Use a custom logger subclass that uses DEBUG level if in debug mode

flask.py

@@ -867,7 +867,7 @@ class Flask(_PackageBoundObject):
     #: .. versionadded:: 0.3
     debug_log_format = (
         '-' * 80 + '\n' +
-        '%(levelname)s in %(module)s, %(pathname)s:%(lineno)d]:\n' +
+        '%(levelname)s in %(module)s [%(pathname)s:%(lineno)d]:\n' +
         '%(message)s\n' +
         '-' * 80
     )
@@ -997,15 +997,19 @@ class Flask(_PackageBoundObject):
         with _logger_lock:
             if self._logger and self._logger.name == self.logger_name:
                 return self._logger
-            from logging import getLogger, StreamHandler, Formatter, DEBUG
+            from logging import getLogger, StreamHandler, Formatter, \
+                                Logger,  DEBUG
+            class DebugLogger(Logger):
+                def getEffectiveLevel(x):
+                    return DEBUG if self.debug else Logger.getEffectiveLevel(x)
             class DebugHandler(StreamHandler):
                 def emit(x, record):
-                    if self.debug:
+                    StreamHandler.emit(x, record) if self.debug else None
-                        StreamHandler.emit(x, record)
             handler = DebugHandler()
             handler.setLevel(DEBUG)
             handler.setFormatter(Formatter(self.debug_log_format))
             logger = getLogger(self.logger_name)
+            logger.__class__ = DebugLogger
             logger.addHandler(handler)
             self._logger = logger
             return logger

tests/flask_tests.py

@@ -718,9 +718,11 @@ class LoggingTestCase(unittest.TestCase):
     def test_debug_log(self):
         app = flask.Flask(__name__)
         app.debug = True
+
         @app.route('/')
         def index():
             app.logger.warning('the standard library is dead')
+            app.logger.debug('this is a debug statement')
             return ''
 
         @app.route('/exc')
@@ -731,9 +733,10 @@ class LoggingTestCase(unittest.TestCase):
         with catch_stderr() as err:
             rv = c.get('/')
             out = err.getvalue()
-            assert 'WARNING in flask_tests,' in out
+            assert 'WARNING in flask_tests [' in out
             assert 'flask_tests.py' in out
             assert 'the standard library is dead' in out
+            assert 'this is a debug statement' in out
 
         with catch_stderr() as err:
             try: