Logging in production in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Flask logging configuration?

Given the following Flask app logging setup, what will be printed to the console when a request is made?

Flask

import logging
from flask import Flask
app = Flask(__name__)

logging.basicConfig(level=logging.WARNING)

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

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

A

[DEBUG] Debug message
[WARNING] Warning message

B [WARNING] Warning message

C No output

D [ERROR] Error message

💡 Hint

Check the logging level set by basicConfig and which messages are shown at that level.

The logging level is set to WARNING, so only WARNING and above messages are printed. DEBUG messages are ignored.

❓ Configuration

intermediate

Which logging configuration writes logs to a file with rotation in Flask?

Select the correct Flask logging setup that writes logs to a file named 'app.log' and rotates the file after it reaches 1MB, keeping 3 backups.

A

from logging.handlers import RotatingFileHandler
handler = RotatingFileHandler('app.log', maxBytes=1048576, backupCount=3)
app.logger.addHandler(handler)

B

from logging.handlers import TimedRotatingFileHandler
handler = TimedRotatingFileHandler('app.log', when='M', interval=1, backupCount=3)
app.logger.addHandler(handler)

C

handler = logging.FileHandler('app.log')
app.logger.addHandler(handler)

D

handler = logging.StreamHandler(open('app.log', 'w'))
app.logger.addHandler(handler)

💡 Hint

Look for a handler that rotates based on file size, not time.

RotatingFileHandler rotates logs based on file size. TimedRotatingFileHandler rotates based on time intervals.

❓ Troubleshoot

advanced

Why are Flask logs not appearing in production with Gunicorn?

You deployed a Flask app with Gunicorn. Your Flask app uses app.logger.warning('Test'). But no logs appear in the console or files. What is the most likely cause?

A Flask app.logger is disabled in production by default.

B The logging level is set to ERROR, so WARNING messages are ignored.

C Gunicorn does not support logging at all.

D Gunicorn overrides Flask logging and needs its own logging configuration.

💡 Hint

Consider how Gunicorn manages logging separately from Flask.

Gunicorn manages its own logging and may override or ignore Flask's logger unless configured properly.

✅ Best Practice

advanced

What is the best practice for logging sensitive information in production Flask apps?

Which option describes the best practice for handling sensitive data in logs for a Flask production app?

A Encrypt logs containing sensitive data and store them publicly.

B Log all data including sensitive info for full traceability.

C Never log sensitive data like passwords or tokens; mask or omit them if needed.

D Disable logging entirely to avoid any data leaks.

💡 Hint

Think about privacy and security risks in logs.

Logging sensitive data can cause security risks. Best practice is to avoid logging such data or mask it.

🔀 Workflow

expert

Order the steps to set up centralized logging for a Flask app in production using Docker and ELK stack.

Arrange the steps in the correct order to enable centralized logging for a Flask app running in Docker containers, sending logs to an ELK (Elasticsearch, Logstash, Kibana) stack.

A 3,1,2,4

B 1,3,2,4

C 1,2,3,4

D 3,2,1,4

💡 Hint

Think about setting up infrastructure before configuring app and forwarding logs.

First deploy ELK stack, then configure app logging format, then set Docker to forward logs, finally visualize in Kibana.

Practice

1. What is the main purpose of logging in a Flask application running in production?

easy

A. To speed up the app response time

B. To record app activity and errors for troubleshooting

C. To store user passwords securely

D. To create user interface themes

Solution

1. Step 1: Understand logging purpose

   Logging records events and errors to help developers understand app behavior.

2. Step 2: Identify production needs

   In production, logs help find and fix issues without stopping the app.

3. Final Answer:

   To record app activity and errors for troubleshooting -> Option B

4. Quick Check:

   Logging = record activity/errors [OK]

Hint: Logging tracks errors and events in production apps [OK]

Common Mistakes:

• Thinking logging speeds up the app
• Confusing logging with security features
• Assuming logging changes UI

2. Which of the following is the correct way to configure logging to write to a file named app.log with level WARNING in Flask?

easy

A. logging.basicConfig(filename='app.log', level=logging.WARNING)

B. logging.setup(filename='app.log', level='WARNING')

C. app.logger.config(filename='app.log', level=logging.WARNING)

D. logging.fileConfig('app.log', level=logging.WARNING)

Solution

1. Step 1: Recall logging setup syntax

   Use logging.basicConfig() with filename and level parameters.

