Trailing slash behavior in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when accessing a Flask route with a trailing slash?

Consider the following Flask route:

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

What will happen if a user visits /hello (without the trailing slash) in their browser?

Flask

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

A Flask will serve the 'Hello!' response without redirect.

B Flask will return a 404 Not Found error.

C Flask will redirect the user to '/hello/' automatically.

D Flask will raise a runtime error due to missing slash.

💡 Hint

Think about how Flask handles routes ending with a slash versus those without.

When a route is defined with a trailing slash, Flask treats it as a directory. Accessing the same route without the slash causes Flask to redirect to the slash version automatically.

❓ component_behavior

intermediate

What is the behavior of a Flask route without a trailing slash when accessed with one?

Given this Flask route:

 @app.route('/about')
 def about():
   return 'About page'

What happens if a user visits /about/ (with a trailing slash)?

Flask

@app.route('/about')
def about():
    return 'About page'

A Flask will return a 404 Not Found error.

B Flask will redirect the user to '/about' (without slash).

C Flask will raise a runtime error due to extra slash.

D Flask will serve the 'About page' response without redirect.

💡 Hint

Consider how Flask treats routes defined without trailing slashes.

When a route is defined without a trailing slash, Flask expects the URL exactly as defined. Accessing it with a trailing slash causes Flask to redirect to the no-slash version automatically.

📝 Syntax

advanced

Which Flask route definition causes a syntax error?

Identify the option that will cause a syntax error when defining a Flask route:

A

@app.route('/profile/'
def profile():
    return 'Profile'

B

@app.route('/contact')
def contact():
    return 'Contact'

C

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

D

@app.route('/dashboard/')
def dashboard():
    return 'Dashboard'

💡 Hint

Check for missing or mismatched parentheses in the route decorator.

Option A is missing a closing parenthesis in the @app.route decorator, causing a syntax error.

❓ state_output

advanced

What is the output when accessing a Flask route with and without trailing slash?

Given this Flask app code:

 @app.route('/item')
 def item():
   return 'Item without slash'

 @app.route('/item/')
 def item_slash():
   return 'Item with slash'

What will be the output when visiting /item and /item/ respectively?

Flask

@app.route('/item')
def item():
    return 'Item without slash'

@app.route('/item/')
def item_slash():
    return 'Item with slash'

A Visiting '/item' returns 'Item without slash'; visiting '/item/' returns 'Item with slash'.

B Both URLs return 'Item without slash'.

C Both URLs return 'Item with slash'.

D Visiting '/item' redirects to '/item/' and returns 'Item with slash'.

💡 Hint

Routes are defined separately for each URL pattern.

Each route is defined explicitly. Flask matches the exact route, so '/item' and '/item/' call different functions and return different responses.

🔧 Debug

expert

Why does this Flask app cause a redirect loop?

Examine the Flask app code:

 @app.route('/loop/')
 def loop():
   return redirect('/loop')

 @app.route('/loop')
 def loop_no_slash():
   return redirect('/loop/')

What is the cause of the redirect loop when visiting '/loop/' or '/loop'?

Flask

@app.route('/loop/')
def loop():
    return redirect('/loop')

@app.route('/loop')
def loop_no_slash():
    return redirect('/loop/')

A Flask automatically resolves the loop and serves the page.

B The app raises a runtime error due to conflicting routes.

C Only '/loop/' causes a redirect loop; '/loop' works fine.

D Both routes redirect to each other endlessly causing a loop.

💡 Hint

Check what each route redirects to and if it matches the other route.

The '/loop/' route redirects to '/loop', and '/loop' redirects back to '/loop/'. This causes an infinite redirect loop between the two URLs.

Practice

1. In Flask, what happens if you define a route with a trailing slash like @app.route('/home/') and a user visits /home without the slash?

easy

A. Flask returns a 404 Not Found error.

B. Flask serves the /home route without redirecting.

C. Flask redirects the user to /home/ automatically.

D. Flask throws a server error.

Solution

1. Step 1: Understand trailing slash behavior in Flask routes

   Routes defined with a trailing slash expect URLs to end with a slash.

2. Step 2: Check Flask's automatic redirect behavior

   If a user visits the URL without the trailing slash, Flask automatically redirects to the URL with the slash.

