Decorator for role requirement in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a decorator in Flask?

A decorator in Flask is a function that wraps another function to add extra behavior without changing the original function's code. It is often used to modify routes or add checks like authentication.

beginner

Why use a role requirement decorator in Flask?

A role requirement decorator checks if a user has the right role before allowing access to a route. It helps protect parts of a web app so only users with certain roles can use them.

intermediate

How does the @wraps decorator help when creating a custom decorator?

The @wraps decorator preserves the original function's name and docstring when it is wrapped by a decorator. This helps with debugging and keeps the function's identity clear.

beginner

What happens if a user without the required role tries to access a Flask route protected by a role requirement decorator?

The decorator will stop the user from accessing the route, usually by returning a 403 Forbidden error or redirecting them to a login or error page.

intermediate

Example: What does this Flask decorator do? @role_required('admin') def admin_panel(): pass

This decorator checks if the current user has the 'admin' role before running the admin_panel function. If the user is not an admin, access is denied.

What is the main purpose of a role requirement decorator in Flask?

A To speed up the Flask app

B To log user activity

C To change the URL of a route

D To check if a user has a specific role before accessing a route

A role requirement decorator ensures only users with the right role can access certain routes.

Which Python module helps preserve function metadata when writing decorators?

A functools

B os

C sys

D json

The functools module provides the @wraps decorator to preserve function metadata.

If a user lacks the required role, what HTTP status code is commonly returned?

A 403 Forbidden

B 200 OK

C 404 Not Found

D 500 Internal Server Error

403 Forbidden indicates the user is not allowed to access the resource.

In Flask, where is the current user's role usually stored for checking in decorators?

A In the request headers

B In the session or user object

C In the URL parameters

D In the database only

User roles are typically stored in the session or user object accessible during the request.

What does the @wraps decorator do inside a custom decorator?

A Changes the function's behavior

B Runs the function twice

C Preserves the original function's name and docstring

D Prevents the function from running

@wraps keeps the original function's metadata intact after decoration.

Explain how to create a Flask decorator that restricts access based on user roles.

Think about wrapping the route function and checking user roles before running it.

Describe what happens when a user without the required role tries to access a protected Flask route.

Focus on the flow of control when role validation fails.

Practice

1. What is the main purpose of a decorator that checks user roles in a Flask app?

easy

A. To log user activity without restrictions

B. To speed up the execution of functions

C. To change the user role dynamically

D. To restrict access to certain functions based on user roles

Solution

1. Step 1: Understand the role of decorators in Flask

   Decorators wrap functions to add extra behavior, like checking conditions before running the function.

2. Step 2: Identify the purpose of role checking

   Role checking ensures only users with the right permissions can access certain parts of the app.

3. Final Answer:

   To restrict access to certain functions based on user roles -> Option D

4. Quick Check:

   Role check decorator = restrict access [OK]

Hint: Decorators add checks before function runs [OK]

Common Mistakes:

• Thinking decorators change user roles
• Assuming decorators speed up code
• Confusing logging with access control

2. Which of the following is the correct syntax to define a decorator that requires a role named 'admin' in Flask?

easy

A. @require_role(admin)\ndef admin_panel():\n pass

B. def require_role('admin'):\n def admin_panel():\n pass

C. @require_role('admin')\ndef admin_panel():\n pass

D. @require_role['admin']\ndef admin_panel():\n pass

Solution

1. Step 1: Recognize decorator syntax in Python

   Decorators use @ followed by the decorator function call with parentheses if it takes arguments.

2. Step 2: Check correct argument passing

   The role name 'admin' must be passed as a string inside parentheses.

