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How Do You Log a Traceback?

00:00 Getting an exception and its resulting Python traceback means that you need to decide what to do about it. Usually, fixing your code is the first step, but sometimes the problem is with unexpected or incorrect input.

00:12 Whilst it’s good to provide for those situations in your code, sometimes it also makes sense to silence or hide the exception by logging the traceback and doing something else.

00:22 Here’s a real-world example of some code that needs to silence some Python tracebacks. This example uses the requests library.

00:30 This code works well. When you run the script, giving a URL as a command-line argument, it will call the URL and then print the HTTP status code and the content from the response. It even works if the response was a HTTP error status.

00:44 However, sometimes the URL your script is given to retrieve doesn’t exist, or the host server is down. In those cases, this script will now cause an uncaught ConnectionError exception and print a traceback.

00:57 The Python traceback here can be very long with many other exceptions being raised and finally resulting in the ConnectionError being raised by the requests itself.

01:06 If you move up the final exceptions traceback, you can see that the problem all started in our code with line 4 of urlcaller.py. If you wrap the offending line in a try and except block, catching the appropriate exception will allow your script to continue to work with more inputs.

01:22 This version of our urlcaller script uses an else clause with the try and except block. If you’re unfamiliar with this feature of Python, then check out the section on the else clause in the Real Python course called Python Exceptions: An Introduction. Now when you run the script with a URL that will result in a ConnectionError being raised, you’ll get a printed -1 for the status code and the content ConnectionError. This works great. However, in most real systems, you don’t want to just silence the exception and the resulting traceback, but you want to log the traceback.

01:55 Logging tracebacks allows you to have a better understanding of what goes wrong in your programs. Let’s take a look at the final version of your script. You can log the traceback in the script by importing the logging package, getting a logger, and calling .exception() function on that logger in an except portion of the try and except block.

02:17 If you want to learn more about Python’s logging system, you should definitely check out the Logging in Python course here at Real Python. Now when you run the script for a problematic URL, it will print the expected -1 and ConnectionError, but it will also log the traceback. By default, Python will send log messages to stderr (standard error). This looks like we haven’t suppressed the traceback output at all. However, if you call it again and then redirect the stderr, you can see that the logging system is working and we can save our logs off for later. And if we cat the log file, we can view the logged traceback.

02:52 Okay! So, now we know how to log the tracebacks from your scripts. In the next lesson, we’ll review everything that you’ve learned in this course.

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