Development server with debug mode in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the Flask development server.

Flask

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

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

Aport

Bhost

Cthreaded

Ddebug

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'host' or 'port' instead of 'debug' to enable debug mode.

Forgetting to set any argument, so debug mode stays off.

The debug=True argument enables debug mode in Flask's development server.

2fill in blank

medium

Complete the code to run the Flask app on all network interfaces.

Flask

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

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

A'127.0.0.1'

B'0.0.0.0'

C'localhost'

D'192.168.1.1'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'localhost' or '127.0.0.1' which only allow local access.

Using a random IP that might not be assigned to the machine.

Using host='0.0.0.0' makes the server listen on all interfaces, allowing access from other devices.

3fill in blank

hard

Fix the error in the code to enable debug mode correctly.

Flask

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

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

ATrue

B'False'

C1

D'True'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 'True' as a string instead of the boolean True.

Passing 1 which works but is less clear than True.

The debug parameter expects a boolean value, not a string.

4fill in blank

hard

Fill both blanks to run the Flask app on port 8080 with debug mode on.

Flask

if __name__ == '__main__':
    app.run(port=[1], debug=[2])

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

A8080

B5000

CTrue

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using the default port 5000 instead of 8080.

Setting debug to False or as a string.

Port 8080 is a common alternative port, and debug=True enables debug mode.

5fill in blank

hard

Fill all three blanks to run the Flask app on all interfaces, port 3000, with debug mode enabled.

Flask

if __name__ == '__main__':
    app.run(host=[1], port=[2], debug=[3])

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

A'0.0.0.0'

B3000

CTrue

D'localhost'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'localhost' instead of '0.0.0.0' for host.

Using port 5000 instead of 3000.

Setting debug to 'True' as a string.

Host '0.0.0.0' listens on all interfaces, port 3000 is specified, and debug=True enables debug mode.

Practice

1. What does setting debug=True in app.run() do in a Flask app?

easy

A. It hides all error messages from the user.

B. It disables the server from running.

C. It makes the app run faster in production.

D. It shows detailed error messages and reloads the server on code changes.

Solution

1. Step 1: Understand debug mode purpose

   Debug mode helps developers by showing detailed error messages and automatically restarting the server when code changes.

2. Step 2: Check what debug=True does in app.run()

   Passing debug=True enables this helpful behavior during development.

3. Final Answer:

   It shows detailed error messages and reloads the server on code changes. -> Option D

4. Quick Check:

   Debug mode = detailed errors + auto reload [OK]

Hint: Debug mode shows errors and reloads automatically [OK]

Common Mistakes:

• Thinking debug mode speeds up production
• Believing debug mode hides errors
• Confusing debug mode with disabling the server

2. Which of the following is the correct way to start a Flask development server with debug mode enabled?

easy

A. app.run(debug=True)

B. app.run(debug=1)

C. app.run(mode='debug')

D. app.run(enable_debug=True)

Solution

1. Step 1: Recall Flask run method parameters

   The app.run() method accepts a debug parameter as a boolean to enable debug mode.

2. Step 2: Identify correct syntax

   Using app.run(debug=True) is the standard and correct way to enable debug mode.

3. Final Answer:

   app.run(debug=True) -> Option A

4. Quick Check:

   Correct debug syntax = debug=True [OK]

Hint: Use debug=True exactly in app.run() [OK]

Common Mistakes:

• Using incorrect parameter names like mode or enable_debug
• Passing debug as a string or number instead of boolean
• Confusing debug with other run parameters

3. What will happen when you run this Flask code?

from flask import Flask
app = Flask(__name__)

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

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

medium

A. The server will not start because the route is missing a template.

B. The server starts but debug mode is off, so no error details are shown.

C. The server starts with debug mode, showing errors and auto-reloading on code changes.

D. The code will raise a syntax error because debug=True is invalid here.

Solution

1. Step 1: Analyze the code structure

   The code defines a Flask app with a simple route returning 'Hello!'. The server is started with debug=True.

2. Step 2: Understand effect of debug=True

   Passing debug=True enables debug mode, so errors show detailed info and the server reloads on code changes.

3. Final Answer:

   The server starts with debug mode, showing errors and auto-reloading on code changes. -> Option C

4. Quick Check:

   app.run(debug=True) = debug mode active [OK]

Hint: debug=True means errors show and server reloads [OK]

Common Mistakes:

• Thinking debug=True causes syntax errors
• Assuming debug mode is off by default
• Believing routes need templates to start server

4. You wrote this code but the Flask server does not reload on code changes:

from flask import Flask
app = Flask(__name__)

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

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

What is the likely fix?

medium

A. Change app = Flask(__name__) to app = Flask().

B. Add debug=True to app.run() to enable auto-reload.

C. Add reload=True parameter to app.run().

D. Remove the if __name__ == '__main__' guard.

Solution

1. Step 1: Identify missing debug mode

   The code calls app.run() without debug=True, so debug mode and auto-reload are off.

2. Step 2: Enable debug mode for auto-reload

   Adding debug=True to app.run() activates debug mode, enabling error messages and auto-reload on code changes.

3. Final Answer:

   Add debug=True to app.run() to enable auto-reload. -> Option B

4. Quick Check:

   Missing debug=True disables auto-reload [OK]

Hint: Add debug=True to app.run() for auto-reload [OK]

Common Mistakes:

• Trying to use reload=True which is invalid
• Removing the main guard which is unrelated
• Changing Flask app initialization unnecessarily

5. You want to run your Flask app with debug mode but only on your local machine, not in production. Which code snippet correctly achieves this?

hard

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

B. app.run(debug=True) # runs debug mode always

C. if __name__ == '__main__':
app.run() # debug mode off

D. app.run() # debug mode on by default

Solution

1. Step 1: Understand the main guard usage

   The if __name__ == '__main__' block ensures the server runs only when the script is executed directly, typically local development.

2. Step 2: Enable debug mode only inside main guard

   Placing app.run(debug=True) inside this block enables debug mode only locally, avoiding debug mode in production.

3. Step 3: Check other options

   Running app.run(debug=True) outside the guard runs debug mode always, which is unsafe for production.

4. Final Answer:

   if __name__ == '__main__': app.run(debug=True) -> Option A

5. Quick Check:

   Main guard + debug=True = safe local debug [OK]

Hint: Use main guard with debug=True for local only [OK]

Common Mistakes:

• Running debug mode outside main guard risks production
• Assuming debug mode is off by default
• Not using main guard and causing unwanted server start