Python: How to Create an Exception Logging Decorator

The other day, I decided I wanted to create a decorator to catch exceptions and log them. I found a rather complex example on Github that I used for some ideas on how to approach this task and came up with the following:

# exception_decor.py

import functools
import logging

def create_logger():
    """
    Creates a logging object and returns it
    """
    logger = logging.getLogger("example_logger")
    logger.setLevel(logging.INFO)

    # create the logging file handler
    fh = logging.FileHandler("/path/to/test.log")

    fmt = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
    formatter = logging.Formatter(fmt)
    fh.setFormatter(formatter)

    # add handler to logger object
    logger.addHandler(fh)
    return logger


def exception(function):
    """
    A decorator that wraps the passed in function and logs 
    exceptions should one occur
    """
    @functools.wraps(function)
    def wrapper(*args, **kwargs):
        logger = create_logger()
        try:
            return function(*args, **kwargs)
        except:
            # log the exception
            err = "There was an exception in  "
            err += function.__name__
            logger.exception(err)

            # re-raise the exception
            raise
    return wrapper

In this code, we have two functions. The first one creates a logging object and returns it. The second function is our decorator function. Here we wrap the passed in function in a try/except and log any exceptions that occur using our logger. You will note that I am also logging the function name the the exception occurred in.

Now we just need to test this decorator out. To do so, you can create a new Python script and add the following code to it. Make sure you save this in the same location that you saved the code above.

from exception_decor import exception

@exception
def zero_divide():
    1 / 0

if __name__ == '__main__':
    zero_divide()

When you run this code from the command line, you should end up with a log file that has the following contents:

2016-06-09 08:26:50,874 - example_logger - ERROR - There was an exception in  zero_divide
Traceback (most recent call last):
  File "/home/mike/exception_decor.py", line 29, in wrapper
    return function(*args, **kwargs)
  File "/home/mike/test_exceptions.py", line 5, in zero_divide
    1 / 0
ZeroDivisionError: integer division or modulo by zero

I thought this was a handy piece of code and I hope you will find it useful too!

UPDATE: An astute reader pointed out that it would be a good idea to generalize this script such that you can pass the decorator a logger object. So let’s look at how that works!

Passing a logger to our decorator

First off, let’s split our logging code off into its own module. Let’s call it exception_logger.py. Here’s the code to put into that file:

# exception_logger.py

import logging

def create_logger():
    """
    Creates a logging object and returns it
    """
    logger = logging.getLogger("example_logger")
    logger.setLevel(logging.INFO)

    # create the logging file handler
    fh = logging.FileHandler(r"/path/to/test.log")

    fmt = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
    formatter = logging.Formatter(fmt)
    fh.setFormatter(formatter)

    # add handler to logger object
    logger.addHandler(fh)
    return logger

logger = create_logger()

Next we need to modify our decorator code so we can accept a logger as an argument. Be sure to save it as exception_decor.py

# exception_decor.py

import functools


def exception(logger):
    """
    A decorator that wraps the passed in function and logs 
    exceptions should one occur
    
    @param logger: The logging object
    """
    
    def decorator(func):
    
        def wrapper(*args, **kwargs):
            try:
                return func(*args, **kwargs)
            except:
                # log the exception
                err = "There was an exception in  "
                err += func.__name__
                logger.exception(err)
            
            # re-raise the exception
            raise
        return wrapper
    return decorator

You will note that we have multiple levels of nested functions here. Be sure to study it closely to understand what’s going on. Finally we need to modify our testing script:

from exception_decor import exception
from exception_logger import logger

@exception(logger)
def zero_divide():
    1 / 0

if __name__ == '__main__':
    zero_divide()

Here we import our decorator and our logger. Then we decorate our function and pass the decorator our logger object. If you run this code, you should see the same file generated as you did in the first example. Have fun!