Writing Error Log in Python Flask Web Application

Writing error log is one of the crucial things when writing a web applications. It’s really helpful when trying to debug where things went wrong. In this tutorial, we’ll have a look at how to write error log in Python Flask web application.

We’ll start by creating a simple web application using Python Flask. Create a project LogApp and a file called app.py.

mkdir LogApp
cd LogApp
vim app.py

Add the following code to app.py.

from flask import Flask
app = Flask(__name__)

@app.route("/log")
def logTest():
    return "Code Handbook !! Log testing."

if __name__ == "__main__":
    app.run()

Writing Error Log in Python Flask

To get started with logging error and other information, all you need to do is import the logging library from python. Once the library has been imported, we’ll be using the RotatingFileHandler handler to handler logging.

import logging
from logging.handlers import RotatingFileHandler

Initialize the log handler when the application in initiated. Set the logging levels to ERROR, INFO or WARNING. Important point to note here is to set both the app.logger level and the RotatingFileHandler level.

if __name__ == "__main__":
    # initialize the log handler
    logHandler = RotatingFileHandler('info.log', maxBytes=1000, backupCount=1)
    
    # set the log handler level
    logHandler.setLevel(logging.INFO)

    # set the app logger level
    app.logger.setLevel(logging.INFO)

    app.logger.addHandler(logHandler)    
    app.run()

Try adding some different level logs to the /log handler and it should be written to the info.log file in the logApp project folder.

@app.route("/log")
def logTest():
    app.logger.warning('testing warning log')
    app.logger.error('testing error log')
    app.logger.info('testing info log')
    return "Code Handbook !! Log testing."

Wrapping It Up

In this tutorial, we saw how to handle error log in Python Flask web application. We used logging python library to write logs in our Flask application. The handler used is RotatingFileHandler which rotates the log file when the size of the file exceeds a certain limit.

Have you faced any other issues while trying to write logs in a Python Flask web application ? Do let us know your thoughts in the comments below.