Service layer pattern in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask service layer call?

Consider a Flask app using a service layer to fetch user data. What will be printed when calling print(user_service.get_user_name(2))?

Flask

class UserService:
    def __init__(self):
        self.users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

    def get_user_name(self, user_id):
        return self.users.get(user_id, 'Unknown')

user_service = UserService()
print(user_service.get_user_name(2))

A Alice

B Bob

C Unknown

D Charlie

💡 Hint

Look at the dictionary key used in the get_user_name call.

The get_user_name method returns the value for key 2, which is 'Bob'.

❓ state_output

intermediate

What is the state of the service after adding a new user?

After running user_service.add_user(4, 'Diana'), what will user_service.users contain?

Flask

class UserService:
    def __init__(self):
        self.users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

    def add_user(self, user_id, name):
        self.users[user_id] = name

user_service = UserService()
user_service.add_user(4, 'Diana')

A {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

B {}

C {4: 'Diana'}

D {1: 'Alice', 2: 'Bob', 3: 'Charlie', 4: 'Diana'}

💡 Hint

Adding a user updates the users dictionary.

The add_user method adds a new key-value pair to the existing dictionary.

📝 Syntax

advanced

Which option correctly defines a Flask service method to delete a user?

Choose the correct method to remove a user by user_id from the users dictionary in the service layer.

Flask

class UserService:
    def __init__(self):
        self.users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

    def delete_user(self, user_id):
        # method to remove user
        pass

A

def delete_user(self, user_id):
    self.users.pop(user_id, None)

B

def delete_user(self, user_id):
    self.users.remove(user_id)

C

def delete_user(self, user_id):
    del self.users[user_id]

D

def delete_user(self, user_id):
    self.users.delete(user_id)

💡 Hint

Use a dictionary method that safely removes a key without error if missing.

pop(key, None) removes the key if present without raising an error. del raises an error if key missing. remove and delete are invalid for dict.

🔧 Debug

advanced

Why does this Flask service method raise a KeyError?

Given this service method, why does calling get_user_name(5) raise a KeyError?

Flask

class UserService:
    def __init__(self):
        self.users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

    def get_user_name(self, user_id):
        return self.users[user_id]

user_service = UserService()
user_service.get_user_name(5)

A Because the method should use self.users.get(user_id) instead of self.users[user_id]

B Because the users dictionary is empty

C Because user_id 5 does not exist in the users dictionary and direct access raises KeyError

D Because the UserService class is missing an __init__ method

💡 Hint

Accessing a missing key in a dictionary directly causes an error.

Direct dictionary access with a missing key raises KeyError. Using get() returns None or default instead.

🧠 Conceptual

expert

What is the main benefit of using a service layer in a Flask app?

Choose the best explanation for why a service layer is used in Flask applications.

A It separates business logic from route handlers, making code easier to maintain and test

B It automatically generates HTML templates for the app

C It replaces the need for a database by storing data in memory

D It handles user authentication and authorization automatically

💡 Hint

Think about code organization and responsibilities.

The service layer keeps business logic separate from routes, improving maintainability and testability. It does not generate templates, replace databases, or handle auth automatically.