Log is an important tool for any developer. it helps in debugging and log important information or exceptions that emits while the code executes

Python provides a logging system as a part of its standard library, so you can quickly add logging to your application

log is a cleaner alternative to writing print statement in the code. You can set up a format and define levels when you have to print the log messages

lt is important to understand at what point in your code you want to log the messages.

If there is a decision point in the code and you want to capture any exception at that point then you can add a DEBUG message there

Another example would be if you just wanted to know what is the value of a variable at the entry and exit of a function then add a log Info there.

There are some messages on which you want to act immediately and trigger an email or slack message when such error occurs, Ensure that you are not setting those log messages as info or Debug instead an Error or a Critical message.

So long story short, you should have done a clear analysis of your code and then add the log messages with different levels

In this post, we will start with basic logging and then take a deep dive on how to set up logging for your project. Here are the topics which we will cover in this post

Basic logging

Python has a logging module which you can import with the simple command

import logging

Now in your code to implement logging initialize logging.basicConfig() class and then anywhere in the code use logging.info(msg) to print the message. the basic configuration creates StreamHandler with a default Formatter and adds it to the root logger

logging.basicConfig(level=logging.DEBUG)

Here is an example where we are logging the division by zero error, Multiplication by zero and addition by zero information

import logging class opr_on_num: def __init__(self,a,b): self.a = a self.b = b def add_two_no(self): if self.a==0 or self.b==0: logging.info("You are adding zero to a number") return self.a+self.b def divide_two_no(self): try: divide=self.a/self.b except Exception as e: logging.info(e) divide = 0 return divide def multiply_two_no(self): if self.a==0 or self.b==0: logging.debug("You are multiplying zero to a number, Result will be a Zero") multi = self.a * self.b return multi if __name__ == '__main__': logging.basicConfig(level=logging.DEBUG) obj=opr_on_num(5,0) obj.divide_two_no() obj.add_two_no() obj.multiply_two_no()

When you run the above code this is the output that you see

INFO:root:division by zero INFO:root:You are adding zero to a number DEBUG:root:You are multiplying zero to a number, Result will be a Zero

Log to a File

In the above code just add filename parameter to basicconfig() settings. This will create a File Handler instead of a default Stream handler and all logs message will be written to my_logs.log file

logging.basicConfig(level=logging.DEBUG,filename='./my_logs.log')

Log Levels

Let’s look at different log levels.

You have to set a log level in your code so that all the log levels above the setup level should be shown

For example if log level is set to Warning then all the info and debug message in the code will not be executed and only levels having value greater than equal to info will be executed. In this case, INFO, Warning, Error and Critical

if you check the above code the log level is set to logging.DEBUG i.e. DEBUG level, so any level equal or above this will be executed by the system

Here is the table you can refer for different log levels and their values

Log Format

This will initialize the format class with a new format string for the log message, if no format is specified then the default %(message)s i.e. the plain message will be shown

Here we are using the format string ‘%(asctime)s : %(levelname)s : %(message)s’ which has following attributes that will be printed in the log message

format='%(asctime)s : %(levelname)s : %(message)s'

%(asctime)s : asctime to show the timestamp when the information is logged, %(levelname)s: Level name is the log level whether it is Info, Debug, Error etc. %(message)s: Finally message will print the Exception message.

In the above code update the logging.basicConfig() code to:

logging.basicConfig(level=logging.DEBUG,format='%(asctime)s : %(levelname)s : %(message)s')

Output:

2019-10-15 17:01:09,739 : ERROR : division by zero 2019-10-15 17:01:09,745 : INFO : You are adding zero to a number 2019-10-15 17:01:09,746 : DEBUG : You are multiplying zero to a number, Result will be a Zero

Logging configuration

basicconfig() uses root logger and its not a good idea to use the root logger when you are working with big projects and different modules where you want to implement the logs. Its better to configure your log at one place and call the logger in every modules with a different name so that each module will log it separately and it would be easier for a developer to understand the source of the log and this way you don’t have to configure the log for each module separately

Configuration and Adding Stream Handler

logger should always be instantiated using module level function logging.getLogger(name). As per the official documentation

The _name_ is potentially a period-separated hierarchical value, like _foo.bar.baz_ (though it could also be just plain _foo_ , for example). Loggers that are further down in the hierarchical list are children of loggers higher up in the list. For example, given a logger with a name of _foo_ , loggers with names of _foo.bar_ , _foo.bar.baz_ , and _foo.bam_ are all descendants of _foo_ . The logger name hierarchy is analogous to the Python package hierarchy, and identical to it if you organise your loggers on a per-module basis using the recommended construction _logging.getLogger(__name__)_ . That’s because in a module, ___name___ is the module’s name in the Python package namespace.

