Route decorator (@app.route) in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @app.route decorator in Flask?

The @app.route decorator tells Flask which URL should trigger a specific function. It connects a web address to the code that runs when that address is visited.

beginner

How do you define a route for the home page using @app.route?

You use @app.route('/') above a function. This means when someone visits the root URL (like 'http://localhost:5000/'), Flask runs that function.

intermediate

Can @app.route handle different HTTP methods? How?

Yes. You can add the methods argument like @app.route('/path', methods=['GET', 'POST']) to tell Flask which HTTP methods the route accepts.

intermediate

What happens if two functions have the same route with @app.route?

Flask will use the last function defined for that route, overwriting the previous one. This can cause unexpected behavior, so routes should be unique.

beginner

How can you capture a variable part of the URL using @app.route?

You use angle brackets in the route like @app.route('/user/<username>'). Flask passes the value in the URL to the function as an argument.

What does @app.route('/about') do in a Flask app?

A Imports the Flask module

B Creates a new Flask app

C Starts the Flask server

D Links the URL '/about' to a function that runs when visited

The route decorator connects the URL '/about' to a function that Flask runs when that URL is accessed.

How do you allow a route to accept both GET and POST requests?

A Add <code>methods=['GET', 'POST']</code> in <code>@app.route</code>

B Use <code>@app.methods(['GET', 'POST'])</code>

C Set <code>app.methods = ['GET', 'POST']</code>

D Flask routes accept all methods by default

You specify allowed HTTP methods by adding the methods argument in the route decorator.

What will Flask do if two functions have the same route?

A Throw an error and stop the app

B Use the last defined function for that route

C Run both functions in order

D Ignore the second function

Flask overwrites the first function with the last one defined for the same route.

How do you capture a username from the URL in Flask?

A Use <code>@app.route('/user/&lt;username&gt;')</code>

B Use <code>@app.route('/user/:username')</code>

C Use <code>@app.route('/user/username')</code>

D Use <code>@app.route('/user?username')</code>

Angle brackets in the route capture parts of the URL as variables passed to the function.

What URL does @app.route('/') represent?

A A 404 error page

B The about page

C The home or root page

D A static file

The '/' route is the root URL, usually the home page of the website.

Explain how the @app.route decorator works in Flask and how it connects URLs to functions.

Think about how you tell Flask what to do when someone visits a web page.

Describe how to create a dynamic route in Flask that accepts a username from the URL.

Imagine you want to greet different users by name in the URL.

Practice

1. What does the @app.route decorator do in a Flask application?

easy

A. It connects a URL path to a specific function that runs when that URL is visited.

B. It creates a database connection for the app.

C. It styles the HTML pages served by Flask.

D. It starts the Flask server automatically.

Solution

1. Step 1: Understand the role of @app.route

   The decorator @app.route is used to link a URL path to a function in Flask.

2. Step 2: Identify what happens when the URL is visited

   When a user visits that URL, Flask runs the linked function to generate a response.

3. Final Answer:

   It connects a URL path to a specific function that runs when that URL is visited. -> Option A

4. Quick Check:

   @app.route links URL to function [OK]

Hint: Remember: @app.route maps URLs to functions [OK]

Common Mistakes:

• Thinking @app.route starts the server
• Confusing it with database setup
• Assuming it styles pages

2. Which of the following is the correct syntax to create a route for the URL path /home in Flask?

easy

A. @app.route('/home')\ndef home():\n return 'Welcome!'

B. app.route('/home')\ndef home():\n return 'Welcome!'

C. @app.route('/home')\nreturn 'Welcome!'

D. @app.route('/home')\ndef home:\n return 'Welcome!'

Solution

1. Step 1: Check the decorator syntax

   The correct decorator syntax uses @app.route('/home') placed immediately before a function definition.

2. Step 2: Verify the function definition and return statement

   The function must be defined with parentheses and a colon, and it returns a string response.

3. Final Answer:

   @app.route('/home')\ndef home():\n return 'Welcome!' -> Option A

4. Quick Check:

   Decorator + function with parentheses = correct syntax [OK]

Hint: Decorator needs @ and function needs () and : [OK]

Common Mistakes:

• Missing @ before app.route
• Omitting parentheses in function definition
• Placing return outside function

3. What will be the output when visiting the URL /user/42 given this Flask code?

@app.route('/user/<int:id>')
def show_user(id):
    return f"User ID is {id}"

medium

A. User ID is <int:id>

B. User ID is 42

C. 404 Not Found error

D. User ID is id

Solution

1. Step 1: Understand the route with variable part

   The route /user/<int:id> captures an integer from the URL and passes it as the argument id to the function.

2. Step 2: Check the function output

   The function returns a string with the captured id value inserted using an f-string.

3. Final Answer:

   User ID is 42 -> Option B

4. Quick Check:

   Variable URL part passes 42 to function [OK]

Hint: Variable parts in < > pass values to function [OK]

Common Mistakes:

• Thinking is printed literally
• Expecting a 404 error without reason
• Confusing variable name with string

4. Identify the error in this Flask route code:

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

medium

A. The return statement is outside the function.

B. The route decorator is missing parentheses.

C. The function name is invalid.

D. The function body is not indented properly.

Solution

1. Step 1: Check indentation inside the function

   Python requires the function body to be indented. Here, print and return are not indented under def hello():.

2. Step 2: Verify decorator and function syntax

   The decorator has correct parentheses and the function name is valid, so no errors there.

3. Final Answer:

   The function body is not indented properly. -> Option D

4. Quick Check:

   Python needs indented function bodies [OK]

Hint: Indent function code under def line [OK]

Common Mistakes:

• Forgetting to indent function body
• Removing parentheses from decorator
• Misnaming the function

5. You want to create a Flask route that accepts a username in the URL like /profile/john and returns Hello, john!. Which code correctly achieves this?

hard

A. @app.route('/profile/<username>')\ndef profile():\n return f'Hello, {username}!'

B. @app.route('/profile/')\ndef profile(username):\n return f'Hello, {username}!'

C. @app.route('/profile/<username>')\ndef profile(username):\n return f'Hello, {username}!'

D. @app.route('/profile/username')\ndef profile():\n return 'Hello, username!'

Solution

1. Step 1: Use variable URL part for username

   The route must capture username as a string variable using <username> in the URL.

2. Step 2: Define function with matching parameter and return greeting

   The function must accept username as a parameter and return the greeting using an f-string.

3. Final Answer:

   @app.route('/profile/<username>')\ndef profile(username):\n return f'Hello, {username}!' -> Option C

4. Quick Check:

   Variable URL + matching param + f-string = correct [OK]

Hint: Match variable name in route and function parameter [OK]

Common Mistakes:

• Using for a string username
• Not passing username parameter to function
• Hardcoding username in route path