Logging debug or error messages is crucial for any application as it helps developers supervise and troubleshoot the application efficiently.<\/p>\n\n\n\n
Python provides a module named In this article, you’ll learn:<\/p>\n\n\n\n Here’s an example of logging a simple message on the console using the The code above runs endlessly, thus when it runs, a warning message will be logged to alert the user.<\/p>\n\n\n\n Here, As you can see, the output begins by logging the warning message “Entering Infinite Loop”<\/strong> in the pattern Similar to this, the There are 5 different levels to log messages of different severity levels.<\/p>\n\n\n\n You can observe that the output is missing the first two messages. Why is it so?<\/strong> The messages with a severity level of You can log The The The When you run the above code, the severity level Similarly, you can log The The message will be logged into the By default, the mode of the file is set to append ( In this example, the file is set to write ( Logging variable data is similar to formatting a string using the In this code, two placeholders ( When you run the code, you’ll get the following output.<\/p>\n\n\n\n You now know how to log variable data; in this section, you’ll utilize it to format the logged messages.<\/p>\n\n\n\n You can choose the format of your logged message to be displayed in the console or a file.<\/p>\n\n\n\n As you can see, the message is logged with the formatting that the You may be wondering now where the Now, this time a new attribute You can take more control of formatting the date and time of the event by passing the The date and time looks more readable now due to the string ( You can add some more attributes such as file name, module name, process ID, name of the logger, and much more.<\/p>\n\n\n\n So far you’ve learned to log messages at the basic level. In the next section, you’ll learn to log messages at an advanced level such as configuring your logger, sending the log messages over multiple destinations, and much more.<\/p>\n\n\n\n In this section, you’ll use classes and functions to log messages. The commonly used classes in the In this example, a custom logger named This is a simple message without the severity level and logger name even though the name of the logger was specified. This is because the custom logger can’t be configured using An important note:<\/p>\n\n\n\n It is a good practice to name your logger based on module like the following.<\/p>\n\n\n\n This will keep track of your module\/package hierarchy. It means if you are using it in same module, the logger name will be The logger object doesn’t log messages of severity level Handler objects are responsible for logging records to the specified destinations. There are several handler types such as The After creating a logger object in the previous section, the next step is to create the handler.<\/p>\n\n\n\n In this code, some modifications have been done to the logger object such as the logger’s name changed and the logger’s level has been set to Two handlers have been created Formatters are responsible for applying the formatting of the developer’s choice to the final output.<\/p>\n\n\n\n It is similar to the formatting you’ve learned while setting up the basic configurations of the messages using Here’s the extension of the previous code.<\/p>\n\n\n\n The formatters To log messages with applied configurations, the final step is to add these handlers ( Now add messages to be logged.<\/p>\n\n\n\n Output<\/strong><\/p>\n\n\n\n Here, the Since the logger’s level is set to The You can see that the name of the logger is logged as Create a new Python file, in this case, When you run this file, your output will look like the following.<\/p>\n\n\n\n The You’ve learned to create the logger and handlers with specified formatting. Now you can create a template for logging the messages and include it in your project code to log messages dynamically.<\/p>\n\n\n\n Here’s a practical example of creating a logger with advanced configurations and then including it in the main script to log messages in a file.<\/p>\n\n\n\n First, create a Python file, in this case, logging<\/code> to log messages. These messages can be written to various destinations, including files, for better monitoring and debugging of programs.<\/p>\n\n\n\n\n
