Logging is an interesting subject. For most developers, most of the time, we
don’t think too hard about logging. Maybe we’ve got some coding standards that
mandate some minimal level of logging, but otherwise we’re just not concerned
about how we log. But one day you’ll find yourself five hours into a marathon
debugging session, throwing print statements in random places and thinking to
yourself “If only I’d put in some logging here, I’d have some idea of what was
going on in this system!”
So you turn to the Python
logging, open the docs and
recoil in horror. All you want to do is log things! What’s this nonsense about
loggers, and handlers, and LogRecords?
It’s OK, I’m here to help.
My Advice? Ignore It All.
The logging module is incredibly powerful, but as a result it’s also
incredibly complicated. The reality is, most of us will never need to touch its
complexity.
Here’s my promise. The following lines of code will solve 95% of your logging
problems.
Libraries
When you’re writing a library and you want it to log, here’s what you do. Find
your top-level __init__.py file and add the following two lines to it:
import logging
logging.getLogger(__name__).addHandler(logging.NullHandler())This essentially enables the default logging behaviour: do nothing.
Then, in each file where you want to emit logs, add this line at the top:
log = logging.getLogger(__name__)That’s it, you’re done. Seriously. To log, you now just call the logging
methods: log.debug(), log.info(), log.warning(), log.error() and
log.critical(), depending on the severity of log you want.
Applications and Executables
What if you’re an application developer, or you’re writing the top-level
script? You know your libraries are logging, and you are too, but how do you
turn it on? Easy:
import logging
logging.basicConfig()That’s it. Done. You’ll now get your default logging to stderr. Want to put
it somewhere else, or log at a different level?
Here are the docs.
Some highlights are logging at a specific level
(logging.basicConfig(level=logging.DEBUG)) and logging to file
(logging.basicConfig(filename='myapp.log')).
That’s it.
Guidelines
Remember, never use the root logger. Always call .debug() and friends on a
logger you got by calling logging.getLogger(__name__).
Otherwise, there’s really nothing else. If you need anything else in the
logging library, you probably already understand it.
See? Logging is easy.
EDIT (31/05/2014): Note that the NullHandler is only available on Python
2.7 or later. If you want an equivalent for Python 2.6, use this to provide a
fallback:
class NullHandler(logging.Handler):
def emit(self, record):
pass