3. Final Answer:

   Flask redirects the user to /home/ automatically. -> Option C

4. Quick Check:

   Trailing slash route redirects missing slash URL [OK]

Hint: Trailing slash routes redirect missing slash URLs [OK]

Common Mistakes:

• Thinking Flask serves both URLs without redirect
• Assuming 404 error occurs without slash
• Believing Flask throws server error

2. Which of the following is the correct syntax to define a Flask route that does NOT accept a trailing slash?

easy

A. @app.route('/about/')

B. @app.route('/about//')

C. @app.route('about/')

D. @app.route('/about')

Solution

1. Step 1: Identify correct route syntax without trailing slash

   To define a route that does not accept a trailing slash, the route string should not end with a slash.

2. Step 2: Check each option

   @app.route('/about') uses '/about' without a trailing slash, which is correct syntax.

3. Final Answer:

   @app.route('/about') -> Option D

4. Quick Check:

   Route without trailing slash = no slash in route string [OK]

Hint: No trailing slash means route string ends without slash [OK]

Common Mistakes:

• Adding extra slashes in route string
• Omitting leading slash in route
• Confusing trailing slash with double slashes

3. Given this Flask code:

@app.route('/page/')
def page():
    return 'Page with slash'

What will happen if a user visits /page in their browser?

medium

A. The user sees 'Page with slash' without URL change.

B. The user is redirected to /page/ and sees 'Page with slash'.

C. The user gets a 404 Not Found error.

D. The server crashes with an error.

Solution

1. Step 1: Analyze route definition with trailing slash

   The route '/page/' expects URLs ending with a slash.

2. Step 2: Understand Flask's redirect behavior

   If the user visits '/page' without the slash, Flask redirects to '/page/' automatically and serves the response.

3. Final Answer:

   The user is redirected to /page/ and sees 'Page with slash'. -> Option B

4. Quick Check:

   Trailing slash route redirects missing slash URL [OK]

Hint: Trailing slash route redirects missing slash URL [OK]

Common Mistakes:

• Assuming no redirect happens
• Expecting 404 error on missing slash
• Thinking server crashes on missing slash

4. Consider this Flask code:

@app.route('/contact')
def contact():
    return 'Contact page'

What happens if a user visits /contact/ with a trailing slash?

medium

A. Flask returns a 404 Not Found error.

B. Flask throws a server error.

C. Flask serves the page normally.

D. Flask redirects to /contact without slash.

Solution

1. Step 1: Identify route without trailing slash

   The route '/contact' is defined without a trailing slash.

2. Step 2: Check Flask behavior on extra trailing slash

   Flask does NOT redirect URLs with an extra trailing slash to the no-slash route; it returns a 404 error.

3. Final Answer:

   Flask returns a 404 Not Found error. -> Option A

4. Quick Check:

   No trailing slash route rejects extra slash URL [OK]

Hint: No trailing slash route rejects URLs with slash [OK]

Common Mistakes:

• Thinking Flask redirects to no-slash URL
• Assuming page loads normally with slash
• Believing server throws error instead of 404

5. You want a single Flask route to accept both /blog and /blog/ URLs without redirecting or 404 errors. Which approach achieves this?

hard

A. Use Flask's strict_slashes=False option on the route decorator.

B. Define one route with trailing slash: @app.route('/blog/') only.

C. Define one route without trailing slash: @app.route('/blog') only.

D. Define two routes: @app.route('/blog') and @app.route('/blog/') pointing to the same function.

Solution

1. Step 1: Understand default Flask behavior

   By default, Flask treats routes with and without trailing slash differently, redirecting or 404ing as needed.

2. Step 2: Use strict_slashes=False to accept both

   Setting strict_slashes=False on the route decorator disables strict trailing slash matching, allowing both URLs without redirect or error.

3. Final Answer:

   Use Flask's strict_slashes=False option on the route decorator. -> Option A

4. Quick Check:

   strict_slashes=False allows both URLs [OK]

Hint: Use strict_slashes=False to accept both slash and no-slash URLs [OK]

Common Mistakes:

• Defining two routes duplicates code
• Relying on default redirect behavior
• Ignoring strict_slashes option