Debug mode error pages in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable debug mode in a Flask app.

Flask

from flask import Flask
app = Flask(__name__)
app.run(debug=[1])

Drag options to blanks, or click blank then click option'

A1

BTrue

C"yes"

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string like "yes" instead of a boolean True.

Setting debug to False disables debug mode.

Setting debug=True enables debug mode in Flask, which shows detailed error pages.

2fill in blank

medium

Complete the code to define a route that triggers an error for testing debug pages.

Flask

@app.route('/error')
def error():
    return 1 / [1]

Drag options to blanks, or click blank then click option'

ANone

B"0"

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 or None which do not cause errors.

Using a string "0" which causes a different error.

Dividing by zero (1 / 0) raises an error to test debug error pages.

3fill in blank

hard

Fix the error in the code to properly enable debug mode when running the Flask app.

Flask

if __name__ == '__main__':
    app.run([1]=True)

Drag options to blanks, or click blank then click option'

Adebug

Bdebug_mode

Crun_debug

Dmode_debug

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like debug_mode or run_debug.

Forgetting to set debug to True.

The correct parameter to enable debug mode in app.run() is debug.

4fill in blank

hard

Fill both blanks to create a custom error handler for 404 errors that shows a debug message.

Flask

@app.errorhandler([1])
def not_found(error):
    return "Error [2]: Page not found", 404

Drag options to blanks, or click blank then click option'

A404

B500

C403

D400

Attempts:

3 left

💡 Hint

Common Mistakes

Using other error codes like 500 or 403 which mean different errors.

Mismatching error codes in decorator and return statement.

The error code for 'Not Found' is 404, so both blanks should be 404.

5fill in blank

hard

Fill all three blanks to log errors and show debug info when an exception occurs.

Flask

import logging

@app.errorhandler([1])
def handle_exception(e):
    logging.[2](f"Error occurred: {e}")
    return "Debug info: " + str(e), [3]

Drag options to blanks, or click blank then click option'

AException

Berror

C500

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'exception' as the error type instead of Exception.

Using logging.error instead of logging.exception for full traceback.

Returning wrong HTTP status codes.

Use Exception to catch all errors, logging.exception to log with traceback, and return HTTP status 500 for server errors.

Practice

1. What does enabling debug mode in a Flask app do?

easy

A. Disables all error messages

B. Automatically fixes bugs in the code

C. Shows detailed error pages with debugging information

D. Prevents the app from restarting on code changes

Solution

1. Step 1: Understand debug mode purpose

   Debug mode helps developers by showing detailed error pages when something goes wrong.

2. Step 2: Compare options with debug mode behavior

   Only Shows detailed error pages with debugging information correctly describes that debug mode shows detailed error pages with debugging info.

3. Final Answer:

   Shows detailed error pages with debugging information -> Option C

4. Quick Check:

   Debug mode = detailed error pages [OK]

Hint: Debug mode shows detailed errors, not hides them [OK]

Common Mistakes:

• Thinking debug mode hides errors
• Believing debug mode fixes bugs automatically
• Confusing debug mode with production mode

2. Which of the following is the correct way to enable debug mode in Flask?

easy

A. app.run(debug=False)

B. app.debug = True

C. app.debug = False

D. app.config['DEBUG'] = False

Solution

1. Step 1: Recall how to enable debug mode

   Debug mode can be enabled by setting app.debug = True or app.config['DEBUG'] = True.

2. Step 2: Identify the correct option

   Only app.debug = True sets debug mode to True correctly.

3. Final Answer:

   app.debug = True -> Option B

4. Quick Check:

   Enable debug = app.debug = True [OK]

Hint: Set app.debug = True to enable debug mode [OK]

Common Mistakes:

• Setting debug to False instead of True
• Confusing app.run debug argument with app.debug property
• Using incorrect config keys

3. Given this Flask code snippet, what will you see if an error occurs?

from flask import Flask
app = Flask(__name__)
app.debug = True

@app.route('/')
def home():
    return 1 / 0  # This will cause an error

app.run()

medium

A. A detailed error page showing the division by zero traceback

B. A blank page with no error message

C. A generic 500 Internal Server Error page without details

D. The app will crash and stop running

Solution

1. Step 1: Analyze debug mode effect on errors

   With app.debug = True, Flask shows detailed error pages including tracebacks.

2. Step 2: Understand the error in the code

   The route causes a division by zero error, triggering the debug error page.

3. Final Answer:

   A detailed error page showing the division by zero traceback -> Option A

4. Quick Check:

   Debug mode shows detailed error page on exceptions [OK]

Hint: Debug True shows detailed error pages on exceptions [OK]

Common Mistakes:

• Expecting a generic error page in debug mode
• Thinking the app crashes completely
• Assuming no error message is shown

4. What is wrong with this Flask app code if you want to see debug error pages?

from flask import Flask
app = Flask(__name__)
app.config['DEBUG'] = False

@app.route('/')
def index():
    return 1 / 0

app.run()

medium

A. Flask apps cannot handle division by zero errors

B. The route function is missing a return statement

C. app.run() is missing the debug=True argument

D. Debug mode is disabled, so detailed error pages won't show

Solution

1. Step 1: Check debug mode setting

   The code sets app.config['DEBUG'] = False, which disables debug mode.

2. Step 2: Understand effect on error pages

   With debug disabled, Flask shows generic error pages, not detailed ones.

3. Final Answer:

   Debug mode is disabled, so detailed error pages won't show -> Option D

4. Quick Check:

   Debug False = no detailed error pages [OK]

Hint: Check if debug is True to get detailed error pages [OK]

Common Mistakes:

• Thinking app.run() debug argument is required if app.config['DEBUG'] is set
• Believing Flask cannot handle division by zero
• Confusing missing return with debug mode issues

5. You want to enable debug mode only during development but disable it in production. Which approach is best?

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

if os.getenv('FLASK_ENV') == 'development':
    app.debug = True
else:
    app.debug = False

@app.route('/')
def home():
    return 'Hello!'

app.run()

hard

A. Use environment variable to toggle debug mode as shown

B. Always set app.debug = True regardless of environment

C. Set app.debug = False and never change it

D. Remove app.debug and rely on app.run(debug=True)

Solution

1. Step 1: Understand environment-based debug control

   Using an environment variable like FLASK_ENV to toggle debug mode is a common and safe practice.

2. Step 2: Evaluate the code logic

   The code sets debug True only if environment is 'development', otherwise disables it, which is correct.

3. Final Answer:

   Use environment variable to toggle debug mode as shown -> Option A

4. Quick Check:

   Env variable controls debug mode safely [OK]

Hint: Use env vars to enable debug only in development [OK]

Common Mistakes:

• Leaving debug always on in production
• Not using environment variables for config
• Relying only on app.run debug argument