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Category-based logging with Flask














Gergely Polonkai

Im in a team who are developing a Flask-based web application, which uses logging extensively. For a while now it spews out a lot of lines so the need arose to index them in ElasticSearch, and more importantly, to search through them for auditing purposes. This latter user story brought up one more question: why dont we categorize our log messages? I quickly came up with an extended log format ([auth] is the new category name):

[2017-01-14 00:55:42,554] [8286] [INFO] [auth] invalid password for [at, in function utils.validate_login]

Here, [auth] is the category name. In the ideal solution, all Id have to do is adding %(category)s to my formatter, and I could call, extra={'auth': 'invalid password'})

to achieve this output. Unfortunately, Flask.logger (and, in the background, the logging module) is not that easy to tame.

As it turns out, a Flask applications logger property is a subclass of logging.Logger, so my first idea was to monkey patch that class. When the apps logger is initialised, it subclasses logging.Logger and tweaks the log level so it goes down to logging.DEBUG if the app is running in debug mode. This is done by using a different logger class depending on the app config. Fortunately it doesnt directly subclass logging.Logger; it calls logging.getLoggerClass() to find which class it should extend. To achieve my goals, all I had to do is to subclass the original logger class, and pass it to logging.setLoggerClass() before I initialise my app, and I have a fail-safe(ish) solution. So far so good, on to the extra category parameter.

Now if you add a new variable to the formatter like my new %(category)s, you get a nifty KeyError saying there is no category in the format expansion dictionary. If you add category='auth' to the calls and its cousins, its fine, because these methods use the magic **kwarg argument to swallow it. Everything goes well until control arrives to the _log() method: it complains about that extra category keyword argument. Taking a peek at Pythons internals, I found two things: info()`, `error(), and co. pass *args and **kwargs to _log() unmodified, and the _log() method doesnt have **kwargs present in its definition to swallow it. A little doc reading later I found that if I want to pass extra arguments for such a formatter, I should do it via the extra keyword argument to _log(). A call like'invalid password', extra={'category': 'auth'}) solved the problem. Now that is tedious.

My first idea was to override all the standard logging methods like info() and error(), and handle category there. But this resulted in lots of repeating code. I changed the specification a bit, so my calls would look like info('message', category='auth) instead of the original plan of info('auth', 'message'): as the logging methods pass all keyword arguments to _log(), I can handle it there. So at the end, my new logger class only patches _log(), by picking out category from the kwarg list, and inserting it to extra before calling super.

As you can see, this is a bit ugly solution. It requires me, the app author, to know about Flasks internals (that I can set my own logging class before the app is created, and so the app will use it.) This means if Flask developers change the way how logging is done, I have to adapt and find a workaround for the new version (well, unless they let me directly set the logging class. That would make it easy.)

What is worse, I must know about Python internals. I know the extra kwarg is documented well (I just failed to notice), but this made adding a new formatter variable so hard. Python version doesnt change as often as Flask version in this project, and I think the standard library wont really change until 4.0, so I dont think my tampering with a “protected” method will cause any trouble in the future. Still, this makes me feel a bit uneasy.

All the above can be circumvented if this class, and the whole solution have some tests. As my class uses the same method as Flask (use logging.getLoggerClass() as a base class instead of using logging.Logger() directly), if the base logger class changes in Python or in the running environment, my app wont care. By checking if the app logger can use my special category variable (ie. it doesnt raise an exception and the category actually gets into the log output), I made sure my class is used as a base in Flask, so if they change the way they construct app.logger, I will know about it when I first run my tests after upgrading Flask.

If you are interested in such functionality (and more), you can grab it from GitHub, or via PyPI.