Token refresh mechanism in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this token refresh simulation?

Consider a simple token refresh function that returns a new access token if the refresh token is valid. What will be the output of the code below?

Rest API

def refresh_token(refresh_token):
    valid_refresh_tokens = {"abc123": "new_access_token_1", "def456": "new_access_token_2"}
    return valid_refresh_tokens.get(refresh_token, "Invalid refresh token")

print(refresh_token("abc123"))

A None

B new_access_token_1

C Invalid refresh token

D new_access_token_2

💡 Hint

Look at the dictionary keys and what the function returns when the key is found.

The function checks if the refresh token exists in the dictionary. Since "abc123" is a key, it returns the corresponding value "new_access_token_1".

🧠 Conceptual

intermediate

Which statement best describes the purpose of a refresh token?

Choose the best description of what a refresh token does in an authentication system.

A It stores user credentials like username and password.

B It grants permanent access to all resources without expiration.

C It replaces the access token and is sent with every API request.

D It is used to obtain a new access token after the old one expires.

💡 Hint

Think about what happens when an access token expires.

A refresh token allows the client to get a new access token without asking the user to log in again, improving security and user experience.

🔧 Debug

advanced

Why does this token refresh code raise an error?

The code below is intended to refresh an access token but raises an error. What is the cause?

Rest API

valid_tokens = {"r1": "token1", "r2": "token2"}

def refresh_token(refresh_token):
    if refresh_token in valid_tokens:
        return valid_tokens[refresh_token]
    else:
        return "Invalid token"

print(refresh_token("r1"))

A NameError because valid_tokens is used before it is defined.

B TypeError because refresh_token is a function parameter and cannot be used as a variable.

C KeyError because 'r1' is not in valid_tokens.

D No error; the code runs and prints 'token1'.

💡 Hint

Check the order of variable and function definitions.

The function uses valid_tokens before it is defined, causing a NameError at runtime.

📝 Syntax

advanced

Which option correctly implements a token refresh function with error handling?

Select the code snippet that correctly returns a new access token or raises an exception if the refresh token is invalid.

A

def refresh_token(token):
    if token not in tokens:
        raise ValueError("Invalid token")
    return tokens[token]

B

def refresh_token(token):
    if token in tokens
        return tokens[token]
    else:
        raise ValueError("Invalid token")

C

def refresh_token(token):
    if token in tokens:
        return tokens[token]
    else
        raise ValueError("Invalid token")

D

def refresh_token(token):
    if token in tokens:
        return tokens[token]
    else:
        return ValueError("Invalid token")

💡 Hint

Look carefully at the syntax of if statements and raising exceptions.

Option A uses correct syntax for if statement and raising an exception. Options B and C miss colons, and D returns an exception object instead of raising it.

🚀 Application

expert

How many tokens will be valid after this refresh sequence?

Given the code below, how many tokens remain valid after refreshing 'r1' and 'r3'?

Rest API

valid_tokens = {"r1": "token1", "r2": "token2", "r3": "token3"}

# Refresh function invalidates old refresh token and issues a new one

def refresh_token(token):
    if token not in valid_tokens:
        return "Invalid token"
    new_token = f"token_new_{token}"
    del valid_tokens[token]
    valid_tokens[f"r_new_{token}"] = new_token
    return new_token

refresh_token("r1")
refresh_token("r3")

A 4

B 2

C 3

D 5

💡 Hint

Count tokens before and after refresh calls carefully.

Initially 3 tokens exist. Each refresh deletes one old token and adds one new token. After two refreshes, total tokens = 3 - 2 + 2 = 3. But keys are different. The question asks how many tokens remain valid, which is 3. However, the new tokens added are different keys, so total keys after refresh are 3 (r2, r_new_r1, r_new_r3). So answer is 3.