolving

Added a django.template logger without a default handler. Added
logging if there is an exception while resolving variables in a
template.

tests/template_tests/test_logging.py

@@ -0,0 +1,71 @@
+from __future__ import unicode_literals
+
+import logging
+
+from django.template import Template, Variable, VariableDoesNotExist
+from django.test import SimpleTestCase
+
+
+class TestHandler(logging.Handler):
+    def __init__(self):
+        super(TestHandler, self).__init__()
+        self.log_record = None
+
+    def emit(self, record):
+        self.log_record = record
+
+
+class VariableResolveLoggingTests(SimpleTestCase):
+    def setUp(self):
+        self.test_handler = TestHandler()
+        self.logger = logging.getLogger('django.template')
+        self.original_level = self.logger.level
+        self.logger.addHandler(self.test_handler)
+        self.logger.setLevel(logging.DEBUG)
+
+    def tearDown(self):
+        self.logger.removeHandler(self.test_handler)
+        self.logger.level = self.original_level
+
+    def test_log_on_variable_does_not_exist_silent(self):
+        class TestObject(object):
+            class SilentDoesNotExist(Exception):
+                silent_variable_failure = True
+
+            @property
+            def template_name(self):
+                return "template"
+
+            @property
+            def template(self):
+                return Template('')
+
+            @property
+            def article(self):
+                raise TestObject.SilentDoesNotExist("Attribute does not exist.")
+
+            def __iter__(self):
+                return iter(attr for attr in dir(TestObject) if attr[:2] != "__")
+
+            def __getitem__(self, item):
+                return self.__dict__[item]
+
+        Variable('article').resolve(TestObject())
+        self.assertEqual(
+            self.test_handler.log_record.msg,
+            'template - Attribute does not exist.'
+        )
+
+    def test_log_on_variable_does_not_exist_not_silent(self):
+        with self.assertRaises(VariableDoesNotExist):
+            Variable('article.author').resolve({'article': {'section': 'News'}})
+
+        self.assertEqual(
+            self.test_handler.log_record.msg,
+            'unknown - Failed lookup for key [author] in %r' %
+            ("{%r: %r}" % ('section', 'News'), )
+        )
+
+    def test_no_log_when_variable_exists(self):
+        Variable('article.section').resolve({'article': {'section': 'News'}})
+        self.assertIsNone(self.test_handler.log_record)