logging<\/strong><\/code> module to log<\/strong> INFO<\/strong><\/code> and<\/strong> DEBUG<\/strong><\/code> level messages and logging events to a file<\/strong><\/li>\n\n\n\nLogging Messages<\/h2>\n\n\n\n
logging<\/code> module in Python.<\/p>\n\n\n\n# Imported logging module\nimport logging\n\nnum = 0\n# Logs a warning message\nlogging.warning(\"Entering Infinite Loop\")\nwhile True:\n if num % 2 == 0:\n print(num)\n num += 1<\/pre><\/div>\n\n\n\n
logging.warning()<\/code> has been used to log the warning message to the console. The warning<\/code> here is the severity level of the message.<\/p>\n\n\n\nWARNING:root:Entering Infinite Loop\n0\n2\n4\n.\n.\n.<\/pre><\/div>\n\n\n\n
WARNING:root:<message><\/code>, indicating the severity level and name assigned by the logger respectively.<\/p>\n\n\n\nlogging<\/code> module offers several levels to show the events’ severity.<\/p>\n\n\n\n# Imported logging module\nimport logging\n\nlogging.info(\"Used to log info message\")\nlogging.debug(\"Used to log debug message\")\nlogging.warning(\"Used to log warning message\")\nlogging.error(\"Used to log error message\")\nlogging.critical(\"Used to log critical message\")<\/pre><\/div>\n\n\n\n
\n
WARNING:root:Used to log warning message\nERROR:root:Used to log error message\nCRITICAL:root:Used to log critical message<\/pre><\/div>\n\n\n\n
WARNING<\/code> and above are logged by the logging<\/code> module.<\/p>\n\n\n\nINFO<\/code> and DEBUG<\/code> messages by applying some configurations.<\/p>\n\n\n\nConfiguring Logging Module<\/h2>\n\n\n\n
logging<\/code> module can be configured to unleash more power to let you do more stuff when logging events.<\/p>\n\n\n\nBasic Configuration<\/h3>\n\n\n\n
logging<\/code> module includes a function called basicConfig()<\/code> that helps in configuring the logging system. Here’s an example of configuring logging<\/code> module to log INFO<\/code> level message.<\/p>\n\n\n\nimport logging\n\n# Configuring logging\nlogging.basicConfig(level=logging.INFO)\nlogging.info(\"This info will be printed\")<\/pre><\/div>\n\n\n\n
logging<\/code> module is configured in the code above to log a INFO<\/code> message by passing the keyword argument level<\/code> to the basicConfig()<\/code> function and setting it to logging.INFO<\/code>.<\/p>\n\n\n\nINFO<\/code> message will now be printed.<\/p>\n\n\n\nINFO:root:This info will be printed<\/pre><\/div>\n\n\n\n
DEBUG<\/code> messages by setting the level<\/code> argument to the value logging.DEBUG<\/code>.<\/p>\n\n\n\nimport logging\n\n# Configuring logging to log DEBUG msg\nlogging.basicConfig(level=logging.DEBUG)\nlogging.debug(\"This is a DEBUG message\")\n\n--------------------\nDEBUG:root:This is a DEBUG message<\/pre><\/div>\n\n\n\n
Logging Event to a File<\/h3>\n\n\n\n
basicConfig()<\/code> function accepts a keyword argument called filename<\/code> to specify the file in which the event will be logged.<\/p>\n\n\n\nimport logging\n\n# Logging to a file\nlogging.basicConfig(filename='debug.log', level=logging.DEBUG)\nlogging.debug(\"This is a DEBUG message\")<\/pre><\/div>\n\n\n\n
debug.log<\/code> file and it will be the same as the previous output but in the file instead of the console.<\/p>\n\n\n\na<\/code>) mode and you can change this by specifying the filemode<\/code> parameter. Additionally, you can use the encoding<\/code> parameter to specify the file’s encoding.<\/p>\n\n\n\nimport logging\n\n# Logging to a file\nlogging.basicConfig(filename='debug_new.log',\n filemode='w',\n encoding='utf-8',\n level=logging.DEBUG)\nlogging.debug(\"Debugging starts from here.\")<\/pre><\/div>\n\n\n\n
