Writing log to file python

Log to a File in Python

There is a built-in module called logging for creating a log file in Python.

To log a message into a separate text file by configuring the logging module’s root logger and logging a debug message:

import logging logging.basicConfig(filename="log.txt", level=logging.DEBUG) logging.debug("Debug logging test. ")

As a result, you see a file called log.txt in the same folder with your program. This file should have a message that looks like this:

DEBUG:root:Debug logging test.

In Python, you can log information about issues in a separate file. This file can later help you diagnose issues. It leaves a trail of breadcrumbs that leads to the source of the problem.

In this comprehensive guide, you learn how to work with the logging module and how to set up a logger.

Logging in Python—A Detailed Tour

Let’s take a more detailed tour of logging and log files in Python.

Logging means tracking the events and the status of your application as it runs. It is a crucial method to develop, debug, and run the software.

If you do not keep the track of any logs of your program, chances are you cannot diagnose issues properly. This is because without logging there is no trail that leads to the possible root cause of a problem.

With proper logging set up, you can follow the trail of breadcrumbs to the source of a problem.

Why No Printing

While printing is an excellent strategy to debug code, it may not always be the best option. This is especially true when the program is more complex.

The problem with printing is that the prints are not stored anywhere. If your application throws an error, there is no trace of that error—unless the error message is logged somewhere.

So you may want to log the data of your program in a separate log file.

In Python, there is a built-in library called logging for this purpose.

How to Start Logging Messages in Python

To start logging messages to a file, you need to know how to set up a logger.

1. Import the logging module
2. Configure the logger using the basicConfig() method. The rest of the steps describe how.
3. Specify the file to which log messages are sent.
4. Define the “seriousness” level of the log messages.
5. Format the log messages.
6. Append or overwrite previous log messages in the file.

Let’s start building a logger while going through these six steps in detail.

1. Import the Python’s Built-In Logging Module

To use Python’s built-in logging functionality, start by importing the logging module.

2. Call basicConfig() to Start Configuring the Logger.

To start logging messages to a file in Python, you need to configure the logging module.

To configure the logging module, call logging.basicConfig() :

import logging logging.basicConfig()

An empty basicConfig() call does nothing useful. In the following steps, you are going to add parameters to this call to actually configure the logger.

3. Log Target File in Python

The basicConfig() method accepts a parameter called filename . This is a string that specifies the target file for log messages.

For example, let’s start logging the messages into a file called log.txt .

import logging logging.basicConfig(filename="log.txt")

Now the logger knows the target file and can already start logging messages into it.

But before we log our first message, we need to specify the “seriousness” level of the messages we want to track.

Let’s have a look at the log message levels in Python.

4. Log Message Levels

There are five main levels of debug messages when logging. These levels describe the “seriousness” of the issue.

1. DEBUG . Used to give detailed information. This level is mostly used for diagnosing issues in code.
2. INFO . Confirms the program works as expected.
3. WARNING . An indication that an unexpected event occured or may occur.
4. ERROR . Serious issue. Indicates that a program was unable to perform some action due to an error.
5. CRITICAL . A serious error. Indicates that the program may be unable to continue running.

The logging levels are integers behind the scenes. Here is a table

About the Author

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Write Logs to a File in Python

1. Write Logs to a File With the logging Module in Python
2. Write Logs to a File With the logging.FileHandler() Function in Python

This tutorial will introduce the methods to write logs to a file in Python.

Write Logs to a File With the logging Module in Python

The logs are used for debugging a program and finding out what went wrong. The logging module is used for logging data to a file in Python. We can use the logging.basicConfig() function to configure the logs to be written to a specific file. By default, we can log 5 types of lines with the logging module. These message types include debug, info, warning, error, and critical. Still, we can increase that number to whatever we need through coding. The following code example shows us how to write logs to a file with the logging.basicConfig() function.

import logging  #Creating and Configuring Logger  Log_Format = "%(levelname)s %(asctime)s - %(message)s"  logging.basicConfig(filename = "logfile.log",  filemode = "w",  format = Log_Format,  level = logging.ERROR)  logger = logging.getLogger()  #Testing our Logger  logger.error("Our First Log Message") 

The file content of logfile.log :

ERROR 2021-06-13 04:14:29,604 - Our First Log Message 

We wrote the log message Our First Log Message of level error with the proper date and time of logging by using the logging.basicConfig() function in the code above. The most important thing while writing the logs is the date and time of their occurrence. So, we first created a format for our logs inside the Log_Format string. This format includes the level of the log, the date and time of occurrence, and the message to be written.

Then, we use the filename parameter of the logging.basicConfig() function to specify the log file. We assigned the value a to the filemode parameter of the logging.basicConfig() function to open the file in append mode so that the previous logs don’t get deleted every time there’s a new log. We then set the format parameter to Log_Format . After that step, we assigned the logging.ERROR to the level parameter to specify the minimum level of the logs to error. We created a logger object with logger = logging.getLogger() to write log messages. In the end, we wrote the error message with logger.error(«Our First Log Message») .

Write Logs to a File With the logging.FileHandler() Function in Python

We can also use the logging.FileHandler() function to write logs to a file in Python. This function takes the file path where we want to write our logs. We can then use the addHandler() function to add this handler to our logger object. The code below demonstrates how to write logs to a file with the logging.FileHandler() function.

import logging  logger = logging.getLogger() handler = logging.FileHandler('logfile.log') logger.addHandler(handler) logger.error('Our First Log Message') 

We wrote the log message, Our First Log Message , to the file logfile.log with the logging.FileHandler() function in the code above. We first created a logger object that we will use to write logs with the logging.getLogger() function. Then, we create a file handler, handler , and assigned logging.FileHandler(‘logfile.log’) to it. After that, we added this new handler to our logger object with logger.addHandler(handler) . In the end, we wrote the error message to our file with logger.error(‘Our First Log Message’) .

Maisam is a highly skilled and motivated Data Scientist. He has over 4 years of experience with Python programming language. He loves solving complex problems and sharing his results on the internet.

Related Article — Python Logging

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