Python logging to text file

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

Hi, I’m Artturi Jalli!

I’m a Tech enthusiast from Finland.

I make Coding & Tech easy and fun with well-thought how-to guides and reviews.

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