Testing with database in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask test with database setup?

Consider this Flask test code snippet that uses a test database. What will be the output when running this test?

Flask

import unittest
from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['TESTING'] = True

db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

class UserTestCase(unittest.TestCase):
    def setUp(self):
        db.create_all()
        user = User(name='Alice')
        db.session.add(user)
        db.session.commit()

    def tearDown(self):
        db.session.remove()
        db.drop_all()

    def test_user_count(self):
        count = User.query.count()
        print(count)

if __name__ == '__main__':
    unittest.main()

A Raises an AttributeError

B 0

C 1

D Raises an OperationalError

💡 Hint

Think about what happens in setUp before the test runs.

The setUp method creates the database tables and adds one user named 'Alice'. So the count of users is 1.

🔧 Debug

intermediate

Why does this Flask test raise an error when accessing the database?

This Flask test raises an error when trying to query the database. What is the cause?

Flask

import unittest
from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['TESTING'] = True

db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

class UserTestCase(unittest.TestCase):
    def test_user_count(self):
        count = User.query.count()
        print(count)

if __name__ == '__main__':
    unittest.main()

A The database tables were not created before the test ran

B The User model is missing a primary key

C The app config is missing the TESTING flag

D The query syntax is incorrect

💡 Hint

Check if the database schema exists before querying.

Without calling db.create_all(), the tables do not exist, so querying raises an error.

📝 Syntax

advanced

Which option correctly uses Flask's test client with database setup?

Select the code snippet that correctly sets up a Flask test client and database for testing a route that queries the database.

A

def setUp(self):
    self.app = app.test_client()
    db.create_all()
    user = User(name='Bob')
    db.session.add(user)
    db.session.commit()

B

def setUp(self):
    self.app = app.test_client()
    user = User(name='Bob')
    db.session.add(user)
    db.session.commit()

C

def setUp(self):
    self.app = app.test_client
    db.create_all()
    user = User(name='Bob')
    db.session.add(user)
    db.session.commit()

D

def setUp(self):
    self.app = app.test_client()
    db.drop_all()
    user = User(name='Bob')
    db.session.add(user)
    db.session.commit()

💡 Hint

Remember to call test_client as a function and create tables before adding data.

Option A correctly calls app.test_client() and creates tables before adding data. Option A misses parentheses on test_client. Option A misses creating tables. Option A drops tables but does not create them before adding data.

❓ state_output

advanced

What is the value of 'user_count' after running this Flask test?

Given this test code, what will be the value of 'user_count' printed?

Flask

import unittest
from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['TESTING'] = True

db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

class UserTestCase(unittest.TestCase):
    def setUp(self):
        db.create_all()
        user1 = User(name='Alice')
        db.session.add(user1)
        db.session.commit()

    def test_add_user(self):
        user2 = User(name='Bob')
        db.session.add(user2)
        db.session.commit()
        user_count = User.query.count()
        print(user_count)

    def tearDown(self):
        db.session.remove()
        db.drop_all()

if __name__ == '__main__':
    unittest.main()

A Raises an error

B 1

C 0

D 2

💡 Hint

Consider the users added in setUp and test_add_user.

setUp adds one user, test_add_user adds another, so total is 2.

🧠 Conceptual

expert

Which statement best explains why Flask tests use an in-memory SQLite database?

Why do Flask tests often configure the database URI as 'sqlite:///:memory:' during testing?

A It stores the database in a file named ':memory:' on disk for persistence

B It creates a temporary database in RAM that is fast and isolated for each test run

C It connects to the production database but disables writes

D It uses a remote database server optimized for testing

💡 Hint

Think about speed and isolation in tests.

Using 'sqlite:///:memory:' creates a fast, temporary database in RAM that is fresh for each test, avoiding side effects.