In the below example we have use __name__ as the module name

logger = logging.getLogger(__name__)

You can use Handler to setup where the log messages should be printed either a File using FileHandler or to a Console using StreamHandler. There are other types of Handlers which you can check in this link

ch = logging.StreamHandler() logger.addHandler(ch)

Finally a custom format string is defined and added to the handler

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

if no other handler is provided in the code then basicConfig() will be called

Here is the Full code:

import logging class opr_on_num: def __init__(self,a,b): self.a = a self.b = b def add_two_no(self): if self.a==0 or self.b==0: logger.info("You are adding zero to a number") return self.a+self.b def divide_two_no(self): try: divide=self.a/self.b except Exception as e: logger.error(e) divide = 0 return divide def multiply_two_no(self): if self.a==0 or self.b==0: logger.debug("You are multiplying zero to a number, Result will be a Zero") multi = self.a * self.b return multi if __name__ == '__main__': logger = logging.getLogger(__name__) # create console handler with a higher log level ch = logging.StreamHandler() # Create Formatter and setup in logging formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s') ch.setFormatter(formatter) # Set the log level and add handler logger.setLevel(logging.DEBUG) logger.addHandler(ch) obj=opr_on_num(5,0) obj.divide_two_no() obj.add_two_no() obj.multiply_two_no()

File Handler

Instead of writing the code to console you can directly write all the log from all the modules to a file using FindHandler.

Only thing you have to do is change the StreamHandler to a FileHandler with the path where you want to save the file.

ch = logging.FileHandler('my_file_1.log') logger.addHandler(ch)

The above code will write the log to a file my_file_1.log in the parent directory

Output in a log file:

Adding Extra Fields to a Log Message

We will add a Filename and **lineno** attribute to the log message just to understand which module and line number is throwing the error or emitting that message

2019-10-15 18:15:07,745 -test.py:19 -main – INFO – division by zero

For the complete list of attributes that can be used for log messages check this link

Just update this format string in the above code:

formatter = logging.Formatter('{"time":%(asctime)s,"file_name": "%(filename)s:%(lineno)d" ,' \ '"level": "%(levelname)s" ,"msg":"%(message)s" }')

Output:

2019-10-15 18:15:07,745 -test.py:19 -main - INFO - division by zero 2019-10-15 18:15:07,746 -test.py:11 -main - INFO - You are adding zero to a number 2019-10-15 18:15:07,747 -test.py:26 -main - DEBUG - You are multiplying zero to a number, Result will be a Zero

Log Format as JSON Object

So far we have seen the format is a String and the output is also a string of log attributes

2019-10-15 18:15:07,745 -test.py:19 -main - INFO - division by zero

if you want a JSON object with Key Value pair for each of these attributes then just update the Format String to this:

formatter = logging.Formatter('{"time":%(asctime)s,"file_name": "%(filename)s:%(lineno)d" ,' '"level": "%(levelname)s" ,"msg":"%(message)s" }')

Output:

> _{"time":2019-10-15 18:20:50,773,"file_name": "test.py:19" ,"level": "INFO" ,"msg":"division by zero" } > {"time":2019-10-15 18:20:50,773,"file_name": "test.py:11" ,"level": "INFO" ,"msg":"You are adding zero to a number" } > {"time":2019-10-15 18:20:50,773,"file_name": "test.py:26" ,"level": "DEBUG" ,"msg":"You are multiplying zero to a number, Result will be a Zero" }_

Custom fields in Log Format

The filter class is used to filter the log records So that a logger will output the desired log message.

You can also add keyword arguments(kwargs) to a log message using the Filter method and print that in the log messages directly

{“time”:2019-10-16 06:48:23,585,”file_name”: “test.py:27” ,”level”: “INFO” ,“msg”:”Division”,”User”:”min2bro” }

Here is an example of Filter which add additional blocks in the message for User and Custom_msg

class AppFilter(logging.Filter): def filter(self, record): record.User = record.args.get("User") record.custommsg = record.args.get("custom_msg") return True

Add the filter to logger instance:

logger.addFilter(AppFilter())

Add kwargs in the log messages:

logger.info("You are adding zero to a number,{"User":'min2bro',"custom_msg":'addition'})

Full code:

import logging # Add Filter class AppFilter(logging.Filter): def filter(self, record): record.User = record.args.get("User") record.custommsg = record.args.get("custom_msg") return True class opr_on_num: def __init__(self,a,b): self.a = a self.b = b def add_two_no(self): if self.a==0 or self.b==0: logger.info("You are adding zero to a number",{"User":'min2bro',"custom_msg":'addition'}) return self.a+self.b def divide_two_no(self): try: divide=self.a/self.b except Exception as e: logger.info(e,{"User":'min2bro',"custom_msg":'Division'}) divide = 0 return divide def multiply_two_no(self): if self.a==0 or self.b==0: logger.debug("You are multiplying zero to a number, Result will be a Zero",{"User":'min2bro',"custom_msg":'Multiplication'}) multi = self.a * self.b return multi if __name__ == '__main__': logger = logging.getLogger(__name__) # create console handler with a higher log level ch = logging.StreamHandler() logger.addFilter(AppFilter()) formatter = logging.Formatter('{"time":%(asctime)s,"file_name": "%(filename)s:%(lineno)d" ,' \ '"level": "%(levelname)s" ,"msg":"%(custommsg)s","User":"%(User)s" }') ch.setFormatter(formatter) logger.setLevel(logging.DEBUG) logger.addHandler(ch) obj=opr_on_num(5,0) obj=opr_on_num(5,0) obj.divide_two_no() obj.add_two_no() obj.multiply_two_no()