filemode='w'<\/code>) mode and encoding is set to 'utf-8'<\/code>. This time the message will be logged into the debug_new.log<\/code> file.<\/p>\n\n\n\nHow to Log Variable Data?<\/h2>\n\n\n\n
%<\/code> operator. This helps when you want to include dynamic data in logged messages.<\/p>\n\n\n\nimport logging\n\n# Logging variable data\nlogging.error(\"%s: Not found - Status: %s\", 404, \"Fail\")<\/pre><\/div>\n\n\n\n
%s<\/code>) are present in the string that will help Python interpolate the string with the specified values i.e., 404<\/code> and 'Fail'<\/code>, and they will be placed in the same position as they appear.<\/p>\n\n\n\nERROR:root:404: Not found - Status: Fail<\/pre><\/div>\n\n\n\n
Formatting the Logged Messages<\/h2>\n\n\n\n
import logging\n\nlogging.basicConfig(format='%(levelname)s - %(message)s')\nlogging.warning(\"Things are getting worse.\")\n\n--------------------\nWARNING - Things are getting worse.<\/pre><\/div>\n\n\n\n
format<\/code> argument set. The severity (levelname<\/code>) is shown first, then the event’s description (message<\/code>).<\/p>\n\n\n\nmessage<\/code> and levelname<\/code> came from. Well, these are the attributes specified by the LogRecord<\/a>. Similarly, LogRecord has more attributes.<\/p>\n\n\n\nimport logging\n\nlogging.basicConfig(\n format='%(levelname)s: %(message)s - %(asctime)s', \n level=logging.DEBUG\n)\nlogging.debug(\"Debugging started at\")<\/pre><\/div>\n\n\n\n
asctime<\/code> is added to the message that logs the date and time of the event. The output looks like the following.<\/p>\n\n\n\nDEBUG: Debugging started at - 2024-06-12 16:37:55,996<\/pre><\/div>\n\n\n\n
datefmt<\/code> argument in the basicConfig()<\/code>.<\/p>\n\n\n\nimport logging\n\nlogging.basicConfig(\n format='%(levelname)s: %(message)s - %(asctime)s',\n datefmt='%d\/%m\/%Y %I:%M:%S %p', \n level=logging.DEBUG\n)\nlogging.debug(\"Debugging started at\")\n\n--------------------\nDEBUG: Debugging started at - 12\/06\/2024 05:55:38 PM<\/pre><\/div>\n\n\n\n
'%d\/%m\/%Y %I:%M:%S %p'<\/code>) set by the datefmt<\/code> argument. You can experiment with the characters within the string just like you would do in the time.strftime()<\/code><\/a>.<\/strong> Here’s an example.<\/p>\n\n\n\nimport logging\n\nlogging.basicConfig(\n format='%(levelname)s: %(message)s - %(asctime)s',\n datefmt='%d %b %Y %a %I:%M:%S %p', \n level=logging.DEBUG\n)\nlogging.debug(\"Debugging started at\")\n\n--------------------\nDEBUG: Debugging started at - 12 Jun 2024 Wed 06:08:20 PM<\/pre><\/div>\n\n\n\n
Logging – Advanced Configurations<\/h2>\n\n\n\n
logging<\/code> module are as follows:<\/p>\n\n\n\n\n
Creating Logger Object<\/h3>\n\n\n\n
import logging\n\n# Create a logger\nlogger = logging.getLogger(\"custom_logger\")\nlogger.warning(\"Warning... Warning...\")<\/pre><\/div>\n\n\n\n
custom_logger<\/code> is created using logging.getLogger()<\/code>. When you run this code, you’ll get the following output.<\/p>\n\n\n\nWarning... Warning...<\/pre><\/div>\n\n\n\n
basicConfig()<\/code>. To format this output, you need to use Handlers<\/strong> and Formatters<\/strong>.<\/p>\n\n\n\n\n
logger = logging.getLogger(__name__)<\/code><\/p>\n\n\n\n__main__<\/code> and if you import this module into another module, then the logger name will the module_name<\/code>, the name of the module in which it is defined.<\/p>\n<\/blockquote>\n\n\n\nDEBUG<\/code> and INFO<\/code>. To log DEBUG<\/code> or INFO<\/code> level messages using the custom logger, you need to create a handler.<\/p>\n\n\n\nCreating Handler<\/h3>\n\n\n\n
StreamHandler<\/code><\/a>, FileHandler<\/code><\/a>, HTTPHandler<\/code><\/a>, SMTPHandler<\/code><\/a> and more<\/a>.<\/p>\n\n\n\naddHandler()<\/code> method is used to add one or more handlers to the logger object. You need to add the individual handler to log messages based on categories to different destinations.<\/p>\n\n\n\n# adv_logging.py\n\nimport logging\n\n# Create a logger\nlogger = logging.getLogger(__name__)\nlogger.setLevel(logging.INFO) # Set the logger's level\n\n# Creating Handlers\n# Console handler\ncs_handler = logging.StreamHandler()\ncs_handler.setLevel(logging.INFO)\n\n# File handler\nfl_handler = logging.FileHandler(\"info.log\")\nfl_handler.setLevel(logging.ERROR)<\/pre><\/div>\n\n\n\n
