Testing with database in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a test client for the Flask app.

Flask

with app.test_client() as [1]:
    response = client.get('/')

Drag options to blanks, or click blank then click option'

Aclient

Bapp

Crequest

Dsession

Attempts:

3 left

💡 Hint

Common Mistakes

Using app instead of client as the variable name.

Using request which is for incoming requests, not test client.

The test_client() method returns a client object used to simulate requests. Naming it client is the common practice.

2fill in blank

medium

Complete the code to set up an in-memory SQLite database for testing.

Flask

app.config['SQLALCHEMY_DATABASE_URI'] = '[1]'

Drag options to blanks, or click blank then click option'

Asqlite:///:memory:

Bsqlite:///test.db

Cpostgresql://localhost/test

Dmysql://localhost/test

Attempts:

3 left

💡 Hint

Common Mistakes

Using a file-based SQLite URI instead of in-memory.

Using URIs for other databases not set up for tests.

Using sqlite:///:memory: creates a temporary in-memory database ideal for fast tests.

3fill in blank

hard

Fix the error in the test setup by completing the code to create all tables before tests.

Flask

with app.app_context():
    [1]

Drag options to blanks, or click blank then click option'

Adb.init_app(app)

Bdb.drop_all()

Cdb.session.commit()

Ddb.create_all()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling drop_all() which deletes tables.

Calling commit() without creating tables.

Calling db.create_all() inside the app context creates all tables needed for tests.

4fill in blank

hard

Fill both blanks to add a new user and commit the change in the test database.

Flask

new_user = User(username='testuser')
db.session.[1](new_user)
db.session.[2]()

Drag options to blanks, or click blank then click option'

Aadd

Bcommit

Cremove

Drollback

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() or rollback() which undo changes.

Forgetting to commit after adding.

Use add() to add the new user to the session, then commit() to save changes.

5fill in blank

hard

Fill all three blanks to query a user by username and check the result in a test.

Flask

user = User.query.filter_by([1]=[2]).[3]()

Drag options to blanks, or click blank then click option'

Ausername

B'testuser'

Cfirst

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using all() which returns a list instead of one user.

Using wrong field names or missing quotes around the username.

Use filter_by(username='testuser') and first() to get the first matching user.

Practice

1. What is the main purpose of using a test database in Flask testing?

easy

A. To store user sessions permanently

B. To isolate tests so they don't affect the real data

C. To speed up the app in production

D. To replace the main database in deployment

Solution

1. Step 1: Understand test database role

   A test database is used to keep test data separate from real data to avoid unwanted changes.

2. Step 2: Identify isolation benefit

   This isolation ensures tests do not affect the real app data or state.

3. Final Answer:

   To isolate tests so they don't affect the real data -> Option B

4. Quick Check:

   Test database isolates data = C [OK]

Hint: Test DB keeps test data separate from real data [OK]

Common Mistakes:

• Thinking test DB speeds up production
• Confusing test DB with session storage
• Assuming test DB replaces main DB in deployment

2. Which Flask testing method is used to prepare the test database before each test runs?

easy

A. tearDown

B. initTest

C. setUp

D. runTest

Solution

1. Step 1: Recall Flask test lifecycle methods

   In Flask testing, setUp runs before each test to prepare the environment.

2. Step 2: Match method to database preparation

   Preparing the test database (like creating tables) happens in setUp.

3. Final Answer:

   setUp -> Option C

4. Quick Check:

   Prepare DB before test = setUp [OK]

Hint: Use setUp to prepare before each test [OK]

Common Mistakes:

• Confusing tearDown with setup
• Using runTest which is not for setup
• Assuming initTest is a Flask method

3. Given this test code snippet, what will print(len(users)) output?

def test_user_creation(self):
    user = User(name='Anna')
    db.session.add(user)
    db.session.commit()
    users = User.query.all()
    print(len(users))

medium

A. 1

B. 0

C. Error: session not initialized

D. Multiple users depending on previous tests

Solution

1. Step 1: Analyze user addition and commit

   The code adds one user and commits to the test database.

2. Step 2: Query all users after commit

   Querying all users returns a list with the one added user.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   One user added and committed = 1 [OK]

Hint: Commit before query to see added users [OK]

Common Mistakes:

• Forgetting to commit before query
• Assuming previous tests add users
• Thinking session is not initialized

4. What is wrong with this test code snippet?

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

medium

A. Missing call to create_all() in tearDown

B. tearDown should not remove session

C. drop_all() should be in setUp, not tearDown

D. No call to rollback() before drop_all()

Solution

1. Step 1: Understand tearDown purpose

   tearDown cleans up after tests by removing session and dropping tables.

2. Step 2: Identify missing rollback before drop_all()

   Without rollback, pending changes may not be discarded, potentially causing errors or inconsistent state.

3. Final Answer:

   No call to rollback() before drop_all() -> Option D

4. Quick Check:

   Rollback before drop_all to discard changes [OK]

Hint: Always rollback before dropping tables in tearDown [OK]

Common Mistakes:

• Thinking create_all belongs in tearDown
• Believing session removal is wrong in tearDown
• Ignoring rollback before drop_all

5. You want to test a Flask app that uses a database. Which sequence correctly sets up and cleans the test database for each test?

hard

A. Call db.create_all() in setUp, run test, then db.drop_all() in tearDown

B. Call db.drop_all() in setUp, run test, then db.create_all() in tearDown

C. Call db.session.commit() in setUp, then db.session.remove() in tearDown

D. Call db.create_all() and db.drop_all() both in setUp

Solution

1. Step 1: Understand test setup and cleanup

   Creating tables before each test ensures a fresh database state.

2. Step 2: Understand test teardown

   Dropping tables after each test cleans up to avoid data carryover.

3. Step 3: Match correct sequence

   Calling create_all() in setUp and drop_all() in tearDown is the correct pattern.

4. Final Answer:

   Call db.create_all() in setUp, run test, then db.drop_all() in tearDown -> Option A

5. Quick Check:

   Create tables before, drop after test = A [OK]

Hint: Create tables before test, drop after test [OK]

Common Mistakes:

• Dropping tables before test instead of after
• Calling create_all and drop_all both in setUp
• Confusing commit with setup/teardown