Using Python Logging Effectively

If you’re writing a Python program that doesn’t have a text-based user interface (either it’s a GUI or runs as part of another program, e.g. a webserver) then you should avoid using the print statement. It’s tempting to use print to fill the console with information about what your program is up to. For code of any size though, this quickly devolves into a hard to navigate mess.

Python’s standard library contains a module, logging, that lets you write code to log as much information as you like and configure what you bits you are interested in at runtime.

There are two concepts that you need to understand with logging. Firstly there is the logging level. This is how you determine how important the message is. The levels range from debug as the least important, through info, warning, error, critical to exception, the most important. Secondly there is the logger. This allows you divide your messages into groups depending on the part of your code they relate to. For example, you might have a gui logger and a data logger.

The logging comes with a series of module level functions by each of the names of the logging levels. These make it quick and easy to log a message using the default logger.

logging.debug("Debug message")
logging.error("Error retrieving %s", url)

The second of these two lines has more than one argument. In this case the logging module will treat the first argument as a format string and the rest as arguments to the format, so that line equivalent to this one.

logging.error("Error retrieving %s" % (url, ))

If you try to treat the logging code like you would a print statement and write logging.error("Error retrieving", url) then you’ll get the following, very unhelpfui, error message.

Traceback (most recent call last):
 File "/usr/lib/python2.6/logging/", line 776, in emit
   msg = self.format(record)
 File "/usr/lib/python2.6/logging/", line 654, in format
   return fmt.format(record)
 File "/usr/lib/python2.6/logging/", line 436, in format
   record.message = record.getMessage()
 File "/usr/lib/python2.6/logging/", line 306, in getMessage
   msg = msg % self.args
TypeError: not all arguments converted during string formatting

Notice how this exception doesn’t tell you where the offending logging statement is in your code! Now you know the type of error that will cause this that will help in tracking the problem down, but there is more than can be done to help you find it. The logging library allows you to specify a global error handle, which combined with the print stack trace function will give you a much better error message.

import logging
import traceback
def handleError(self, record):
logging.Handler.handleError = handleError

Loggers are created by calling logging.getLogger('loggername'). This returns an object with the same set of log level functions as the module, but which can be controlled independently. For example:

gui_log = logging.getLogger('gui')
gui_log.debug("created window")
data_log = logging.getLogger('gui')
data_log.debug("loaded file")

Where this comes in really handy is when you set the level of messages that you want to see independently for each logger. In the next code block we set the logging module so we’ll see lots of debugging messages from the GUI and only errors from the data layer. Although here we’re setting the levels directly in code, it’s not a big jump to make them configurable using a command line option.


The logging module also lets you configure how your messages are formatted, and to direct them to files rather than the console. Hopefully this short guide is useful, let me know in the comments!


logging.error("Error retrieving %s", url)
[...] so that line equivalent to this one.
logging.error("Error retrieving %s" % (url, ))

Not exactly and the wrong usage can lead to serious performance problems.

In the first statement, the log message and parameters are pushed on the stack, then the public log method is called. This method first checks if the desired log level is actually enabled before creating a LogRecord for further processing. When the log level is enabled, the LogRecord is passed to the configured log handlers (e.g. StreamHandler). The actual string formatting is done at each handlers emit() method.

With the second statement, string formatting is done first. The formatted string will then be passed to the logger and thrown away if the desired log level is disabled, effectively wasting computing resources.

Mario Steinhoff

12 Sep 2011