Debug mode error pages in Flask

Introduction

Debug mode error pages help you see detailed error information when your Flask app has a problem. This makes fixing bugs easier.

When you are building a Flask app and want to find mistakes quickly.

When you want to see the exact error and where it happened in your code.

When testing new features to catch errors early.

When you want to see helpful debugging tools in your browser.

When running your app locally before making it live.

Syntax

Flask

app = Flask(__name__)
app.debug = True

# or
app = Flask(__name__)
app.config['DEBUG'] = True

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

Setting app.debug = True or app.config['DEBUG'] = True turns on debug mode.

Debug mode shows detailed error pages with stack traces and interactive debugger.

Examples

This example turns on debug mode by setting app.debug = True. If an error happens, Flask shows a detailed error page.

Flask

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

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

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

This example uses app.config['DEBUG'] = True and intentionally causes an error to show the debug error page.

Flask

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

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

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

Sample Program

This Flask app runs with debug mode on. When you visit the home page, it causes a division by zero error. Flask shows a detailed debug error page with the error message and stack trace.

Flask

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

@app.route('/')
def home():
    # Intentional error to show debug page
    return 1 / 0

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

Important Notes

Never use debug mode on a live website because it can show sensitive information to visitors.

Debug mode automatically reloads the app when you change code, so you don't need to restart manually.

If you see the debug error page, you can use the interactive debugger to inspect variables and fix the problem.

Summary

Debug mode shows detailed error pages to help find bugs quickly.

Turn it on by setting app.debug = True or app.config['DEBUG'] = True.

Use debug mode only during development, never in production.

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