INFO<\/code>.<\/p>\n\n\n\ncs_handler<\/code> and fl_handler<\/code>. The first handler (cs_handler<\/code>) is responsible for sending messages in the console and its level is set to INFO<\/code>. The second handler (fl_handler<\/code>) sends the messages to the file name info.log<\/code> which is set to ERROR<\/code> level.<\/p>\n\n\n\nCreating Formatters<\/h3>\n\n\n\n
basicConfig()<\/code>. But here, you need to use a Formatter<\/code> class of the logging<\/code> module.<\/p>\n\n\n\n...\n\n# Creating Formatters\n# Formatter for console\ncs_formatter = logging.Formatter(\n '%(asctime)s: %(name)s - %(levelname)s: %(message)s',\n datefmt='%d %b %Y %a %I:%M:%S %p'\n)\n# Setting formatter to handler\ncs_handler.setFormatter(cs_formatter)\n\n# Formatter for file\nfl_formatter = logging.Formatter(\n '%(asctime)s: %(name)s - %(levelname)s: %(message)s',\n datefmt='%d %b %Y %a %I:%M:%S %p'\n)\n# Setting formatter to handler\nfl_handler.setFormatter(fl_formatter)<\/pre><\/div>\n\n\n\n
cs_formatter<\/code> and fl_formatter<\/code> have been created. The formatting style of both formatters is identical, and setFormatter()<\/code> has been used to assign each formatter to its corresponding handler.<\/p>\n\n\n\ncs_handler<\/code> and fl_handler<\/code>) to the logger.<\/p>\n\n\n\n...\n\n# Adding handler to logger\nlogger.addHandler(cs_handler)\nlogger.addHandler(fl_handler)<\/pre><\/div>\n\n\n\n
...\n\nlogger.info(\"Process started...\")\nlogger.warning(\"Warning... Warning...\")\nlogger.error(\"Unusual activity detected...\")\nlogger.critical(\"System security compromised...\")<\/pre><\/div>\n\n\n\n
13 Jun 2024 Thu 03:55:24 PM: __main__ - INFO: Process started...\n13 Jun 2024 Thu 03:55:24 PM: __main__ - WARNING: Warning... Warning...\n13 Jun 2024 Thu 03:55:24 PM: __main__ - ERROR: Unusual activity detected...\n13 Jun 2024 Thu 03:55:24 PM: __main__ - CRITICAL: System security compromised...<\/pre><\/div>\n\n\n\n
logging.info()<\/code>, logging.warning()<\/code>, logging.error()<\/code>, logging.critical()<\/code> passed the information to the handlers (cs_haldler<\/code> and fl_handler<\/code>).<\/p>\n\n\n\nINFO<\/code> and cs_handler<\/code> is a StreamHandler<\/code> set to INFO<\/code> level printed the messages with specified formatting of INFO<\/code> level and above in the console.<\/p>\n\n\n\nfl_handler<\/code> is a FileHandler<\/code> which is set to ERROR<\/code> level will log the messages in the info.log<\/code> file of the level ERROR<\/code> and above.<\/p>\n\n\n\n13 Jun 2024 Thu 03:55:24 PM: __main__ - ERROR: Unusual activity detected...\n13 Jun 2024 Thu 03:55:24 PM: __main__ - CRITICAL: System security compromised...<\/pre><\/div>\n\n\n\n
__main__<\/code> which means the source file is executed directly. If you import this module to another file and then run it, the name of the logger will change.<\/p>\n\n\n\nsample_log.py<\/code>. Import the source file into it, and then run the file.<\/p>\n\n\n\n# sample_log.py\nimport adv_logging<\/pre><\/div>\n\n\n\n
13 Jun 2024 Thu 03:58:12 PM: adv_logging - INFO: Process started...\n13 Jun 2024 Thu 03:58:12 PM: adv_logging - WARNING: Warning... Warning...\n13 Jun 2024 Thu 03:58:12 PM: adv_logging - ERROR: Unusual activity detected...\n13 Jun 2024 Thu 03:58:12 PM: adv_logging - CRITICAL: System security compromised...<\/pre><\/div>\n\n\n\n
info.log<\/code> file will look like the following.<\/p>\n\n\n\n13 Jun 2024 Thu 03:55:24 PM: __main__ - ERROR: Unusual activity detected...\n13 Jun 2024 Thu 03:55:24 PM: __main__ - CRITICAL: System security compromised...\n13 Jun 2024 Thu 03:58:12 PM: adv_logging - ERROR: Unusual activity detected...\n13 Jun 2024 Thu 03:58:12 PM: adv_logging - CRITICAL: System security compromised...<\/pre><\/div>\n\n\n\n
Practical Example<\/h2>\n\n\n\n
log_temp.py<\/code>. This file holds a template for logging a message.<\/p>\n\n\n\n