3. Final Answer:

   @require_role('admin')\ndef admin_panel():\n pass -> Option C

4. Quick Check:

   Decorator with parentheses and string argument = correct [OK]

Hint: Decorator with argument uses parentheses and quotes [OK]

Common Mistakes:

• Missing parentheses for decorator with arguments
• Passing role without quotes
• Using square brackets instead of parentheses

3. Given this decorator code snippet, what will be printed when a user with role 'user' tries to access the decorated function?

def require_role(role):
    def decorator(func):
        def wrapper(user_role):
            if user_role == role:
                return func()
            else:
                return 'Access Denied'
        return wrapper
    return decorator

@require_role('admin')
def secret_data():
    return 'Secret Info'

print(secret_data('user'))

medium

A. Secret Info

B. Access Denied

C. TypeError

D. None

Solution

1. Step 1: Understand the decorator logic

   The wrapper checks if the passed user_role matches the required role 'admin'. If yes, it calls the function; otherwise, returns 'Access Denied'.

2. Step 2: Analyze the function call with 'user' role

   The user_role 'user' does not match 'admin', so the wrapper returns 'Access Denied'.

3. Final Answer:

   Access Denied -> Option B

4. Quick Check:

   User role mismatch = Access Denied [OK]

Hint: Mismatch role returns 'Access Denied' [OK]

Common Mistakes:

• Assuming function runs regardless of role
• Confusing return values
• Ignoring wrapper function logic

4. Identify the error in this Flask role-checking decorator code:

def require_role(role):
    def decorator(func):
        def wrapper(*args, **kwargs):
            if current_user.role == role:
                func(*args, **kwargs)
            else:
                return 'Forbidden'
        return wrapper
    return decorator

medium

A. Missing return statement before func call inside wrapper

B. Incorrect use of *args and **kwargs

C. current_user.role should be current_user.roles

D. Decorator should not have nested functions

Solution

1. Step 1: Check the wrapper function's return behavior

   The wrapper calls func(*args, **kwargs) but does not return its result, so the decorated function returns None.

2. Step 2: Confirm correct fix

   Adding 'return' before func call ensures the wrapper returns the function's output properly.

3. Final Answer:

   Missing return statement before func call inside wrapper -> Option A

4. Quick Check:

   Always return function result in wrapper [OK]

Hint: Wrapper must return the wrapped function's result [OK]

Common Mistakes:

• Forgetting to return function output
• Misusing argument unpacking
• Incorrect attribute name for user role

5. You want to create a Flask decorator that allows access if the user role is either 'admin' or 'editor'. Which of these decorator implementations correctly achieves this?

hard

A. def require_roles(*roles):
def decorator(func):
def wrapper(user_role):
if user_role in roles:
return func()
return 'Access Denied'
return wrapper
return decorator

@require_roles('admin', 'editor')
def edit_page():
return 'Edit Page'

B. def require_roles(roles):
def decorator(func):
def wrapper(user_role):
if user_role == roles:
return func()
return 'Access Denied'
return wrapper
return decorator

@require_roles(['admin', 'editor'])
def edit_page():
return 'Edit Page'

C. def require_roles(*roles):
def decorator(func):
def wrapper(user_role):
if user_role == roles:
return func()
return 'Access Denied'
return wrapper
return decorator

@require_roles('admin', 'editor')
def edit_page():
return 'Edit Page'

D. def require_roles(roles):
def decorator(func):
def wrapper(user_role):
if user_role in roles:
func()
return 'Access Denied'
return wrapper
return decorator

@require_roles(['admin', 'editor'])
def edit_page():
return 'Edit Page'

Solution

1. Step 1: Understand how to accept multiple roles

   Using *roles collects multiple role arguments as a tuple, allowing easy membership checks.

2. Step 2: Check membership and return function result

   The wrapper checks if user_role is in roles and returns func() result; otherwise returns 'Access Denied'.

3. Final Answer:

   Implementation using *roles and 'in' check with return func() -> Option A

4. Quick Check:

   Use *args and 'in' for multiple roles [OK]

Hint: Use *args and 'in' to check multiple roles [OK]

Common Mistakes:

• Comparing user_role to tuple instead of membership
• Not returning function result
• Passing roles as list but comparing with == instead of 'in'