2. Step 2: Check each option

   Only logging.basicConfig(filename='app.log', level=logging.WARNING) uses correct function and parameters.

3. Final Answer:

   logging.basicConfig(filename='app.log', level=logging.WARNING) -> Option A

4. Quick Check:

   basicConfig sets file and level [OK]

Hint: Use logging.basicConfig() to set file and level [OK]

Common Mistakes:

• Using wrong function names like setup or fileConfig
• Passing level as string instead of logging constant
• Trying to configure app.logger directly

3. Given this Flask route code, what will be logged when a user visits /hello?

import logging
from flask import Flask
app = Flask(__name__)

logging.basicConfig(filename='app.log', level=logging.INFO)

@app.route('/hello')
def hello():
    app.logger.info('Hello route accessed')
    return 'Hello!'

medium

A. A log entry with message 'Hello route accessed' at INFO level

B. No log will be written because app.logger is not configured

C. An error log because logging.basicConfig is missing a format

D. A warning log with message 'Hello route accessed'

Solution

1. Step 1: Check logging configuration

   logging.basicConfig sets INFO level and file 'app.log'.

2. Step 2: Analyze app.logger usage

   app.logger.info writes an INFO level log with given message.

3. Final Answer:

   A log entry with message 'Hello route accessed' at INFO level -> Option A

4. Quick Check:

   app.logger.info logs message at INFO [OK]

Hint: app.logger.info writes INFO logs if level allows [OK]

Common Mistakes:

• Assuming app.logger needs extra setup beyond basicConfig
• Confusing log levels INFO and WARNING
• Thinking format is mandatory for logging

4. You set up logging in Flask with logging.basicConfig(filename='app.log', level=logging.ERROR) but notice that INFO logs still appear in app.log. What is the likely cause?

medium

A. The file app.log is not writable

B. logging.basicConfig does not control log levels

C. app.logger.info() does not respect logging levels

D. The root logger level was set before basicConfig and overrides it

Solution

1. Step 1: Understand basicConfig behavior

   basicConfig only works if root logger has no handlers yet.

2. Step 2: Identify cause of wrong log levels

   If root logger was configured earlier, basicConfig call is ignored, so level stays lower.

3. Final Answer:

   The root logger level was set before basicConfig and overrides it -> Option D

4. Quick Check:

   Previous root logger config blocks basicConfig [OK]

Hint: basicConfig ignored if root logger already configured [OK]

Common Mistakes:

• Believing basicConfig cannot set levels
• Thinking app.logger ignores levels
• Assuming file permission causes wrong logs

5. You want to log errors and warnings to a file error.log and info messages to info.log in your Flask app. Which setup correctly achieves this?

hard

A. Use print statements instead of logging for info and error messages

B. Call logging.basicConfig twice with different filenames and levels

C. Use two handlers: one FileHandler for error.log with level WARNING, another for info.log with level INFO, added to app.logger

D. Set app.logger level to INFO and write all logs to error.log only

Solution

1. Step 1: Understand multiple log files need multiple handlers

   One handler per file with its own level filters logs accordingly.

2. Step 2: Check options for multiple handlers

   Use two handlers: one FileHandler for error.log with level WARNING, another for info.log with level INFO, added to app.logger correctly uses two FileHandlers with different levels added to app.logger.

3. Final Answer:

   Use two handlers: one FileHandler for error.log with level WARNING, another for info.log with level INFO, added to app.logger -> Option C

4. Quick Check:

   Multiple handlers = multiple log files [OK]

Hint: Use multiple handlers for different log files and levels [OK]

Common Mistakes:

• Calling basicConfig multiple times (only first works)
• Logging all levels to one file without filtering
• Using print instead of logging for production