Category-based logging with Flask
#################################

:date: 2017-03-26T20:00:52Z
:category: blog
:tags: development,python,flask
:url: 2017/03/27/category-based-logging-with-flask/
:save_as: 2017/03/27/category-based-logging-with-flask/index.html
:status: published
:author: Gergely Polonkai

I’m 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 don’t we categorize our log messages?  I quickly came up with an extended
log format (``[auth]`` is the new category name):

.. code-block:: log

   [2017-01-14 00:55:42,554] [8286] [INFO] [auth] invalid password for john@example.com [at __init__.py:12, in function utils.validate_login]

Here, ``[auth]`` is the category name.  In the ideal solution, all I’d have to
do is adding ``%(category)s`` to my formatter, and I could call

.. code-block:: python

   app.logger.info(message, 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 application’s ``logger`` property is a subclass of ``logging.Logger``, so
my first idea was to monkey patch that class.  When the app’s 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 doesn’t 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 ``app.logger.info()`` calls and its cousins, it’s 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 Python’s internals, I found two things: ``info()`, `error()``, and co. pass
``*args`` and ``**kwargs`` to ``_log()`` unmodified, and the ``_log()`` method doesn’t 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 ``app.logger.info('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 Flask’s 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
doesn’t change as often as Flask version in this project, and I think the standard library won’t
really change until 4.0, so I don’t 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 won’t care.  By checking if the app logger can use my special ``category``
variable (ie. it doesn’t 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
<https://github.com/gergelypolonkai/flask-logging-extras>`_, or via `PyPI
<https://pypi.python.org/pypi/Flask-Logging-Extras/>`_.
