Implement sprockets.logging.ContextFilter

docs/history.rst

@@ -3,4 +3,4 @@ Version History
 
 Next Release
 ------------
- - implement greatness
+ - Added :class:`sprockets.logging.ContextFilter`

sprockets/logging.py

@@ -1,5 +1,39 @@
 """
 Make good log output easier.
+
+- :class:`ContextFilter` adds fixed properties to a log record
+
 """
+from __future__ import absolute_import
+
+import logging
+
+
 version_info = (0, 0, 0)
 __version__ = '.'.join(str(v) for v in version_info)
+
+
+class ContextFilter(logging.Filter):
+    """
+    Ensures that properties exist on a LogRecord.
+
+    :param list|None properties: optional list of properties that
+        will be added to LogRecord instances if they are missing
+
+    This filter implementation will ensure that a set of properties
+    exists on every log record which means that you can always refer
+    to custom properties in a format string.  Without this, referring
+    to a property that is not explicitly passed in will result in an
+    ugly ``KeyError`` exception.
+
+    """
+
+    def __init__(self, name='', properties=None):
+        logging.Filter.__init__(self, name)
+        self.properties = list(properties) if properties else []
+
+    def filter(self, record):
+        for property_name in self.properties:
+            if not hasattr(record, property_name):
+                setattr(record, property_name, None)
+        return True

tests.py

@@ -0,0 +1,44 @@
+import logging
+import uuid
+import unittest
+
+import sprockets.logging
+
+
+class Prototype(object):
+    pass
+
+
+class RecordingHandler(logging.FileHandler):
+    def __init__(self):
+        logging.FileHandler.__init__(self, filename='/dev/null')
+        self.log_lines = []
+
+    def format(self, record):
+        log_line = logging.FileHandler.format(self, record)
+        self.log_lines.append(log_line)
+        return log_line
+
+
+class ContextFilterTests(unittest.TestCase):
+
+    def setUp(self):
+        super(ContextFilterTests, self).setUp()
+        self.logger = logging.getLogger(uuid.uuid4().hex)
+        self.handler = RecordingHandler()
+        self.logger.addHandler(self.handler)
+
+    def test_that_filter_blocks_key_errors(self):
+        formatter = logging.Formatter('%(message)s [%(context)s]')
+        self.handler.setFormatter(formatter)
+        self.handler.addFilter(sprockets.logging.ContextFilter(
+            properties=['context']))
+        self.logger.info('hi there')
+
+    def test_that_filter_does_not_overwrite_extras(self):
+        formatter = logging.Formatter('%(message)s [%(context)s]')
+        self.handler.setFormatter(formatter)
+        self.handler.addFilter(sprockets.logging.ContextFilter(
+            properties=['context']))
+        self.logger.info('hi there', extra={'context': 'foo'})
+        self.assertEqual(self.handler.log_lines[-1], 'hi there [foo]')