Why not take a dive in the code and see...
The module we land on is flask.logging.py, which defines a function named create_logger(app). Inspecting that function will give a few clues as to potential culprits when troubleshooting logging issues with Flask.
EDIT: this answer was meant for Flask prior to version 1. The flask.logging.py module has considerably changed since then. The answer still helps with some general caveats and advices regarding python logging, but be aware that some of Flask's peculiarities in that regard have been addressed in version 1 and might no longer apply.
The first possible cause of conflicts in that function is this line:
logger = getLogger(app.logger_name)
Let's see why:
The variable app.logger_name is set in the Flask.__init__() method to the value of import_name, which is itself the receiving parameter of Flask(__name__). That is app.logger_name is assigned the value of __name__, which will likely be the name of your main package, let's for this example call it 'awesomeapp'.
Now, imagine that you decided to configure and create your own logger manually. What do you think the chances are that if your project is named "awesomeapp" you would also use that name to configure your logger, I think it's pretty likely.
my_logger = logging.getLogger('awesomeapp') # doesn't seem like a bad idea
fh = logging.FileHandler('/tmp/my_own_log.log')
my_logger.setLevel(logging.DEBUG)
my_logger.addHandler(fh)
It makes sense to do this... except for a few problems.
When the Flask.logger property is invoked for the first time it will in turn call the function flask.logging.create_logger() and the following actions will ensue:
logger = getLogger(app.logger_name)
Remember how you named your logger after the project and how app.logger_name shares that name too? What happens in the line of code above is that the function logging.getLogger() has now retrieved your previously created logger and the following instructions are about to mess with it in a way that will have you scratching your head later. For instance
del logger.handlers[:]
Poof, you just lost all the handlers you may have previously registered with your logger.
Other things that happen within the function, without going much into details. It creates and registers two logging.StreamHandler objects that can spit out to sys.stderr and/or Response objects. One for log level 'debug' and another for 'production'.
class DebugLogger(Logger):
def getEffectiveLevel(self):
if self.level == 0 and app.debug:
return DEBUG
return Logger.getEffectiveLevel(self)
class DebugHandler(StreamHandler):
def emit(self, record):
if app.debug and _should_log_for(app, 'debug'):
StreamHandler.emit(self, record)
class ProductionHandler(StreamHandler):
def emit(self, record):
if not app.debug and _should_log_for(app, 'production'):
StreamHandler.emit(self, record)
debug_handler = DebugHandler()
debug_handler.setLevel(DEBUG)
debug_handler.setFormatter(Formatter(DEBUG_LOG_FORMAT))
prod_handler = ProductionHandler(_proxy_stream)
prod_handler.setLevel(ERROR)
prod_handler.setFormatter(Formatter(PROD_LOG_FORMAT))
logger.__class__ = DebugLogger
logger.addHandler(debug_handler)
logger.addHandler(prod_handler)
With the above details to light it should become clearer why our manually configured logger and handlers misbehave when Flask gets involved. The new information gives us new options though. If you still want to keep separate handlers, the simplest approach is to name your logger to something different than the project (e.g. my_logger = getLogger('awesomeapp_logger') ). Another approach, if you want to be consistent with the logging protocols in Flask, is to register a logging.FileHandler object on Flask.logger using a similar approach to Flask.
import logging
def set_file_logging_handler(app):
logging_path = app.config['LOGGING_PATH']
class DebugFileHandler(logging.FileHandler):
def emit(self, record):
# if your app is configured for debugging
# and the logger has been set to DEBUG level (the lowest)
# push the message to the file
if app.debug and app.logger.level==logging.DEBUG:
super(DebugFileHandler, self).emit(record)
debug_file_handler = DebugFileHandler('/tmp/my_own_log.log')
app.logger.addHandler(debug_file_handler)
app = Flask(__name__)
# the config presumably has the debug settings for your app
app.config.from_object(config)
set_file_logging_handler(app)
app.logger.info('show me something')
