In part one of our Python Logging Guide Overview, we covered the basics of Python logging, which included the default logging module, log levels, and several Python logging best practices.
Now, in part two, we’ll build on those basics to cover more advanced Python logging topics:
- Logging to multiple destinations
- Python tracebacks and exceptions
- Structured versus unstructured log data
python-json-loggerto structure log data as JSON objects
If you’re new to Python logging, we recommend reading Part One and brushing up on general logging best practices before proceeding. If you’re already up to speed, let’s dive in!
Logging to Multiple Destinations
In part one, we demonstrated how you can log to single destinations such as:
- The console
- A file
Any one of those may be sufficient for basic use cases. However, you may have a scenario in which you need to emit log messages to multiple destinations. Logging to the console and a file is a common example, so let’s start there.
In general, you can target multiple logging destinations by specifying different log handlers.
basicConfig provides a simple way to specify multiple handlers. In the script below, we use
basicConfig to log the output to the console (stderr) and to a log file called
# import logging import logging # Use basic config to send logs to a file and console logging.basicConfig(handlers=[logging.FileHandler("PythonDemo.log"),logging.StreamHandler()]) # Emit a Warning message logging.warning('You are learning Python logging!')
When you run the script, you should see this output in the console:
WARNING:root:You are learning Python logging!
At the same time, the log file you specified will contain the same text:
With this same approach, we can log to other destinations and handle more advanced use cases as well. For example, borrowing from the official Python Logging Cookbook, we can build a script that logs messages with a level of
DEBUG and above to a file. At the same time, we can log those with a level of
WARNING and above to the console. The code to do this looks like this:
# import logging import logging # Use basic config to send logs to a file at DEBUG level logging.basicConfig(level=logging.DEBUG, filename='PythonDemo.log', filemode='w') # Create a StreamHandler and set it to WARNING level console = logging.StreamHandler() console.setLevel(logging.WARNING) # Add the console handler to the root logger logging.getLogger('').addHandler(console) logging.debug('This is a debug message!') logging.warning('Danger! This is a warning message')
When you run that script, only the
WARNING-level message is logged to the console. However, the log file contains both the
Understanding Exceptions and Tracebacks in Python
Often, you log messages so you can debug unexpected failures. Understanding exceptions and tracebacks are two of the most important aspects of debugging in Python.
What is a Python exception?
A Python exception is an error that occurs during the execution of a program. Exceptions disrupt the normal operation of a Python program. If the program does not have a handler for the exception—for example, by using a try statement—then it will exit.
What is a Python Traceback?
A Python traceback is a report that contains function calls from a specific point in a program. The content in a Python traceback is similar to traditional stacktraces (sometimes called backtraces), except the most recent function calls are at the bottom of the trace. This difference adds some context to the famous
Traceback (most recent call last): messages you may see when a Python program “errors out.”
To understand how tracebacks work, let’s create one. The script below attempts to divide the integer
1 by the string egg:
# Do some bad math 1 / "egg"
When you run this script, you should see an error similar to this in the console:
Here’s a breakdown of how to read that output:
Traceback (most recent call last): This statement tells us that the traceback of the error is beginning.
File "c:\Path\to\your\script\divideByEgg.py", line 2, in <module>1 / "egg": This points us to the specific line where the exception occurred.
TypeError: unsupported operand type(s) for /: 'int' and 'str': This explains to us that the exception is a TypeError, which means there was a problem with our data types. In this case, we can’t perform division using an integer (
'int') and a string (
How to Log Python Tracebacks
Given that tracebacks provide granular detail on why a program failed, logging them is often useful. The default Python logging module includes a
logging.exception function we can use to log exceptions and add custom error messages. The default severity for
To catch and log our example traceback, we can wrap it in a
try except statement. Below is a modified example of our previous script, now set to log the exception to the console and a log file called
# Import the default logging module import logging # Use basic config to send logs to a file and console logging.basicConfig(handlers=[logging.FileHandler("BadMathDemo.log"),logging.StreamHandler()]) # Do some bad math try: 1 / "egg" except: logging.exception('Your math was bad')
Now when we run the script, we’ll see an error message in the console, and the message includes the tracebook. The console output and the contents of
BadMathDemo.log should look similar to the following:
Unstructured versus structured data in Python logs
While humans will often read log messages, in many cases other programs need to ingest logs. When we send logs to other programs, the structure—or lack of structure—in the formatting can make a world of difference.
What is unstructured data?
Unstructured data is data that does not follow a specific pattern or module. Strings that are human-readable sentences—like “Oops! An error occurred. Please check your math and try again!”—are examples of unstructured data. People have no problem parsing the meaning, but things get complex when a computer needs to parse them.
What is structured data?
Structured data is data formatted using objects that follow a specific pattern. JSON, XML, and YAML are common examples of structured data formats. By using data objects (as opposed to strings) and following a specific pattern, structured data makes it much easier for computers to read data and to automate and scale log parsing. As of Python 3.2, Python’s default logging module supports structured data.
python-json-logger is a popular library that makes it easy to emit Python logs as JSON objects. You can install it using
pip with this command:
pip install python-json-logger
To use it, we import the json logger and add
jsonlogger.JsonFormatter() as a formatter in a program. For example, to make our earlier example use JSON formatting and output to a log file called
BadMathDemo.json file, we can use this script:
# Import the default logging module import logging # Import the json logger from pythonjsonlogger import jsonlogger # Get the logger logger = logging.getLogger() # Point filehandler to the output file logHandlerJson = logging.FileHandler("BadMathDemo.json") # Set the formatter formatter = jsonlogger.JsonFormatter() logHandlerJson.setFormatter(formatter) # Add the handler logger.addHandler(logHandlerJson) # Wrap our bad math in a try except statement try: 1 / "egg" except: logging.exception('Your math was bad')
When we run the script, the file should contain a stringified JSON object:
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