When writing a script or trying to debug a larger program, logging is your friend. Have you ever found yourself adding lots of print statements to try and find the information you need? By properly setting up Python’s logging module, you can save yourself some major headaches!

When I first started writing scripts, I wrote my own logging “system” in every program. It usually ended up looking something like this:

This is definitely useful, but even for quick debugging it’s pretty iffy. If we invoke Python’s built-in logging module, we can reproduce this behavior in about the same number of lines:

Logging has five levels of severity: debug, info, warning, error and critical. By default, Python’s logging module only prints warning,error and critical messages, so only the second message would print to the console in the above example.

If you keep it at the default configuration, it looks like this:

WARNING:root:This is a warning message!

By itself, that’s not tremendously pretty, nor does it give us more information than we started with. Let’s see what happens if we configure the logger ourselves:

Important note: when sending a message to the logger, you use the lowercase version (logger.info(“this is my message!”)). When setting a level (logger.setLevel(logging.INFO)), you need the uppercase version. This is a common “gotcha” for people new to the logging library!

Now we get a lot more information, just by that one change:

[2016-08-09 11:21:02,326] - [INFO] - This is an info message!

This is definitely good information to have, and as a bonus, it’s clear and easy to understand! In the above example, the only formatting bits the logger actually cares about are %(asctime)s, %(levelname)s and %(message)s.

Those aren’t the only ones, though; what happens if we put %(funcName)s into our logger?

I’ll give you a hint; it looks like this!

[DEBUG] - [test_module] - This is a debug message!
[WARNING] - [second_module] - This is a warning message!

If you’re working with scripts that are all in one file, having logging of this type is incredibly useful. But what if you’re working with something even larger?

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The root logger is fairly limited; it can only log one thing with one format. For example, that means you can’t write the log to a file and to the console at the same time. Fortunately, there’s a graceful way to overcome that in the form of “handlers.” Let’s look at adding a handler so we can log to file only, then having both the console and log file report the same thing:

The above will create a file in the same directory as the script with the same type of output as before. What if we want to log the same thing to the file and the console? If we add the following lines after logger.setLevel(logging.DEBUG)

…then we get the same output in the file, but now we get the following on the console:

This is an info message!
This is a warning message!

Notice that the formatting didn’t carry over; we have to declare our own formatting for the new handler. In this case, we’ll just use the “NEW_FORMAT” variable that we declared earlier.

Now we get the same formatting (date, time, level and message) in both the console and the file!

There’s a lot more you can do with Python’s logging module, but I’d like to leave you with my favorite optimization tweak for logging. When creating a bunch of statements in one block, you can perform one check for all of them to see if they should be printed.

Proper logging is absolutely necessary in any decently sized application; even small apps benefit from knowing exactly what variable is getting passed where or finding out what’s broken. At SmartFile, we log everything our applications do and utilize many of the features of Python’s logging module — there’s no job too big or too small!

If you’re interested in learning more about Python’s logging library, here are some good resources:

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