Testing with database in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a test client for FastAPI.

FastAPI

from fastapi.testclient import TestClient
from main import app

client = [1](app)

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

AAppClient

BClient

CFastClient

DTestClient

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong client class name.

Forgetting to import TestClient.

The TestClient class from fastapi.testclient is used to create a test client for the FastAPI app.

2fill in blank

medium

Complete the code to override the database dependency in FastAPI for testing.

FastAPI

from fastapi import Depends

def override_get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

app.dependency_overrides[[1]] = override_get_db

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

Adatabase

Bdb_session

Cget_db

Dget_database

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong dependency name.

Not overriding the dependency before tests.

The original database dependency function is usually named get_db. We override it with our test version.

3fill in blank

hard

Fix the error in the test function to correctly use the test client and database session.

FastAPI

def test_create_item():
    response = client.post('/items/', json={'name': 'Test Item'})
    assert response.status_code == [1]
    data = response.json()
    assert data['name'] == 'Test Item'

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

A201

B404

C400

D200

Attempts:

3 left

💡 Hint

Common Mistakes

Expecting 200 instead of 201.

Confusing client errors with success codes.

When creating a new item, the API should return status code 201 Created to indicate success.

4fill in blank

hard

Fill both blanks to create a test database session and override the app dependency.

FastAPI

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args=[1])
TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

def override_get_db():
    db = TestingSessionLocal()
    try:
        yield db
    finally:
        db.close()

app.dependency_overrides[[2]] = override_get_db

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

A{"check_same_thread": False}

B{"check_same_thread": True}

Cget_db

Ddatabase_session

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong connect_args for SQLite.

Overriding the wrong dependency.

SQLite requires check_same_thread=False for testing with multiple threads. The dependency to override is get_db.

5fill in blank

hard

Fill all three blanks to write a test that creates an item and checks the database directly.

FastAPI

def test_create_and_check_db():
    response = client.post('/items/', json=[1])
    assert response.status_code == 201
    data = response.json()
    with TestingSessionLocal() as db:
        item = db.query(Item).filter(Item.name == [2]).first()
        assert item is not None
        assert item.name == [3]

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

A{'name': 'Test Item'}

B"Test Item"

C'Test Item'

D{'name': 'Another Item'}

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong data type for JSON payload.

Mismatching quotes in string literals.

Not querying the database correctly.

The test posts JSON with the item name, then queries the database for the same name as a string. The JSON uses a dictionary, and the query uses string literals.

Practice

1. Why should you use a separate test database when testing FastAPI applications that interact with a database?

easy

A. To skip database setup during tests

B. To make tests run slower and more realistic

C. To avoid changing real data and keep tests isolated

D. To share data between tests and production

Solution

1. Step 1: Understand test isolation

   Tests should not affect real data to avoid accidental data loss or corruption.

2. Step 2: Use separate test database

   Using a separate database keeps test data separate and allows safe repeated testing.

3. Final Answer:

   To avoid changing real data and keep tests isolated -> Option C

4. Quick Check:

   Separate test database = safe isolated tests [OK]

Hint: Always isolate test data from real data [OK]

Common Mistakes:

• Using production database for tests
• Not cleaning test data after tests
• Assuming tests don't affect real data

2. Which of the following is the correct way to override the database dependency in FastAPI tests?

easy

A. app.replace_dependency(get_db, override_get_db)

B. app.override_dependency(get_db, override_get_db)

C. app.set_dependency(get_db, override_get_db)

D. app.dependency_overrides[get_db] = override_get_db

Solution

1. Step 1: Recall FastAPI dependency override syntax

   FastAPI uses the dictionary app.dependency_overrides to replace dependencies during tests.

2. Step 2: Match correct syntax

   Assigning override_get_db function to app.dependency_overrides[get_db] is the correct method.

3. Final Answer:

   app.dependency_overrides[get_db] = override_get_db -> Option D

4. Quick Check:

   Override dependencies via app.dependency_overrides dict [OK]

Hint: Use app.dependency_overrides dictionary to override [OK]

Common Mistakes:

• Using non-existent methods like set_dependency
• Trying to override dependencies without assignment
• Confusing syntax with other frameworks

3. Given this test code snippet, what will be the output of response.json()?

def test_create_item(client):
    response = client.post('/items/', json={'name': 'Book', 'price': 10})
    return response.json()

medium

A. {'id': 1, 'name': 'Book', 'price': 10}

B. {'name': 'Book', 'price': 10}

C. An error because POST is not allowed

D. Empty response {}

Solution

1. Step 1: Understand POST /items/ behavior

   Assuming the endpoint creates an item and returns it with an assigned id.

2. Step 2: Analyze response.json()

   The response should include the new item's id plus the sent data.

3. Final Answer:

   {'id': 1, 'name': 'Book', 'price': 10} -> Option A

4. Quick Check:

   POST creates item and returns it with id [OK]

Hint: POST usually returns created object with id [OK]

Common Mistakes:

• Expecting only sent data without id
• Assuming POST is disallowed
• Ignoring response content

4. What is the main issue with this test code snippet?

def override_get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

app.dependency_overrides[get_db] = override_get_db

def test_read_items(client):
    response = client.get('/items/')
    assert response.status_code == 200
    assert response.json() == []

medium

A. The dependency override is missing

B. The test database session is not properly created or isolated

C. The test client is not defined

D. The test expects wrong status code

Solution

1. Step 1: Check override_get_db function

   It uses SessionLocal but does not ensure a separate test database or transaction rollback.

2. Step 2: Identify test isolation problem

   Without a test database or rollback, tests may affect real data or each other.

3. Final Answer:

   The test database session is not properly created or isolated -> Option B

4. Quick Check:

   Test DB session must be isolated and separate [OK]

Hint: Ensure test DB session is isolated and rolled back [OK]

Common Mistakes:

• Not using a test database
• Not overriding dependencies correctly
• Assuming client is auto-defined

5. You want to write a test that creates an item, then reads it back from the test database in FastAPI. Which approach correctly ensures the test uses a separate database and cleans up after itself?

hard

A. Use a test database URL, override get_db to yield a session from it, and rollback after each test

B. Use the production database URL and delete created items manually after test

C. Use an in-memory list to mock database and skip actual DB calls

D. Run tests without overriding get_db and rely on default DB

Solution

1. Step 1: Use separate test database URL

   This prevents tests from affecting production data.

2. Step 2: Override get_db to yield test session and rollback

   This ensures each test runs isolated and cleans up changes automatically.

3. Final Answer:

   Use a test database URL, override get_db to yield a session from it, and rollback after each test -> Option A

4. Quick Check:

   Separate DB + override + rollback = clean isolated tests [OK]

Hint: Always isolate tests with separate DB and rollback [OK]

Common Mistakes:

• Using production DB for tests
• Not cleaning test data after tests
• Skipping dependency overrides