Output:

> _{"time":2019-10-16 06:48:23,585,"file\_name": "test.py:27" ,"level": "INFO" ,"msg":"Division","User":"min2bro" } > {"time":2019-10-16 06:48:23,589,"file\_name": "test.py:19" ,"level": "INFO" ,"msg":"addition","User":"min2bro" } > {"time":2019-10-16 06:48:23,590,"file\_name": "test.py:34" ,"level": "DEBUG" ,"msg":"Multiplication","User":"min2bro" }_

Logging Variable Data

This can help to format the log message with a variable data. You can either use a style parameter with the formatter and set it to either ‘{‘ or ‘$’

formatter = logging.Formatter('{asctime} {name} {levelname:8s} {message}', style='{')

Or use the f-string which is introduced in Python 3.6, and it works great for the string formatting. Here **num** and **example** are the variables which are directly embedded in the log message without setting any style for the formatter string

logger.error(f'This is message {num} for {example}')

Full Code:

import logging class opr_on_num: def __init__(self,a,b): self.a = a self.b = b def add_two_no(self): return self.a+self.b def divide_two_no(self): try: divide=self.a/self.b except Exception as e: logger.error(f'This is message {num} for {example}') divide = 0 return divide def multiply_two_no(self): multi = self.a * self.b return multi if __name__ == '__main__': num=10 example = 'logging_example' logger = logging.getLogger(__name__) # create console handler with a higher log level ch = logging.StreamHandler() formatter = logging.Formatter('{asctime} {name} {levelname:8s} {message}', style='{') ch.setFormatter(formatter) logger.setLevel(logging.INFO) logger.addHandler(ch) obj=opr_on_num(5,0) obj=opr_on_num(5,0) obj.divide_two_no() obj.add_two_no() obj.multiply_two_no()

Output:

2019-10-15 19:26:22,464 main ERROR This is message 10 for logging_example

Loading the log configuration from a JSON file

Here is an example of logging configuration dictionary which can be stored in a flat json file

log_config = { "version": 1, "disable_existing_loggers": False, "formatters": { "simple": { "format": "%(asctime)s - %(name)s - %(levelname)s - %(message)s" } }, "handlers": { "file_handler": { "class": "logging.FileHandler", "level": "DEBUG", "formatter": "simple", "filename": "my_file.log", "encoding": "utf8" } }, "root": { "level": "DEBUG", "handlers": ["file_handler"] } }

And this dictionary is passed to the dictconfig() to setup the log configuration

with open('C:/Personal/Kanoki/logging/log_config.json', 'rt') as f: config = json.load(f) logging.config.dictConfig(config)

Full code:

import logging,json import logging.config class opr_on_num: def __init__(self,a,b): self.a = a self.b = b def add_two_no(self): return self.a+self.b def divide_two_no(self): try: divide=self.a/self.b except Exception as e: logger.error(f'This is message {num} for {example}') divide = 0 return divide def multiply_two_no(self): multi = self.a * self.b return multi if __name__ == '__main__': num=10 example = 'logging_example' with open('C:/Personal/Kanoki/logging/log_config.json', 'rt') as f: config = json.load(f) logging.config.dictConfig(config) logger = logging.getLogger(__name__) # create console handler with a higher log level # ch = logging.StreamHandler() # formatter = logging.Formatter('{asctime} {name} {levelname:8s} {message}', # style='{') # ch.setFormatter(formatter) # logger.setLevel(logging.INFO) # logger.addHandler(ch) obj=opr_on_num(5,0) obj=opr_on_num(5,0) obj.divide_two_no() obj.add_two_no() obj.multiply_two_no()

Rotating File Handlers

Rotating file handler is used to rotate the log files. if a log files exceeds a certain defined size then a new log file is created.

_logging.handlers.RotatingFileHandler_ (filename, mode=’a’, maxBytes=0, backupCount=0, encoding=None, delay=False)

**maxByte** size is set to write the logs to a new file when existing log file size goes beyond that limit

backupCount when set to non-zero the system will append the existing files with extension 1, 2 etc.

Here is the code you have to replace in the above code to create a rotating file handler

handler = RotatingFileHandler('my_log.log', maxBytes=2000, backupCount=10) logger.addHandler(handler)

Read the official Python Logging documentation here to understand how the log files are renamed and what are the other parameters used for

Conclusion

Logging is an important feature for any project and implementing logs in an efficient way really helps to debug the code and catch errors and exceptions without much hassle. This post gives a very detailed overview of the logging module in Python and demonstrates various features with the help of examples. For further reading you can refer to the logging official documentation here. Hope you enjoyed this blog and if you have any comments or suggestions please drop a note in the comments section below