Logging configuration in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Flask logging setup?

Consider this Flask app snippet configuring logging:

import logging
from flask import Flask

app = Flask(__name__)

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

@app.route('/')
def home():
    app.logger.debug('Debug message')
    app.logger.warning('Warning message')
    return 'Hello'

What will be printed to the console when accessing the '/' route?

Flask

import logging
from flask import Flask

app = Flask(__name__)

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

@app.route('/')
def home():
    app.logger.debug('Debug message')
    app.logger.warning('Warning message')
    return 'Hello'

A WARNING:Warning message

B DEBUG:Debug message\nWARNING:Warning message

C No output printed

D ERROR:Warning message

💡 Hint

Remember the logging level set in basicConfig controls which messages appear.

The logging level is set to WARNING, so only messages with level WARNING or higher are shown. The debug message is ignored.

❓ Configuration

intermediate

Which logging configuration writes logs to a file with INFO level?

Choose the correct Flask logging setup that writes all INFO and above messages to a file named app.log.

A logging.basicConfig(filename='app.log', level=logging.INFO, format='%(asctime)s %(levelname)s %(message)s')

B logging.basicConfig(filename='app.log', level=logging.DEBUG, format='%(message)s')

C logging.basicConfig(file='app.log', level=logging.INFO, format='%(levelname)s %(message)s')

D logging.basicConfig(filename='app.log', level='INFO', format='%(asctime)s %(message)s')

💡 Hint

Check the parameter names and level types in basicConfig.

Option A correctly sets filename, level as logging.INFO, and a proper format string. Option A uses wrong parameter 'file'. Option A sets level as string instead of logging constant. Option A sets level DEBUG, which is more verbose than requested.

❓ Troubleshoot

advanced

Why does Flask not log messages below WARNING despite setting DEBUG level?

A developer sets Flask logging level to DEBUG using:

logging.basicConfig(level=logging.DEBUG)

But only WARNING and above messages appear in console. What is the most likely cause?

A The format string is missing, so DEBUG messages are suppressed

B Flask's default logger level overrides basicConfig and remains at WARNING

C The app.logger.debug() method is deprecated and does not log messages

D The logging module does not support DEBUG level in Flask

💡 Hint

Flask uses its own logger instance which may have a different level than root logger.

Flask's app.logger has its own level set to WARNING by default. Setting root logger level with basicConfig does not change app.logger's level. You must set app.logger.setLevel(logging.DEBUG) explicitly.

🔀 Workflow

advanced

Order the steps to configure Flask logging to send ERROR logs to a file and INFO logs to console.

Arrange these steps in the correct order to achieve the logging setup:

A 2,1,4,3

B 4,1,2,3

C 1,2,4,3

D 4,2,1,3

💡 Hint

Think about setting the logger level before adding handlers and the order of creating handlers.

First set logger level to INFO to allow INFO and above. Then create StreamHandler for INFO logs, FileHandler for ERROR logs, then add both handlers to logger.

✅ Best Practice

expert

Which is the best practice for Flask logging in production to avoid duplicate log entries?

In production, Flask logs sometimes appear duplicated in console and files. Which practice prevents this?

A Add multiple handlers without removing existing ones

B Call logging.basicConfig() multiple times to reset handlers

C Disable Flask's default logger propagation by setting app.logger.propagate = False

D Set logging level to DEBUG to reduce duplicates

💡 Hint

Propagation causes logs to be passed to parent loggers, causing duplicates.

By default, Flask's logger propagates messages to the root logger, which may also have handlers. Disabling propagation prevents duplicate logs.