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Error Handling in Python

try / except

The foundation of error handling in Python is based on appropriate use of try and except statements in your code. You try to run your functions and routines as you would expect to, but you must be ready when an except occurs, i.e. a non-zero result that triggers an error in Python.

Here is a simple framework:

try:
  # do something
except Exception as e:
  # handle an error, where e is now an object for that error
  # and Exception is a base class for all errors.

The try stanza, if successful, will continue to run your process. However, in the except stanza you may want to do something with the raised exception:

  1. Print it to the screen, if your script is run interactively.
  2. Stop the process completely.
  3. Log it to an error log for later review. (See below)

Here is a revised example, incorporating options 1 and 2 above:

import sys

try:
  # do something
except Exception as e:
  # print out the error
  print(e)

  # stop the process and exit with a non-zero status
  sys.exit(1)

Logging

Logging is a useful way of capturing errors and other informative output from your code. Broadly interpreted, logging can mean many things:

  • Appending a log message to a file
  • Sending a log message to a database
  • Sending a log message to a remote logging service
  • Notifying a user via email or other messaging service

Logging to files is made simple with the logging package. This package is generally part of standard Python distributions and does not need to be installed. The logging package provides a Logger class that can be instantiated and used to log messages to a file. Here are the basic options:

Level

Decide the level of events you want to capture. The level will be flagged in each line of your log, which is useful for future filtering and sorting. The developer can choose what level to use with each actual logging call:

  • Debug
  • Info
  • Warning
  • Error
  • Critical

Filename

This is the full path to your error log file.

Filemode

The mode in which your filename is opened. The default is a for append.

Format

The format of the log message. This is a string that can contain select output from your code.

import logging

logging.basicConfig(filename='/var/log/app-error.log', filemode='a', format='%(asctime)s - %(levelname)s - %(message)s')
logging.warning("This is a warning message in the log file")
logging.error("LOOK OUT! This is an error!!")

The log results from above:

2023-09-18 14:09:28,328 - WARNING - This is a warning
2023-09-18 14:10:26,890 - ERROR - LOOK OUT! This is an error!!

Passing in the e exception message to be logged is then simple:

import sys
import logging

# set up logging
logging.basicConfig(filename='/var/log/app-error.log', filemode='a', format='%(asctime)s - %(levelname)s - %(message)s')

# try/except things:
try:
  # do something
except Exception as e:
  # log the error
  logging.error(e)
  # stop the process and exit
  sys.exit(1)

Stack Traces

The logging module also allows you to capture stack traces into the log by passing the exc_info parameter as True:

import logging

x = 23
y = 0

try:
  z = x / y
except Exception as e:
  logging.error("EXCEPTION", exc_info=True)

Which results in the following log message:

ERROR:root:EXCEPTION
Traceback (most recent call last):
  File "logging-demo.py", line 7, in <module>
    z = x / y
ZeroDivisionError: division by zero
[Finished in 0.1s]

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